**October 29 2018**

**New article: Optimal storage of a single photon by a single intra-cavity atom**

Our article **Optimal storage of a single photon by a single intra-cavity atom** has been published as: New Journal of Physics, vol. 20 (2018).

#### Optimal storage of a single photon by a single intra-cavity atom

L. Giannelli, T. Schmit, T. Calarco, C. Koch, S. Ritter, and G. MorigiNew Journal of Physics, vol. 20 (2018)

arxiv:1804.10558

We theoretically analyse the efficiency of a quantum memory for single photons. The photons propagate along a transmission line and impinge on one of the mirrors of a high-finesse cavity. The quantum memory is constituted by a single atom within the optical resonator. Photon storage is realised by the controlled transfer of the photonic excitation into a metastable state of the atom and occurs via a Raman transition with a suitably tailored laser pulse, which drives the atom. Our study is supported by numerical simulations, in which we include the modes of the transmission line and we use the experimental parameters of existing experimental setups. It reproduces the results derived using input-output theory in the corresponding regime and can be extended to compute dynamics where the input-output formalism cannot be straightforwardly applied. Our analysis determines the maximal storage efficiency, namely, the maximal probability to store the photon in a stable atomic excitation, in the presence of spontaneous decay and cavity parasitic losses. It further delivers the form of the laser pulse that achieves the maximal efficiency by partially compensating parasitic losses. We numerically assess the conditions under which storage based on adiabatic dynamics is preferable to non-adiabatic pulses. Moreover, we systematically determine the shortest photon pulse that can be efficiently stored as a function of the system parameters.

**October 09 2018**

**09.10.2018: We congratulate B.Sc. Peter-Maximilian Ney!**

We congratulate Peter-Maximilian Ney for his Bachelor degree with the thesis "Solid State Quantum Memory for Single Photons".

**October 09 2018**

**09.10.2018: We congratulate B.Sc. Jonas Cassel!**

We congratulate Jonas Cassel for his Bachelor degree with the thesis "Large deviation techniques applied to systems with long-range interactions".

**August 24 2018**

**24.08.2018: We congratulate M.Sc. Lukas Himbert!**

We congratulate Lukas Himbert for his Masters degree on "Quantum phases of long-range interacting bosons".

**July 10 2018**

**11-12.07: Visit of Prof. Pierbiagio Pieri (Uni Camerino)**

Prof. Pierbiagio Pieri (University of Camerino) visits us on July 11th and 12th. He will present a talk on Thursday 12th in the Physikalisches Kolloqium at 16.00 in C6 4, Hörsaal II.

**June 11 2018**

**14.06: Visit of Prof. Dr. Martin Weitz (Uni Bonn)**

Prof. Dr. Martin Weitz (Uni Bonn) visits us on Thursday, June 14th. He will present a talk in the Physikalisches Kolloqium at 16.00 in C6 4, Hörsaal II.

**June 04 2018**

**07-08.06: Visit of Prof. Dr. Christof Wunderlich (Uni Siegen)**

Prof. Dr. Christof Wunderlich (Uni Siegen) visits us on June 7th and 8th. He will present a talk on Thursday 7th in the Physikalisches Kolloqium at 16.00 in C6 4, Hörsaal II.

**May 11 2018**

**11.05.2018: Adipta Pal (IISER Kolkata) is our guest for three months**

Adipta Pal (IISER Kolkata) is a guest in our group for the next three months. His visit is funded by the DAAD WISE project.

**May 04 2018**

**07-08.05.2018: Visit of Davide Giambastiani (Università di Pisa)**

Davide Giambastiani (Università di Pisa) is visiting us from Monday 07.05 to Tuesday 08.05. He will give a talk entitled "Interaction Range Effects and Universality in the BCS-BEC Crossover of Spin-Orbit Coupled Fermi Gases" on Monday at 14:30 in room 4.18.

**April 16 2018**

**16-17.04.2018: Visit of Cliò Efthimia Agrapidis (IFW Dresden)**

Cliò Efthimia Agrapidis (IFW Dresden) is visiting us and will give a talk about "Frustration induced topological order" in the group seminar "Many-body physics with light" on 17.04 at 10:30 in room 4.18.

**March 26 2018**

**27.03.2018: Visit of Dr. Nicolo Defenu (Uni Heidelberg)**

Dr. Nicolo Defenu (Uni Heidelberg) will visit us on 27.03.2018 and he will give a talk entitled "Kibble-Zurek mechanism in long-range interacting systems" in our group meeting at 10:30 in room 4.18.

**February 09 2018**

**New article: Quenches across the self-organization transition in multimode cavities**

Our article **Quenches across the self-organization transition in multimode cavities** has been published as: New Journal of Physics, vol. 20 (2018).

#### Quenches across the self-organization transition in multimode cavities

T. Keller, V. Torggler, S. B. Jäger, S. Schütz, H. Ritsch and G. MorigiNew Journal of Physics, vol. 20 (2018)

arxiv:1709.07833

A cold dilute atomic gas in an optical resonator can be radiatively cooled by coherent scattering processes when the driving laser frequency is tuned close to but below the cavity resonance. When the atoms are sufficiently illuminated, their steady state undergoes a phase transition from a homogeneous distribution to a spatially organized Bragg grating. We characterize the dynamics of this self-ordering process in the semi-classical regime when distinct cavity modes with commensurate wavelengths are quasi-resonantly driven by laser fields via scattering by the atoms. The lasers are simultaneously applied and uniformly illuminate the atoms; their frequencies are chosen so that the atoms are cooled by the radiative processes, and their intensities are either suddenly switched or slowly ramped across the self-ordering transition. Numerical simulations for different ramp protocols predict that the system will exhibit long-lived metastable states, whose occurrence strongly depends on the initial temperature, ramp speed, and the number of atoms.

**January 31 2018**

**05-06.02.2018: Visit of Prof. Dr. Andreas Buchleitner and collaborators (Uni Freiburg)**

Prof. Dr. Andreas Buchleitner (Uni Freiburg) and his collaborators will visit us from Monday 05.02.2018 to Tuesday 06.02.2018.

Dr. L. de Forges de Parny (Uni Freiburg) will give a talk in our group seminar on Tuesday 06.02.2018, at 10:00 in room 4.18.

## Supersolid Phase in Ultracold Gases

The supersolid phase has attracted a great attention since its theoretical prediction by Andreev and Liftshitz in 1969. This exotic phase is characterized by both long-range phase coherence (a superfluid property) and spatial ordering (a solid property). In other words, a supersolid is able to move without friction while retaining a rigid shape! Many different unsuccessful experiments have focused on the observation of this phase ... until 2016. For the first time, the supersolid phase has been unambiguously observed in two independent ultracold gases experiments [1,2]. We have focused on the model investigated in the T. Esslinger's experiment, in which an optical cavity allows the establishment of long-range interactions between bosons in an optical lattice [1]. Using quantum Monte Carlo simulations, we obtained a similar - but more precise - phase diagram than the experimental one, and confirmed the existence of the supersolid phase using finite-size scaling techniques. Our theoretical study also elucidates the nature of the quantum phase transitions.[1] R. Landig, L. Hruby, N. Dogra, M. Landini, R. Mottl, T. Donner, and T. Esslinger, Nature 532, 476 (2016).

**January 23 2018**

**25.01.2018: Visit of Guido Pupillo (Uni Strasbourg)**

Guido Pupillo (Uni Strasbourg, France) visits us and will give a talk in the Physikalisches Kolloqium at 16.00 in C6 4, Hörsaal II.

**January 19 2018**

**19.01.2018: Visit of Haggai Landa (IPhT)**

Haggai Landa (IPhT, CEA Saclay, France) visits us.

**January 05 2018**

**12.01.2017: Visit of Dr. Nicolo Defenu (Uni Heidelberg)**

Dr. Nicolo Defenu (Uni Heidelberg) is visiting us.

**December 01 2017**

**1.12.2017: Visit of Dr. Nicolo Defenu (Uni Heidelberg)**

Dr. Nicolo Defenu (Uni Heidelberg) is visiting us.

**November 29 2017**

**29.11.2017: Opening lecture of "Purer Zufall"**

# Ein Quantum Zufall

## Prof. Dr. Frank Wilhelm-Mauch, Theoretische Physik

Prof. Dr. Jürgen Eschner, Experimentalphysik

Universität des Saarlandes

### 29. Nov. 2017, 18:30 Uhr, Filmhaus

Die Quantenphysik hat das mechanistische Weltbild der klassischen Physik erschüttert: Im physikalischen Geschehen auf mikroskopischer Ebene führt kein Weg um den Zufall herum. Mehr noch: Dies ist, so weit wir wissen, der einzig echte Zufall. Dennoch kann man mit den Gesetzen der Quantenphysik arbeiten und Vorhersagen machen. Wir können sogar Quanten-Verschlüsselungsverfahren anwenden, die ohne diesen Zufall überhaupt nicht funktionieren würden.

Link: Purer Zufall

**November 09 2017**

**09.11.17-10.11.17: Visit by Prof. Paz**

Prof. Paz from Buenos Aires University is visiting us. On Thursday, at 4 pm in C6 4, lecture room II, he will give a talk on "Quantum thermodynamics and tests of non equilibrium fluctuation theorems".

**October 17 2017**

**03.11.2017: Inaugural lecture of P.D. Dr. Marc Bienert.**

P.D. Dr. Marc Bienert will give his inaugural lecture on Friday 03.11.2017 at 17.00 in room E.04, building E2.6.

## Das Leuchten von Störstellen in Diamant: Eine elegante Beschreibung

Ein einzelnes angeregtes Atom emittiert durch die Ankopplung an das freie Strahlungsfeld spontan ein Photon, dessen Frequenz der des beteiligten elektronichen Übergangs entspricht. In der Quantenoptik wird diese Dämpfung des elektronichen Dipols durch eine einfache Mastergleichung beschrieben. Ist der Dipol in einer Festkörpermatrix eingebaut, ändert sich dieses Bild: Während des Emissionsprozesses können Schwingungsquanten im Festkörper erzeugt oder vernichtet werden, was sich im Emissionsspektrum durch phononische Seitenbänder manifestiert. In dieser Vorlesung wird vorgestellt, wie sich die quantenoptische Mastergleichung erweitern lässt, um die wichtigsten Merkmale eines Festkörper-Quantenemitters wiederzugeben und gleichzeitig so handlich bleibt, dass eine vollständig analytische Lösung möglich ist. Damit lassen sich Quantenemitter in Festkörpern bis hin zu hybriden Quantensystemen untersuchen.

**October 16 2017**

**23.10.-27.10.2017: Visit by Dr. Jake Taylor**

From the 23th to 27th of October, Dr. Jake Taylor (QuICS/JQI/NIST) is visiting our group. He will give a talk on Tuesday, 24.10. at 10.00 in room 4.18 with the title "Exploring quantum phases of matter with light", and a further talk on Wednesday, 25.10., in Ctrl-Q.

**October 16 2017**

We welcome Alexey Konovalov as a new group member. Alexey will make his PhD studies at Saarland University.

**October 16 2017**

**20.10.2017: Dr. Arezoo Mokhberi (University of Mainz) visits us.**

Dr. Arezoo Mokhberi (University of Mainz) visits us on Friday, October 20th. She will give a talk at 10:15 in seminar room 4.18, building E2.6.

## Cold, trapped ions excited to Rydberg states

Excitation of cold, trapped ions to electronically high-laying Rydberg states offers a unique opportunity for observing novel effects arising from the interplay between the Coulomb interaction and their giant dipole moments. Rydberg ions exhibits the potential for quantum simulation and quantum computation. There are also exciting proposals for investigating phase transitions in these systems. However, experimental challenges had impeded the progress on this project for about ten years. The Mainz ion-trapping group was pioneering to demonstrate the first excitation of trapped ions to Rydberg states. I will talk about our experiment, and will give a picture of the current status of this technology as well as future plans.

**September 29 2017**

**04-05.10: Dr. Hiroki Takahashi (University of Sussex) visits us.**

Dr. Hiroki Takahashi (University of Sussex) visits us from Wednesday, October 4th to Thursday, October 5th. He will give a talk on Wednesday, October 4th at 9:00 in our Ctrl-Q seminar (room E.04, building E2.6).

## Coupling a single ion to an optical fibre cavity: Towards strong coupling

Cavity QED with single trapped ions has been pursued for long time as a promising avenue for quantum network but has not yet achieved strong coupling with a single ion. This is due to the technical difficulty in combining ion traps and dielectric cavity mirrors in a small volume. The development of fiber-based Fabry-Perot cavities has offered a new perspective for integrating small optical cavities in ion traps. Their reduced size and possibility of tight integration and electrical shielding has the potential to achieve a small cavity-mode volume without seriously compromising the trapping stability. Recently we succeeded in coupling a single trapped ion to an optical fibre cavity. We observed a Purcell effect caused by the cavity which strongly alters the photon emission property of a Calcium 40 ion (Takahashi et al. PRA 96, 023824 (2017)). Through analysis of the measurement results, we have obtained an ion-cavity coupling of 5.3 +/- 0.1 MHz. the largest reported so far for a single ion in the infrared domain. The coupling is mainly limited by the spatial overlap between the ion and cavity field. However this can be further optimised by manipulating the rf field in the trap. We report on the most recent development in the experiment towards achieving strong coupling of a single ion.

**September 28 2017**

**28.9.2017: We congratulate Sascha Wald for the successful defence of his PhD thesis!**

## We congratulate Sascha Wald for the successful defence of his PhD thesis!

On September 28th Sascha defended his thesis with the title "Thermalization and relaxation of quantum systems" at the University of Lorraine in Nancy. His PhD studies took place within a cotutelle agreement between Saarland University and University of Lorraine in Nancy. Directors of the thesis were Prof. Malte Henkel for Nancy and Prof. Giovanna Morigi for Saarbrücken.**September 26 2017**

**26.09.2017: We congratulate B.Sc. Marc Hermes!**

We congratulate Marc Hermes for his Bachelor degree on "Quantum memory for photons".

**September 26 2017**

**26.09.2017: We congratulate B.Sc. Marvin Schnubel!**

We congratulate Marvin Schnubel for his Bachelor degree on "Kommensurabel-Inkommensurabel Phasenübergang bei endlicher Temperatur".

**September 26 2017**

**26.09.2017: We congratulate B.Sc. Julia Brunkert!**

We congratulate Julia Brunkert for his Bachelor degree on "Kollektive Bewegung zweier Atome in einem Resonator".

**September 18 2017**

Our article **Enhanced Second-Order Nonlinearity for THz Generation by Resonant Interaction of Exciton-Polariton Rabi Oscillations with Optical Phonons** has been published as: Phys. Rev. Lett. 119, 127401 (2017)

#### Enhanced Second-Order Nonlinearity for THz Generation by Resonant Interaction of Exciton-Polariton Rabi Oscillations with Optical Phonons

K. Rojan, Y. Leger, G. Morigi, M. Richard, and A. MinguzziPhys. Rev. Lett. 119, 127401 (2017)

arXiv:1706.03650

Semiconductor microcavities in the strong-coupling regime exhibit an energy scale in the terahertz (THz) frequency range, which is fixed by the Rabi splitting between the upper and lower exciton-polariton states. While this range can be tuned by several orders of magnitude using different excitonic media, the transition between both polaritonic states is dipole forbidden. In this work, we show that, in cadmium telluride microcavities, the Rabi-oscillation-driven THz radiation is actually active without the need for any change in the microcavity design. This feature results from the unique resonance condition which is achieved between the Rabi splitting and the phonon-polariton states and leads to a giant enhancement of the second-order nonlinearity.

**September 11 2017**

**26.09.-27.09.: Visit of Haggai Landa**

Haggai Landa (IPhT, CEA Saclay, France) visits us from Tuesday, September 26th to Wednesday, September 27th. On Tuesday, September 26, at 1 pm in room E.04, he will give a talk on "Ultracold active matter".

Abstract:

Ultracold active matter

-------------------------------------

The "active" aspect of active matter, from the statistical-physics perspective, is the breaking of detailed balance in the microscopic dynamics. Hence, modelling of nonequilibrium microscopic conditions and their implications, in particular on the macroscopic dynamics (such as the appearance of emergent equilibrium), is now active as a field of research. Surprisingly, recent theory studies and experiments with ultracold ions trapped in vacuum, make contact with these questions;

(i) A fundamental model of transport in a noisy environment is that of the Brownian ratchet, or Brownian motor. It models the emergence of nonvanishing currents in a noisy system despite the vanishing of all mean forces. Crucially based on symmetry breaking, it is a basic model for some of the physics underlying, e.g., biological molecular motors. I will discuss self-organized ion crystals featuring transport of ratchet-like discrete solitons. The rate and direction can be described as a Kramer's escape applied to a collective coordinate, with an emergent effective temperature.

(ii) In recent years ion traps are microfabricated with electrode-ion distances down to tens of micrometers, whence ion dynamics are ruled by an interplay of nonlinearity, chaos, stochastic heating and laser cooling. A detailed understanding of these dynamics is interesting for practical and theoretical reasons, and at the same time, the ion trap offers a system with excellent experimental accessibility to nonequilibrium, microscopic stochastic processes. I will present a study of the structure of phase space and a unified analysis of the Hamiltonian and stochastic dynamics in terms of action angle coordinates.

**August 30 2017**

**New article: Spectral properties of single photons from quantum emitters**

Our article **Spectral properties of single photons from quantum emitters** has been published as: Phys. Rev. A 96, 023861 (2017)

#### Spectral properties of single photons from quantum emitters

P. Müller, T. Tentrup, M. Bienert, G. Morigi, and J. EschnerPhys. Rev. A 96, 023861 (2017)

arXiv:1705.02489

Quantum networks require flying qubits that transfer information between the nodes. This may be implemented by means of single atoms (the nodes) that emit and absorb single photons (the flying qubits) and requires full control of photon absorption and emission by the individual emitters. In this paper, we theoretically characterize the wave packet of a photon emitted by a single atom undergoing a spontaneous Raman transition in a three-level scheme. We investigate several excitation schemes that are experimentally relevant and discuss control parameters that allow one to tailor the spectrum of the emitted photon wave packet.

**August 14 2017**

**17.08.2017: Visit of Timo Felser**

Timo Felser will visit us on 17.08.2017 and he will give a talk about "Symmetries in Tensor Networks" in room 4.18 at 11.00.

**July 11 2017**

**11.07.2017: Visit of Dr. Nicolo Defenu (Uni Heidelberg)**

Dr. Nicolo Defenu (Uni Heidelberg) is visiting us.

**July 03 2017**

**New article: Semiclassical theory of synchronization-assisted cooling**

Our article **Semiclassical theory of synchronization-assisted cooling** has been published as: Phys. Rev. A 95, 063852 (2017)

#### Semiclassical theory of synchronization-assisted cooling

S. B. Jäger, M. Xu, S. Schütz, M. J. Holland, and G. MorigiPhys. Rev. A 95, 063852 (2017)

arXiv:1702.01561.

We analyze the dynamics leading to radiative cooling of an atomic ensemble confined inside an optical cavity when the atomic dipolar transitions are incoherently pumped and can synchronize. Our study is performed in the semiclassical regime and assumes that cavity decay is the largest rate in the system dynamics. We identify three regimes characterizing the cooling. At first hot atoms are individually cooled by the cavity friction forces. After this stage, the atoms' center-of-mass motion is further cooled by the coupling to the internal degrees of freedom while the dipoles synchronize. In the latest stage dipole-dipole correlations are stationary and the center-of-mass motion is determined by the interplay between friction and dispersive forces due to the coupling with the collective dipole. We analyze this asymptotic regime by means of a mean-field model and show that the width of the momentum distribution can be of the order of the photon recoil. Furthermore, the internal excitations oscillate spatially with the cavity standing wave forming an antiferromagnetic-like order.

**July 03 2017**

**We welcome Nahuel Freitas, who joins our group as Postdoctoral fellow!**

We welcome Nahuel Freitas, who joins our group as Postdoctoral fellow!

**July 03 2017**

**05-07.07: Dr. Vedran Dunjko (MPI of Quantum Optics) visits us.**

Dr. Vedran Dunjko (MPI of Quantum Optics) visits us from Wednesday, July 5th to Friday, July 7th. He will give a talk on Thursday, July 6th at 10:30 (room E.11, building E2.6).

## "Advances in quantum reinforcement learning"

### Abstract:

Quantum machine learning explores the interaction between quantum computing and machine learning, in both directions of influence. This emerging field has been trending in recent times for multiple reasons. First is due to promises that quantum computing can speed up big data analyses. The second stems from the converse direction of influence, as ML methods have been shown promising for the control quantum experiments by handling hard optimization tasks However, machine learning and artificial intelligence machinery can do more than analyze data and perform optimization. Indeed, progress in more general learning methods, such as reinforcement learning have driven some of the most exciting technological and scientific trends of recent times, such as the AlphaGo system.In this overview talk, we will present some of the results of the intersection between reinforcement learning and quantum computing. We will explain the basic ideas behind reinforcement learning on the example of Projective Simulation (PS), which is a physics-inspired learning model. Following this, we will review some of the main results exploring the mutually beneficial exchange between reinforcement learning and quantum computing including: quantum speed-ups of the PS model, generic quantum improvements in learning efficiency for reinforcement learning, but also most recent results showing how learning agents can be used to discover new quantum experiments.

We will finish off with a brief discussion of what consequences these (predominantly theoretical) results may have on machine learning, artificial intelligence and quantum information processing in the near (and not so near) term.

### Some references:

-Projective simulation for artificial intelligence, H. J. Briegel & G. De las Cuevas Sci. Rep. 2, Article number: 400 (2012)-Projective simulation with generalization, A. A. Melnikov, A. Makmal, V. Dunjko, Hans J. Briegel, arXiv:1504.02247

- Meta-learning within Projective Simulation, A. Makmal, A. A. Melnikov, V. Dunjko, and H. J. Briegel, IEEE Access 4, 2110 (2016) [arXiv:1602.08017].

-Quantum speed-up for active learning agents, G. Paparo, V. Dunjko, A. Makmal, M. A. Martin-Delgado, and H. J. Briegel, Phys. Rev. X 4, 031002 (2014) [arXiv:1401.4997]

- Quantum-Enhanced Machine Learning, V. Dunjko, J. M. Taylor, and H. J. Briegel, Phys. Rev. Lett. 117, 130501 (2016).

-Active learning machine learns to create new quantum experiments, A. A. Melnikov, H. Poulsen Nautrup, M. Krenn, V. Dunjko, M. Tiersch, A. Zeilinger, H. J. Briegel

**June 14 2017**

**14.6.17: We congratulate Dr. rer. nat. Katharina Rojan for the successful defence of her PhD thesis!**

Saarbrücken, June 14th. We congratulate Katharina Rojan, who successfully defended her PhD thesis in Saarbrücken in front of a jury of internationally recognized scientists. The PhD project is a cotutelle agreement between the University of Grenoble Alpes and Saarland University, the studies were performed under the joint supervision of Anna Minguzzi, Research Director at CNRS in Grenoble, and Giovanna Morigi.

**June 12 2017**

**14.06.07: Ctrl-Q Workshop "Semiconductor cavity quantum electrodynamics"**

Ctrl-Q Workshop on "Semiconductor cavity quantum electrodynamics".

Organizers: G. Morigi, QSAAR

Date: June 14th, 2017

Conference room in the Graduate Centre (Gebäude C9 3)

Program:

9:00 Welcome

9:10-9:50 Dario Gerace, Uni Pavia

"Quantum simulation with hybrid spin-photon qubits"

9:50-10:30 Maxime Richard, LANEF Grenoble

"nonequilibrium phenomena in exciton-polariton quantum fluid"

10:30-11:00 Coffee Break

11:00-11:40 Anna Minguzzi, CNRS Grenoble

"Persistent currents in excitons-polaritons on a ring"

11:40-12:20 Jonathan Keeling, Uni St. Andrews

"Modelling organic condensates from weak to strong coupling"

**June 12 2017**

**New article: Phases of cold atoms interacting via photon-mediated long-range forces**

Our article **Phases of cold atoms interacting via photon-mediated long-range forces** has been published as: J. Stat. Mech. (2017) 064002.

#### Phases of cold atoms interacting via photon-mediated long-range forces

Tim Keller, Simon B. Jager, and Giovanna MorigiJ. Stat. Mech. (2017) 064002.

Atoms in high-finesse optical resonators interact via the photons they multiply scatter into the cavity modes. The dynamics is characterized by dispersive and dissipative optomechanical long-range forces, which are mediated by the cavity photons, and exhibits a steady state for certain parameter regimes. In standing-wave cavities the atoms can form stable spatial gratings. Moreover, their asymptotic distribution is a Maxwell–Boltzmann whose effective temperature is controlled by the laser parameters. In this work we show that in a two-mode standing-wave cavity the stationary state possesses the same properties and phases of the generalized Hamiltonian mean field model in the canonical ensemble. This model has three equilibrium phases: a paramagnetic, a nematic, and a ferromagnetic one, which here correspond to different spatial orders of the atomic gas and can be detected by means of the light emitted by the cavities. We further discuss perspectives for investigating in this setup the ensemble inequivalence predicted for the generalized Hamiltonian mean field model.

**June 12 2017**

**12.06.2017: Visit of Dr. Nicolo Defenu (Uni Heidelberg)**

Dr. Nicolo Defenu (Uni Heidelberg) is visiting us.

**June 12 2017**

**Congratulations to Marc Bienert for his venia legendi in theoretical physics!**

Congratulations to Marc Bienert for his venia legendi in theoretical physics!

**June 07 2017**

**New article: Multimode Bose-Hubbard model for quantum dipolar gases in confined geometries**

Our article **Multimode Bose-Hubbard model for quantum dipolar gases in confined geometries** has been published in the 1 June 2017 issue of Physical Review A (Vol.95, No.6).

#### Multi-mode Bose-Hubbard model for quantum dipolar gases in confined geometries

F. Cartarius, A. Minguzzi, and G. Morigi

Phys. Rev. A 95, 063603 (2017).

We theoretically consider ultracold polar molecules in a wave guide. The particles are bosons: They experience a periodic potential due to an optical lattice oriented along the wave guide and are polarized by an electric field orthogonal to the guide axis. The array is mechanically unstable by opening the transverse confinement in the direction orthogonal to the polarizing electric field and can undergo a transition to a double-chain (zigzag) structure. For this geometry we derive a multimode generalized Bose-Hubbard model for determining the quantum phases of the gas at the mechanical instability, taking into account the quantum fluctuations in all directions of space. Our model limits the dimension of the numerically relevant Hilbert subspace by means of an appropriate decomposition of the field operator, which is obtained from a field theoretical model of the linear-zigzag instability. We determine the phase diagrams of small systems using exact diagonalization and find that, even for tight transverse confinement, the aspect ratio between the two transverse trap frequencies controls not only the classical but also the quantum properties of the ground state in a nontrivial way. Convergence tests at the linear-zigzag instability demonstrate that our multimode generalized Bose-Hubbard model can catch the essential features of the quantum phases of dipolar gases in confined geometries with a limited computational effort.

**March 29 2017**

** 5.4: Ctrl-Q Lecture on "Quantum Machine Learning" by V. Dunjko**

Dr. Vedran Dunjko, University of Innsbruck

Wednesday April 5th at 11:00, in HS001 E1.3, Computer Science Department

Title: Machine learning and Quantum Information Processing: a perfect match

Abstract:

The nascent field of Quantum Machine Learning has been generating a substantial buzz in the last few years.

The broad theme of this field is the interplay between the disciplines of quantum information processing (QIP) and of machine learning (ML). The research is thus typically driven by one of two basic questions. The first question focuses on the ways in which QIP can help in ML problems. The complementary line of research studies the extent to which ML can be beneficially applied in QIP tasks. In this overview talk, I will present the basic ideas behind quantum computing and information processing. I will draw parallels between features of QIP and aspects of machine learning, which suggest that quantum effects may play an integral role in improved learning algorithms. While some of the features require full blown quantum computers, some can be addressed using near term devices.

This will be illustrated through a selection of recent results which probe the potential and limitations of quantum-enhanced learning, followed by a snapshot of fresh proposals addressing the complementary question of exploiting ML techniques in quantum experiments.

These results suggest not only that (Q)ML applications may be among the best reasons to build quantum computers in the first place (barring perhaps quantum simulations and cryptography), but also that ML may significantly help bringing about large-scale quantum computers. I will finish the talk with a perspective on the field we have developed in Innsbruck, which also touches the arguably broader topic of the interplay of artificial general intelligence and quantum mechanics.

--

**March 29 2017**

**4.4.-6.4.: Dr. Vedran Dunjko (University of Innsbruck) visits uns.**

Dr. Vedran Dunjko (University of Innsbruck) visits uns and will give a lecture on April 5th on Quantum Machine Learning.

**February 28 2017**

We welcome Francesco Rosati as a new group member. Francesco will make his PhD studies at Saarland University within the European ITN ColOpt.

**February 24 2017**

**1-4.03.2017: Visit of Dr. Giulia De Rosi (University of Trento)**

Dr. Giulia De Rosi (University of Trento) will be visiting us from 01.03 to 04.03. On Thursday 02.03 at 10.30 in room 4.18 she will give a talk about: "Collective oscillations of trapped atomic gases in low dimensions: a tool for the investigation of collisional processes".

Abstract:

Since 20 years, both theoretical and experimental investigation of collective oscillations has been carried out in trapped quantum gases. Fermionic and bosonic gases at different interaction, temperature, dimensions and geometrical configurations have been studied. We show a unified description of collective modes for all above atomic gases. All collective frequencies have been calculated by solving a single equation for the fl ow velocity derived starting from the hydrodynamic equations. Moreover, by using the sum- rule approach, we predict a different excitation signal at high temperature for the dipole compression mode in the hydrodynamic (single frequency) and collisionless (beating of 2 frequencies) regime for a one dimension (1D) harmonically trapped Bose gas. This theoretical prediction opens promising perspectives for the experimental investigation of collisional effects in 1D.

References:

[1] G. De Rosi and S. Stringari,
"Collective oscillations of a trapped quantum gas in low dimensions",

Phys. Rev. A 92 , 053617 (2015)

[2] G. De Rosi and S. Stringari,
"Hydrodynamic versus collisionless dynamics of a one-dimensional
harmonically trapped Bose gas" ,

Phys. Rev. A 94 , 063605 (2016)

**February 20 2017**

**24.02.2017: Visit by Nahuel Freitas (Universidad de Buenos Aires, Argentina)**

Nahuel Freitas (Universidad de Buenos Aires, Argentina) will visit us on Friday 24.02.2017. He will give a talk at 10.00 in room 4.18 on "Fundamental limits for cooling of linear quantum refrigerators".

Abstract:

I study the asymptotic dynamics of a network of oscillators whose frequencies and couplings are periodically driven while coupled with a number of bosonic reservoirs. I obtain exact results for the heat currents coming into the system from each reservoir (valid beyond the usual weak coupling, weak driving or Markovian approximations). I use these expressions to rigorously prove the validity of the dynamical version of the third law of thermodynamics (Nernst unattainability principle) in this context. The fundamental limit for cooling is imposed by a heating process which is present at zero temperature. It consists of the non resonant creation of pairs of excitations in the reservoirs by the driving field. It is intrinsically quantum, it is linked to the dynamical Casimir effect and it is not captured by usual perturbative treatments. Thus, for any cooling strategy there is a minimum attainable temperature, that we estimate for some relevant examples. Experimental proposals will also be discussed.

**February 15 2017**

**15.02.2017: Visit by Julian Leonard (ETH Zürich)**

Julian Leonard from ETH Zürich is visiting us.

**February 07 2017**

** 7.2.2017: Visit of Dr. Nicolo Defenu and Prof. Dr. TIlman Enns, University of Heidelberg**

Dr. Nicolo Defenu and Prof. Dr. TiIlman Enns from the University of Heidelberg are visiting us today.

**February 01 2017**

**We welcome Prof. Dr. Simone Montangero!**

We welcome Prof. Dr. Simone Montangero, who joins the physics department as Heisenberg fellow of the German Research Foundation.

**January 26 2017**

**26.01.2017 "Rechnen mit Nichts", Prof. Dr. Giovanna Morigi, Saarland University**

#### "Rechnen mit Nichts"

Prof. Dr. Giovanna Morigi, Saarland University.Physikalisches Kolloquium und Ringvorlesung über Nichts.

Stadtgalerie Saarbrücken, 18:30.

Alle physikalischen Systeme besitzen einen stabilen Zustand, der die minimal mögliche Energie hat. Für das elektromagnetische Feld ist dieses der Zustand mit Null Energiequanten, das "Vakuum". Während mit diesem Begriff in der klassischen Physik der leere Raum - das Nichts - bezeichnet wurde, ist das Elektromagnetische Vakuum in der Quantenphysik keineswegs ohne Eigenschaften und lässt sich zum Beispiel sogar zum Rechnen mit quantenmechanischen Zuständen in einem Quantencomputer nutzen, sowie um hochpräzise Messungen durchzuführen.

**January 19 2017**

**19.01.2017 "Reise bis zum Urknall", Prof. Dr. Johanna Stachel, University of Heidelberg**

#### "Unser dunkles Universum"

Prof. Dr. Johanna Stachel, University of Heidelberg.Physikalisches Kolloquium und Ringvorlesung über Nichts.

Stadtgalerie Saarbrücken, 18:30.

Die Physik kann die Entwicklung des Universums um ca. 14 Milliarden Jahre zurückverfolgen, bis zu Sekundenbruchteilen nach dem Urknall. Diese Zeitreise liefert gleichzeitig einen Blick auf die kleinsten Bestandteile der Materie und auf die Kraftfelder, von denen der leere Raum zwischen ihnen erfüllt ist.

**January 18 2017**

**18.01.2017: We congratulate B.Sc. Frederic Folz!**

We congratulate Frederic Folz for his Bachelor degree "Ueber ein auf verzoegerter Rueckkopplung basierendes Modell zur axonalen Laengenregulation".

**January 11 2017**

**11-13.01.2017 visit of Prof. George Batrouni (University Sophia Antipolis, Nice)**

Prof. George Batrouni (University Sophia Antipolis, Nice) visits our group from January 11th till January 13th. On January 12th he will give a colloquium with the title "Quantum Monte Carlo Study Of The Rabi-Hubbard and Dicke models"

Abstract:

Many-body physics with light is attracting increasing interest for quantum technological applications and for the novel insights it offers on photon-matter interactions at the limits. In this talk I will first recall and review the dynamics of periodic photonic structures, which are theoretically described by the extension of the Rabi and the related Jaynes-Cummings and Dicke models. I will then present the Rabi lattice (RLM) and the Jaynes-Cummings (JC) lattice models. After a brief review of the JC model phase diagram, I will present our work on the phase diagram of the RLM using mean field and QMC simulations. I will also discuss our work in progress on a realization of the Dicke model, and compare our predictions with experimental results recently obtained by the group of Tilmann Esslinger at ETH in Zürich.

**January 10 2017**

**We congratulate Dr. rer. nat. Stefan Schütz!**

We congratulate Stefan Schütz for the successful defence of his PhD thesis!

**January 02 2017**

**Visit by Christian Arenz (Princeton University)**

Christian Arenz, a former member of our group and now PostDoc at Princeton University, will visit us on Friday 06.01.2017. He will give a talk at 12 am in room 4.18 on 'Universal control induced by noise'.

Abstract:

Typically the interaction of a quantum system with its environment is considered to be detrimental for quantum information processing. Quantum features one wants to use for quantum information tasks are washed out quickly so that the implementation of quantum gates becomes noisy. In recent decades, however, it has been observed that the environment can also have a beneficial effect. Rather than fighting against the environment, dissipative state preparation and dissipative quantum computing are valuable alternatives to unitary gate designs.

Here we show that the environment can be used as a resource to increase the set of operation that can be implemented with a set of controls. Instead of decreasing the fidelity for implementing a unitary operation, on the basis of the quantum Zeno effect we show that a strong noise process exhibiting a decoherence free subspace can raise the fidelity. The action of the strong dissipation allows the implementation of gate operations that cannot be realized without the help of the dissipation and even full control can be achieved. We discuss our findings on several examples and study how the process fidelity scales with the noise strength.

**December 13 2016**

**13.12.2016: We congratulate M.Sc. Tim Keller!**

We congratulate Tim Keller to his Masters degree on "Atomic self-organization in multi-mode cavities".

**December 13 2016**

**13.12.2016: We congratulate M.Sc. Rebecca Kraus!**

We congratulate Rebecca Kraus to her Masters degree on "Atom localization in quasi-periodic light potentials".

**December 05 2016**

**08.12.2016: Colloquium by Prof. Joachim Weickert**

This Thursday, 08.12.2016 at 4 pm (ct) in C6.4 HS II, Prof. Joachim Weickert from the Faculty of Mathematics and Computer Science, Saarland University, will give a talk on "Image Processing and Computer Graphics with Models from Physics".

**November 26 2016**

**29.11: "The Nobel Prize in Physics 2016", colloquium by Prof. F. Iglói (Budapest)**

Prof. Ferenc Iglói (Wigner Research Center for Physics, Budapest, Hungary) will give a talk with the title "The Nobel Prize in Physics 2016: Topology in condensed matter physics" on Tuesday 29.11.2016 in Hörsaal 1 (Gebäude C 6.4).

**November 23 2016**

**29.11 Visit of Dr. Andrey Grankin (Institut d'Optique, Palaiseau)**

Dr. Andrey Grankin (Institut d'Optique, Palaiseau) will visit us on 29.11 and will give a talk is the seminar session "Many-body physics with Light" on Tuesday at 12.00 in room 4.18 with the title "Theoretical studies of optical non-linear effects in ultracold Rydberg gases".

**November 23 2016**

**28.11: Visit of Dr. Anahit Gogyan (IPR-NAS)**

Dr. Anahit Gogyan (IPR-NAS: Institute for Physical Research of the National Academy of Sciences of Armenia) will visit us on Monday 28.11 and will give a talk at 14.00 in room 4.18 in the seminar session "Many-body physics with Light" with the title "Generation of quantum states in single-atom-cavity QED and in atomic ensembles".

**November 22 2016**

**27.-29.11: Visit of Francesco Rosati (Uni Trento and Pisa)**

Francesco Rosati (Uni Trento and Pisa) will visit us from 27.11 to 29.11 and will give a talk is the seminar session "Many-body physics with Light" on Monday 28.11 at 10.00 in room 4.18 with the title "Goldstino mode in supersymmetric Bose-Fermi mixtures".

**November 21 2016**

**25.11: Visit of Prof. Tommaso Calarco (Uni Ulm)**

Prof. Tommaso Calarco (Uni Ulm) will be visiting us on 25.11.

**November 17 2016**

Our article **Laser and cavity cooling of a mechanical resonator with a nitrogen-vacancy center in diamond** has been published in the 1 November 2016 issue of Physical Review A (Vol.94, No.5).

#### Laser and cavity cooling of a mechanical resonator with a nitrogen-vacancy center in diamond

Luigi Giannelli, Ralf Betzholz, Laura Kreiner, Marc Bienert, and Giovanna MorigiPhys. Rev. A 94, 053835

We theoretically analyze the cooling dynamics of a high-Q mode of a mechanical resonator, when the structure is also an optical cavity and is coupled with a nitrogen-vacancy (NV) center. The NV center is driven by a laser and interacts with the cavity photon field and with the strain field of the mechanical oscillator, while radiation pressure couples the mechanical resonator and cavity field. Starting from the full master equation we derive the rate equation for the mechanical resonator's motion, whose coefficients depend on the system parameters and on the noise sources. We then determine the cooling regime, the cooling rate, the asymptotic temperatures, and the spectrum of resonance fluorescence for experimentally relevant parameter regimes. For these parameters, we consider an electronic transition, whose linewidth allows one to perform sideband cooling, and show that the addition of an optical cavity in general does not improve the cooling efficiency. We further show that pure dephasing of the NV center's electronic transitions can lead to an improvement of the cooling efficiency.

**November 14 2016**

**20-25.11: Visit of Dr. Magdalena Stobińska (Uni Gdansk)**

Dr. Magdalena Stobińska (Uni Gdansk) will visit us from 20.11 to 25.11 and will give a talk is the seminar session "Many-body physics with Light" on Tuesday 22.11 at 9.00 in room 4.18.

**November 08 2016**

**Visit of Prof. Dr. Michael Kastner (Stellenbosch, South Africa)**

Prof. Dr. Michael Kastner (Stellenbosch, South Africa) is visiting us.

**November 02 2016**

**3.11.2016 "Unser dunkles Universum", Prof. Dr. Matthias Bartelmann, University of Heidelberg.**

#### "Unser dunkles Universum"

Prof. Dr. Matthias Bartelmann, University of Heidelberg.Physikalisches Kolloquium und Ringvorlesung über Nichts.

Stadtgalerie Saarbrücken, 18:30.

Unsere Vorstellung vom Universum ist im Lauf der letzten etwa 15 Jahre erheblich präziser geworden. Ausgehend von Albert Einsteins allgemeiner Relativitätstheorie und zwei einfachen Annahmen wurde schon in den 20er Jahren des letzten Jahrhunderts das moderne kosmologische Weltmodell konstruiert, das nun durch eine Vielzahl verschiedener Beobachtungen als glänzend bestätigt gelten kann. Zwei sehr ernst zu nehmende Schlussfolgerungen daraus sind höchst rätselhaft: Bei weitem die meiste Materie im Universum muss aus einer unbekannten, dunklen Materie bestehen, die nicht mit Licht wechselwirken kann. Dazu kommt, dass das Universum etwa seit der Hälfte seines heutigen Alters von einer abgebremsten in eine beschleunigte Ausdehnung übergegangen ist. Wir machen dafür eine weitere dunkle Substanz verantwortlich, die dunkle Energie, von der wir aber noch weniger wissen als von der dunklen Materie.

**November 02 2016**

3.-4.11.2016: Simone Montangero (University of Ulm) visits the physics department. On Thursday, November 3rd, he gives a colloquium with the title Extreme simulations for quantum technologies at the MPI of Computer Science.

The colloquium takes place on Thursday 3.11 at 13:00 in Building E1.4, MPI-INF, Room 024.

**October 25 2016**

**Visit by David Mukamel (Weizmann Institute, Israel) and Haggai Landa (LPTMS, Paris)**

David Mukamel and Haggai Landa are visiting us for a discussion.

**October 24 2016**

**New article: Resonance fluorescence of a laser-cooled atom in a non-harmonic potential**

Our article **Resonance fluorescence of a laser-cooled atom in a non-harmonic potential** has been published in the most recent volume of European Physical Journal D.

#### Resonance fluorescence of a laser-cooled atom in a non-harmonic potential

Ralf Betzholz and Mark BienertEur. Phys. J. D (2016) 70: 215

We investigate a single laser driven atom trapped in a non-harmonic potential. We present the performance of ground-state laser cooling and Doppler cooling and the signatures of the center-of-mass motion in the power spectrum of the scattered light. In order to illustrate the results we provide two explicit examples for the confining potential: the infinite square well and the Morse potential.

**October 18 2016**

**New article: Master equation for high-precision spectroscopy**

Our article **Master equation for high-precision spectroscopy** has been published in the most recent volume of Physical Review A.

#### Master equation for high-precision spectroscopy

Andreas Alexander Buchheit and Giovanna MorigiPhys. Rev. A 94, 042111 (2016)

The progress in high-precision spectroscopy requires one to verify the accuracy of theoretical models such as the master equation describing spontaneous emission of atoms. For this purpose, we apply the coarse-graining method to derive a master equation of an atom interacting with the modes of the electromagnetic field. This master equation naturally includes terms due to quantum interference in the decay channels and fulfills the requirements of the Lindblad theorem without the need of phenomenological assumptions. We then consider the spectroscopy of the 2S-4P line of atomic Hydrogen and show that these interference terms, typically neglected, significantly contribute to the photon count signal. These results can be important in understanding spectroscopic measurements performed in recent experiments for testing the validity of quantum electrodynamics.

**October 17 2016**

**A Lecture Series on "Nichts" ("Nothing") for the "Jahr des Nichts 2016"**

November 3 2016 - January 26 2017

Taking place on Thursday 18:30 Uhr, in the Stadtgalerie or Rathausfestsaal, Saarbrücken.

Event program

Nov. 3 2016, 18:30 Uhr, Stadtgalerie: The first talk is given by Prof. Dr. Matthias Bartelmann, Zentrum für Astronomie, Universität Heidelberg on "Unser Dunkles Universum".

**October 07 2016**

**Visit by Nicolo Defenu (SISSA, Trieste) and Valentin Torggler (Innsbruck), October 4-7th **

**October 07 2016**

**We congratulate Dr. rer. nat. Ralf Betzholz for the successful defence of his PhD thesis.**

**September 28 2016**

**4.-6. October 2016: Project Meeting "Quantum Crystals of Matter and Light"**

Taking place in Saarbrücken, Campus E2.6, room E.04.

**September 23 2016**

**We congratulate Dr. rer. nat. Florian Cartarius for the successful defence of his PhD thesis**

Grenoble, September 22nd. We congratulate Florian Cartarius, who successfully defended his PhD thesis in Grenoble in front of a jury of international recognized scientists. The PhD project is a cotutelle agreement between the University of Grenoble Alpes and Saarland University, the studies were performed under the joint supervision of Anna Minguzzi, Research Director at CNRS in Grenoble, and Giovanna Morigi.

**September 08 2016**

**New article: Ultracold bosons with cavity-mediated long-range interactions**

Our article **Ultracold bosons with cavity-mediated long-range interactions: A local mean-field analysis of the phase diagram** has been published in the most recent volume of Physical Review A.

#### Ultracold bosons with cavity-mediated long-range interactions: A local mean-field analysis of the phase diagram

Astrid E. Niederle, Giovanna Morigi, and Heiko RiegerPhys. Rev. A 94, 033607 (2016)

Ultracold bosonic atoms in optical lattices self-organize into a variety of structural and quantum phases when placed into a single-mode cavity and pumped by a laser. Cavity optomechanical effects induce an atom density modulation at the cavity-mode wavelength that competes with the optical lattice arrangement. Simultaneously short-range interactions via particle hopping promote superfluid order such that a variety of structural and quantum coherent phases can occur. We analyze the emerging phase diagram in two dimensions by means of an extended Bose-Hubbard model using a local mean-field approach combined with a superfluid cluster analysis. For commensurate ratios of the cavity and external lattice wavelengths, the Mott insulator-superfluid transition is modified by the appearance of charge density wave and supersolid phases, at which the atomic density supports the buildup of a cavity field. For incommensurate ratios, the optomechanical forces induce the formation of Bose-glass and superglass phases, namely, nonsuperfluid and superfluid phases, respectively, displaying quasiperiodic density modulations, which in addition can exhibit structural and superfluid stripe formation. The onset of such structures is constrained by the on-site interaction and is favorable at fractional densities. Experimental observables are identified and discussed.

**August 29 2016**

**New article: Optomechanical many-body cooling to the ground state using frustration**

Our article **Optomechanical many-body cooling to the ground state using frustration** has been published in the most recent volume of Physical Review A and has been highlighted as an editor's suggestion.

#### Optomechanical many-body cooling to the ground state using frustration

Thomás Fogarty, Haggai Landa, Cecilia Cormick, and Giovanna MorigiPhys. Rev. A 94, 023844 (2016)

We show that the vibrations of an ion Coulomb crystal can be cooled to the zero-point motion through the optomechanical coupling with a high-finesse cavity. Cooling results from the interplay between coherent scattering of cavity photons by the ions, which dynamically modifies the vibrational spectrum, and cavity losses, that dissipate motional energy. The cooling mechanism we propose requires that the length scales of the crystal and the cavity are mismatched so that the system is intrinsically frustrated, leading to the formation of defects (kinks). When the pump is strong enough, the anti-Stokes sidebands of all vibrational modes can be simultaneously driven. These dynamics can be used to prepare ground-state chains of dozens of ions within tens of milliseconds in state-of-the-art experimental setups. In addition, we identify parameter regimes of the optomechanical interactions where individual localized modes can be selectively manipulated, and monitored through the light at the cavity output. These dynamics exemplify robust quantum reservoir engineering of strongly correlated mesoscopic systems and could find applications in optical cooling of solids.

**August 26 2016**

**New article: Buckling Transitions and Clock Order of Two-Dimensional Coulomb Crystals**

Our article **Buckling Transitions and Clock Order of Two-Dimensional Coulomb Crystals** has been published in the most recent volume of Physical Review X.

#### Buckling Transitions and Clock Order of Two-Dimensional Coulomb Crystals

Daniel Podolsky, Efrat Shimshoni, Giovanna Morigi, and Shmuel FishmanPhys. Rev. X 6, 031025

Crystals of repulsively interacting ions in planar traps form hexagonal lattices, which undergo a buckling instability towards a multilayer structure as the transverse trap frequency is reduced. Numerical and experimental results indicate that the new structure is composed of three planes, whose separation increases continuously from zero. We study the effects of thermal and quantum fluctuations by mapping this structural instability to the six-state clock model. A prominent implication of this mapping is that at finite temperature, fluctuations split the buckling instability into two thermal transitions, accompanied by the appearance of an intermediate critical phase. This phase is characterized by quasi-long-range order in the spatial tripartite pattern. It is manifested by broadened Bragg peaks at new wave vectors, whose line shape provides a direct measurement of the temperature-dependent exponent η(T) characteristic of the power-law correlations in the critical phase. A quantum phase transition is found at the largest value of the critical transverse frequency: Here, the critical intermediate phase shrinks to zero. Moreover, within the ordered phase, we predict a crossover from classical to quantum behavior, signifying the emergence of an additional characteristic scale for clock order. We discuss experimental realizations with trapped ions and polarized dipolar gases, and propose that within accessible technology, such experiments can provide a direct probe of the rich phase diagram of the quantum clock model, not easily observable in condensed matter analogues. Therefore, this work highlights the potential for ionic and dipolar systems to serve as simulators for complex models in statistical mechanics and condensed matter physics.

**August 16 2016**

**New article: Dissipation-Assisted Prethermalization in Long-Range Interacting Atomic Ensembles**

Our article **Dissipation-Assisted Prethermalization in Long-Range Interacting Atomic Ensembles** has been published in the most recent volume of Physical Review Letters.

#### Dissipation-Assisted Prethermalization in Long-Range Interacting Atomic Ensembles

Stefan Schütz, Simon B. Jäger, and Giovanna MorigiPhys. Rev. Lett. 117, 083001

We theoretically characterize the semiclassical dynamics of an ensemble of atoms after a sudden quench across a driven-dissipative second-order phase transition. The atoms are driven by a laser and interact via conservative and dissipative long-range forces mediated by the photons of a single-mode cavity. These forces can cool the motion and, above a threshold value of the laser intensity, induce spatial ordering. We show that the relaxation dynamics following the quench exhibits a long prethermalizing behavior which is first dominated by coherent long-range forces and then by their interplay with dissipation. Remarkably, dissipation-assisted prethermalization is orders of magnitude longer than prethermalization due to the coherent dynamics. We show that it is associated with the creation of momentum-position correlations, which remain nonzero for even longer times than mean-field predicts. This implies that cavity cooling of an atomic ensemble into the self-organized phase can require longer time scales than the typical experimental duration. In general, these results demonstrate that noise and dissipation can substantially slow down the onset of thermalization in long-range interacting many-body systems.

**August 04 2016**

**New article: Mean-field theory of atomic self-organization in optical cavities**

Our article **Mean-field theory of atomic self-organization in optical cavities** has been published in the most recent volume of Physical Review A.

#### Mean-field theory of atomic self-organization in optical cavities

Simon B. Jäger, Stefan Schütz, and Giovanna MorigiPhys. Rev. A. 94, 023807

Photons mediate long-range optomechanical forces between atoms in high-finesse resonators, which can induce the formation of ordered spatial patterns. When a transverse laser drives the atoms, the system undergoes a second-order phase transition that separates a uniform spatial density from a Bragg grating maximizing scattering into the cavity and is controlled by the laser intensity. Starting from a Fokker-Planck equation describing the semiclassical dynamics of the N-atom distribution function, we systematically develop a mean-field model and analyze its predictions for the equilibrium and out-of-equilibrium dynamics. The validity of the mean-field model is tested by comparison with the numerical simulations of the N-body Fokker-Planck equation and by means of a Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy. The mean-field theory predictions well reproduce several results of the N-body Fokker-Planck equation for sufficiently short times and are in good agreement with existing theoretical approaches based on field-theoretical models. The mean field, on the other hand, predicts thermalization time scales which are at least one order of magnitude shorter than the ones predicted by the N-body dynamics. We attribute this discrepancy to the fact that the mean-field ansatz discards the effects of the long-range incoherent forces due to cavity losses.

**July 29 2016**

**New article: Localization transition in the presence of cavity backaction**

Our article **Localization transition in the presence of cavity backaction** has been published in the most recent volume of Physical Review A.

#### Localization transition in the presence of cavity backaction

Katharina Rojan, Rebecca Kraus, Thomás Fogarty, Hessam Habibian, Anna Minguzzi, and Giovanna MorigiPhys. Rev. A. 94, 013839

We study the localization transition of an atom confined by an external optical lattice in a high-finesse cavity. The atom-cavity coupling yields an effective secondary lattice potential, whose wavelength is incommensurate with the periodicity of the optical lattice. The cavity lattice can induce localization of the atomic wave function analogously to the Aubry-André localization transition. Starting from the master equation for the cavity and the atom we perform a mapping of the system dynamics to a Hubbard Hamiltonian, which can be reduced to the Harper's Hamiltonian in appropriate limits. We evaluate the phase diagram for the atom's ground state and show that the transition between extended and localized wave function is controlled by the strength of the cavity nonlinearity, which determines the size of the localized region and the behavior of the Lyapunov exponent. The Lyapunov exponent, in particular, exhibits resonancelike behavior in correspondence with the optomechanical resonances. Finally we discuss the experimental feasibility of these predictions.

**July 15 2016**

**We congratulate Dr. Jens Baltrusch!**

We congratulate Dr. rer. nat. Jens Baltrusch for the successful defense of his PhD thesis with the title "Quenches across structural transitions in ion Coulomb crystals".

**July 12 2016**

Alexey Konovalov (St. Petersburg University) is participating in a traineeship at our group for the next three weeks.

**June 28 2016**

**24.06-14.07.16: Visit of Cecilia Cormick **

Cecilia Corm (Universidad de Córdoba, Argentina) is visiting us. On Wednesday 07.07.2016 in seminar room 2 in builing E2.5, she will give a talk on "Simulating spin-bosons with trapped ions".

**June 03 2016**

**New Article: Crossover from Classical to Quantum Kibble-Zurek Scaling**

Our article **Crossover from Classical to Quantum Kibble-Zurek Scaling** has been published in the most recent volume of Physical Review Letters.

#### Crossover from Classical to Quantum Kibble-Zurek Scaling

Pietro Silvi, Giovanna Morigi, Tommaso Calarco, and Simone MontangeroPhys. Rev. Lett. 116, 225701

The Kibble-Zurek (KZ) hypothesis identifies the relevant time scales in out-of-equilibrium dynamics of critical systems employing concepts valid at equilibrium: It predicts the scaling of the defect formation immediately after quenches across classical and quantum phase transitions as a function of the quench speed. Here, we study the crossover between the scaling dictated by a slow quench, which is ruled by the critical properties of the quantum phase transition, and the excitations due to a faster quench, where the dynamics is often well described by the classical model. We estimate the value of the quench rate that separates the two regimes and support our argument using numerical simulations of the out-of-equilibrium many-body dynamics. For the specific case of a ϕ^4 model we demonstrate that the two regimes exhibit two different power-law scalings, which are in agreement with the KZ theory when applied to the quantum and classical cases. This result contributes to extending the prediction power of the Kibble-Zurek mechanism and to providing insight into recent experimental observations in systems of cold atoms and ions.

**May 10 2016**

**16-18.05.2016: Visit of Shamik Gupta**

Shamik Gupta (Max Planck Institute for the Physics of Complex Systems, Dresden, Germany) is visiting us. On Tuesday 17.05.2016 at 9 am in room 4.18 he will give a talk on 'Long-range interacting systems driven out of equilibrium'.

Abstract: Systems with long-range interactions have an inter-particle
interaction potential that decays slower than 1/r^d in d dimensions.
Examples are widespread, from plasmas, dipolar ferroelectrics and
ferromagnets, to gravitational systems. After a brief introduction to
unusual static and dynamic properties of long-range systems, I will dwell
on the question: What happens when a long-range system is driven out of
thermal equilibrium by, e.g., an impulsive kick? In similar situations,
short-range systems would typically relax to another thermal equilibrium
with a uniform temperature across the system. By contrast, a long-range
system relaxes to non-Boltzmann nonequilibrium stationary states that
support a non-uniform temperature profile across the system. More striking
and counterintuitive is the observation of temperature inversion in such a
state: denser parts of the system are colder than dilute ones. Such
inversions occur in nature, e.g., in the solar corona and in interstellar
molecular clouds. We demonstrate how an interplay of wave-particle
interaction and spatial inhomogeneity offers a simple and appealing
mechanism to explain temperature inversion in generic long-range systems.

**May 10 2016**

**16-17.05.2016: Visit of Haggai Landa**

Haggai Landa (Laboratoire de Physique Théorique et Modèles Statistiques (LPTMS), Université Paris Sud, Orsay, France) is visiting us.

**April 27 2016**

**New Article: Supercooling of Atoms in an Optical Resonator**

Our recent article **Supercooling of Atoms in an Optical Resonator** has been highlighted as an Editors' Suggestion in the most recent volume of Physical Review Letters.

#### Supercooling of Atoms in an Optical Resonator

Minghui Xu, Simon B. Jäger, S. Schütz, J. Cooper, Giovanna Morigi, and M. J. HollandPhys. Rev. Lett. 116, 153002 (2016)

We investigate laser cooling of an ensemble of atoms in an optical cavity. We demonstrate that when atomic dipoles are synchronized in the regime of steady-state superradiance, the motion of the atoms may be subject to a giant frictional force leading to potentially very low temperatures. The ultimate temperature limits are determined by a modified atomic linewidth, which can be orders of magnitude smaller than the cavity linewidth. The cooling rate is enhanced by the superradiant emission into the cavity mode allowing reasonable cooling rates even for dipolar transitions with ultranarrow linewidth.

**March 09 2016**

**10.03.2016: Talk of Thomás Fogarty**

Thursday 10.03.2016 at 9:30 in room 4.18 Dr. Thomas Fogarty (OIST, Okinawa) is giving a lecture on "static friction for optical refrigeration of solids".

**February 15 2016**

**17-19.02.16 Visit of Nicolò Defenu**

Nicolò Defenu (SISSA Trieste) is visiting us and will give a talk on Wednesday 17th of February at 13.30 in room 4.18. He will talk about "Anisotropic Long Range Systems".

**February 01 2016**

**04.02.16: Visit of Florian Marquardt**

Florian Marquardt (Friedrich-Alexander Universität Erlangen-Nürnberg) is visiting us and will give a talk on Thursday 4th of February in the Physikalisches Kolloquium at 16.15.

**January 26 2016**

**New Article: Formation of helical ion chains**

#### Formation of helical ion chains

R. Nigmatullin, A. del Campo, G. De Chiara, G. Morigi, M. B. Plenio, and A. Retzker

Phys. Rev. B 93, 014106 (2016)

We study the nonequilibrium dynamics of the linear-to-zigzag structural phase transition exhibited by an ion chain confined in a trap with periodic boundary conditions. The transition is driven by reducing the transverse confinement at a finite quench rate, which can be accurately controlled. This results in the formation of zigzag domains oriented along different transverse planes. The twists between different domains can be stabilized by the topology of the trap, and under laser cooling the system has a chance to relax to a helical chain with nonzero winding number. Molecular dynamics simulations are used to obtain a large sample of possible trajectories for different quench rates. The scaling of the average winding number with different quench rates is compared to the prediction of the Kibble-Zurek theory, and a good quantitative agreement is found.

**January 26 2016**

**28.01.2016: Visit of Tilman Pfau**

Tilman Pfau (Universität Stuttgart) is visiting us.

**January 25 2016**

**25.01.2016: Visit of Haggai Landa**

Haggai Landa (Laboratoire de Physique Théorique et Modèles Statistiques (LPTMS), Université Paris Sud, Orsay, France) is visiting us.

**January 06 2016**

**13-14.01.2016: Visit of Leticia F. Cugliandolo**

Leticia F. Cugliandolo (Université Pierre et Marie Curie, Paris) is visiting us and will give a talk on Thursday 14th of January in the Physikalisches Kolloquium at 16.00.

She will talk about phase ordering kinetics in two dimensions.

**January 06 2016**

**13-14.01.2016: Visit of Simone Montangero and Pietro Silvi**

Simone Montangero and Pietro Silvi (Institute for Complex Quantum Systems - University of Ulm) are visiting us.

**December 11 2015**

**New Article: Dynamical depinning of a Tonks Girardeau gas**

#### Dynamical depinning of a Tonks Girardeau gas

Florian Cartarius, Eiji Kawasaki, Anna Minguzzi

Phys. Rev. A 92, 063605 (2015)

We study the dynamical depinning following a sudden turn off of an optical lattice for a gas of impenetrable bosons in a tight atomic waveguide. We use a Bose-Fermi mapping to infer the exact quantum dynamical evolution. At long times, in the thermodynamic limit, we observe the approach to a nonequilibrium steady state, characterized by the absence of quasi-long-range order and a reduced visibility in the momentum distribution. Similar features are found in a finite-size system at times corresponding to half the revival time, where we find that the system approaches a quasisteady state with a power-law behavior.

**December 11 2015**

**14.12.2015: Visit of Christiane Koch**

Christiane Koch is visiting us.

**December 11 2015**

**14.-15.12.2015: Visit of Francesco Piazza**

Francesco Piazza visits us and will give a talk on Tuesday 15.12.2015 in the group seminar @09.00 in 4.18.

**December 07 2015**

**New Article: Thermodynamics and dynamics of atomic self-organization in an optical cavity**

#### Thermodynamics and dynamics of atomic self-organization in an optical cavity

Stefan Schütz, Simon B. Jäger, and Giovanna Morigi

Phys. Rev. A 92, 063808 (2015)

Pattern formation of atoms in high-finesse optical resonators results from the mechanical forces of light associated with superradiant scattering into the cavity mode. It occurs when the laser intensity exceeds a threshold value such that the pumping processes counteract the losses. We consider atoms driven by a laser and coupling with a mode of a standing-wave cavity and describe their dynamics with a Fokker-Planck equation, in which the atomic motion is semiclassical but the cavity field is a full quantum variable. The asymptotic state of the atoms is a thermal state, whose temperature is solely controlled by the detuning between the laser and the cavity frequency and by the cavity loss rate. From this result we derive the free energy and show that in the thermodynamic limit self-organization is a second-order phase transition. The order parameter is the field inside the resonator to which one can associate a magnetization in analogy to ferromagnetism, the control field is the laser intensity, but the steady state is intrinsically out of equilibrium. In the symmetry-broken phase, quantum noise induces jumps of the spatial density between two ordered patterns: We characterize the statistical properties of this temporal behavior at steady state and show that the thermodynamic properties of the system can be extracted by detecting the light at the cavity output. The results of our analysis are in full agreement with previous studies; we extend them by deriving a self-consistent theory which is valid also when the cavity field is in the shot-noise limit and elucidate the nature of the self-organization transition.

**December 06 2015**

**10.12.2015:"Licht-Reflektionen" :Talk of A.Langenbucher**

## Lecture series “Licht-Reflektionen” for the International Year of Light

**Achim Langenbucher**

Medizinische Fakultät der Universität des Saarlandes

**Donnerstag, 10. Dezember 2015, 19:00 Uhr
Haus der Zukunft (Richard-Wagner-Str. 14-16, 66111 Saarbrücken)**

*Licht, medizinisch gesehen *

Kurzdarstellung:

Medizinisch gesehen hat Licht zwei komplementäre Aspekte: Das Auge als Licht

wahrnehmendes Organ ist Untersuchungsgegenstand der Ophthalmologie, in welcher

Medizin, Biologie und Physik zusammenfließen.

Gleichzeitig wird Licht, weit über die Augenheilkunde hinaus, als Werkzeug zur

Diagnose und Heilung verwendet. Wir beleuchten einige dieser Aspekte anhand

konkreter Beispiele aus der wissenschaftlichen und klinischen Praxis.

**December 04 2015**

**07-11.12.2015: Visit of Valentin Torggler**

Valentin Torggler (Innsbruck) is visiting us and will give a talk on Monday 7th of December in our group meeting at 11.00.

**December 02 2015**

**New Article: Nanofriction in Cavity Quantum Electrodynamics**

#### Nanofriction in Cavity Quantum Electrodynamics

T. Fogarty, C. Cormick, H. Landa, Vladimir M. Stojanović, E. Demler, and Giovanna Morigi

Phys. Rev. Lett. 115, 233602 (2015)

The dynamics of cold trapped ions in a high-finesse resonator results from the interplay between the long-range Coulomb repulsion and the cavity-induced interactions. The latter are due to multiple scatterings of laser photons inside the cavity and become relevant when the laser pump is sufficiently strong to overcome photon decay. We study the stationary states of ions coupled with a mode of a standing-wave cavity as a function of the cavity and laser parameters, when the typical length scales of the two self-organizing processes, Coulomb crystallization and photon-mediated interactions, are incommensurate. The dynamics are frustrated and in specific limiting cases can be cast in terms of the Frenkel-Kontorova model, which reproduces features of friction in one dimension. We numerically recover the sliding and pinned phases. For strong cavity nonlinearities, they are in general separated by bistable regions where superlubric and stick-slip dynamics coexist. The cavity, moreover, acts as a thermal reservoir and can cool the chain vibrations to temperatures controlled by the cavity parameters and by the ions’ phase. These features are imprinted in the radiation emitted by the cavity, which is readily measurable in state-of-the-art setups of cavity quantum electrodynamics.

**November 29 2015**

**05.11.2015:"Licht-Reflektionen" :Talk of R. Bennewitz and J. Eschner**

## Lecture series “Licht-Reflektionen” for the International Year of Light

** Roland Bennewitz**

Leibnitz-Institut für neue Materialien GmbH, Saarbrücken

**& Jürgen Eschner **

Experimentalphysik, Universität des Saarlandes

**Donnerstag, 03. Dezember 2015, 19:00 Uhr
Haus der Zukunft (Richard-Wagner-Str. 14-16, 66111 Saarbrücken)**

*Einzelne Atome im Licht der Physik und der Philosophie *

Kurzdarstellung:

Was bedeutet “Sehen” in der mikroskopischen Welt? Physiker zeigen gerne Bilder von

Atomen, es wird sogar daran geforscht, einzelne Atome zur Speicherung von

Information einzusetzen. Aber können wir einzelne Atome oder Moleküle tatsächlich

sehen? Was “sieht” man auf solchen Bildern? Darf man aus dem Gesehenen schließen,

dass es Atome gibt? Wir beleuchten diese Fragen aus physikalischer und aus

philosophischer Sicht.

**November 22 2015**

**05.11.2015:"Licht-Reflektionen" :Talk of F. Lautenschläger**

## Lecture series “Licht-Reflektionen” for the International Year of Light

**Franziska Lautenschläger**

Experimentalphysik, Universität des Saarlandes

**Donnerstag, 26. November 2015, 19:00 Uhr
Haus der Zukunft (Richard-Wagner-Str. 14-16, 66111 Saarbrücken)**

*Farbiges Licht verschafft Einblicke in Zellen des Immunsystems *

Kurzdarstellung:

Wie schauen sich Biologen lebende Zellen an? Eine Möglichkeit sind Lichtmikroskope.

Aber was sieht man, wenn man eine Zelle unter ein Lichtmikroskop legt? Erstmal nicht

viel. Das Mikroskop vergrößert die Zellen ca. 100 - 2000x. Aber selbst dann sieht man

höchstens die Umrisse der Zellen, und mit ganz viel Glück vielleicht noch die Zellkerne.

Aber in Zellen steckt so viel mehr! Wir nutzen farbiges Licht und das Phänomen der

Fluoreszenz um möglichst viele dieser Zellbestandteile anzufärben und zu untersuchen,

ganz speziell Immunzellen. Neu sind sogenannte Superresolutionsverfahren, mit denen

man die Auflösung von zwei Punkten über die bisher berechnete, theoretisch mögliche

Grenze hinaus messen kann (Nobelpreis 2014)! Ich werde diese Verfahren erklären und

zeigen, wie weit man damit ins Innere von Zellen blicken kann. Außerdem nutzen wir

Licht nicht nur zum Betrachten von Zellen, sondern auch, um Kräfte auf diese Zellen

auszuüben, um die Dynamik von Partikeln innerhalb von Zellen zu messen oder sogar

um kleine Löcher in lebende Zellen zu schneiden. Wie und warum? Erfahren Sie im

Vortrag!

**November 15 2015**

**18.11.2015:"Licht-Reflektionen" :Talk of J. Dalibard**

## Lecture series “Licht-Reflektionen” for the International Year of Light

**Jean Dalibard**

Collège de France, Paris

**Mittwoch, 18. November 2015, 17:30 Uhr
Campus E 2.2, Günter-Hotz-Hörsaal**

*Atoms cooled and controlled by light *

Abstract:

Forty years ago appeared the idea that laser light could be used to reduce the thermal

motion of an atomic vapour. Since that date, the manipulation and the cooling of atomic

gases with laser light has undergone spectacular developments, which went far beyond

the most optimistic initial predictions. Light beams with well-chosen characteristics can

bring an atomic assembly down to a temperature of only a few nanokelvins above the

absolute zero.

The behaviour of theses cold gases is governed by Quantum Mechanics: The velocity

distribution of the atoms is dramatically reduced and their wavelength is increased

correspondingly, which allows one to realize atomic clocks and interferometric matterwave

sensors with an unprecedented precision. In addition, by concentrating these

atoms in a small volume one realizes a novel type of “quantum matter”, which

constitutes a simulator of other many-body systems, such as the electronic fluid of some

families of superconductors or of samples exhibiting the Quantum Hall effect.

In this talk I will briefly present the physical principles that are at the basis of the

cooling. I will then describe some recent developments, dealing either with precision

measurements at the single atom level or with collective phenomena related to

condensed matter physics. I will close with a short summary of the many perspectives

that are opened in this research field.

**November 12 2015**

**New Article: Dissipative quantum control of a spin chain**

#### Dissipative quantum control of a spin chain

G. Morigi, J. Eschner, C. Cormick, Y. Lin, D. Leibfried, and D. J. Wineland

Phys. Rev. Lett. 115, 200502 (2015)

A protocol is discussed for preparing a spin chain in a generic many-body state in the asymptotic limit of tailored nonunitary dynamics. The dynamics require the spectral resolution of the target state, optimized coherent pulses, engineered dissipation, and feedback. As an example, we discuss the preparation of an entangled antiferromagnetic state, and argue that the procedure can be applied to chains of trapped ions or Rydberg atoms.

**November 08 2015**

**12.11.2015:"Licht-Reflektionen" :Talk of O. Müller**

## Lecture series “Licht-Reflektionen” for the International Year of Light

**Olaf Müller**

Institut für Philosophie, Universität Berlin

**Donnerstag, 12. November 2015, 19:00 Uhr
Haus der Zukunft (Richard-Wagner-Str. 14-16, 66111 Saarbrücken)**

*Mehr Licht: Goethe im Streit mit Newton um die Farben *

Kurzdarstellung:

Goethes Protest gegen Newtons Theorie des Lichts und der Farben ist besser, als man

gemeinhin denkt. Man kann diesem Protest in den wichtigsten Elementen folgen, ohne

Newton in der physikalischen Sache unrecht zu geben. Laut meiner Interpretation hat

Goethe in Newtons wissenschaftsphilosophischer Selbsteinschätzung eine entscheidende

Schwäche aufgedeckt: Newton glaubte, mithilfe prismatischer Experimente beweisen zu

können, dass das Licht der Sonne aus Lichtstrahlen verschiedener Farben

zusammengesetzt sei. Goethe zeigt, dass dieser Übergang vom Beobachtbaren zur

Theorie problematischer ist, als Newton wahrhaben wollte. Und diese Einsicht Goethes

gewinnt eine überraschende Schärfe, weil Goethe plausibel machen kann, dass sich alle

entscheidenden prismatischen Experimente Newtons ebenso gut mit einer alternativen

Theorie vereinbaren lassen. Wenn ich recht sehe, war Goethe der erste

Wissenschaftsphilosoph, der mindestens eine empirisch äquivalente Alternative zu

einer wohletablierten physikalischen Theorie gesehen hat: Damit war Goethe seiner Zeit

um ein gutes Jahrhundert voraus.

**November 08 2015**

**New Article: Phys. Rev. A 92, 043822 (2015)**

#### Suppression of Rabi oscillations in hybrid optomechanical systems

Timo Holz, Ralf Betzholz and Marc Bienert

Phys. Rev. A 92, 043822 (2015)

In a hybrid optomechanical setup consisting of a two-level atom in a cavity with a pendular end mirror, the interplay between the light field's radiation pressure on the mirror and the dipole interaction with the atom can lead to an effect, which manifests itself in the suppression of Rabi oscillations of the atomic population. This effect is present when the system is in the single-photon strong-coupling regime and has an analogy in the photon blockade of optomechanics.

**November 04 2015**

**04.11.2015: We congratulate B.Sc. Lukas Himbert!**

We congratulate Lukas Himbert for his Bachelor degree on "Structures in classical dipolar gases".

**November 01 2015**

**05.11.2015:"Licht-Reflektionen" :Talk of W. P. Schleich**

## Lecture series “Licht-Reflektionen” for the International Year of Light

**Wolfgang P. Schleich**

Institut für Quantenphysik, Universität Ulm

**Donnerstag, 05. November 2015, 19:00 Uhr
Haus der Zukunft (Richard-Wagner-Str. 14-16, 66111 Saarbrücken)**

*Welle-Teilchen Dualismus:
Du sollst Dir kein Bild vom Mikrokosmos machen *

Kurzdarstellung:

Der Titel dieses Vortrages drückt in Anlehnung an die Zehn Gebote nach Exodus 20:4-6

die Kopenhagener Interpretation der Quantenmechanik aus, wie sie von Niels Bohr

formuliert wurde. In der Tat verbietet diese heute weitgehend akzeptierte

Formulierung, sich ein Bild von der Quantenwelt zu machen. Es ist verblüffend, dass

diese Theorie Vorhersagen macht, die mit hoher Genauigkeit getestet werden können,

aber keine klare klassische Vorstellung des Mikrokosmos liefert. Der amerikanische

Physiker N. David Mermin fasste diesen Gegensatz in der prägnanten Phrase zusammen:

“Halte den Mund und rechne!”

In diesem Vortrag geben wir einen kurzen Überblick über die Quantentheorie

ausgehend von Max Plancks Entdeckung des Wirkungsquantums im Jahr 1900, über die

Matrizenmechanik von Werner Heisenberg zur Wellenmechanik von Erwin Schrödinger.

Wir behandeln dann die Frage nach der Interpretation des Formalismus anhand des

Doppelspalt-Experimentes von Thomas Young, der Einstein-Podolsky-Rosen Situation

und der Schrödinger-Katze. Neuere Entwicklungen auf dem Gebiet beschließen den

Vortrag.

**October 26 2015**

**29.10.2015:"Licht-Reflektionen" :Talk of E. K. Grebel **

## Lecture series “Licht-Reflektionen” for the International Year of Light

**Eva K. Grebel**

Zentrum für Astronomie, Universität Heidelberg

**Donnerstag, 29. Oktober 2015, 19:00 Uhr
Haus der Zukunft (Richard-Wagner-Str. 14-16, 66111 Saarbrücken)**

*Galaxienentwicklung:
Neue Erkenntnisse aus altem Licht *

Kurzdarstellung:

Astronomische Beobachtungen beruhen hauptsächlich auf dem Licht, das andere Himmelskörper aussenden. Aufgrund der endlichen Ausbreitungsgeschwindigkeit des Lichts ist jede astronomische Beobachtung ein Blick in die Vergangenheit - umso weiter zurück, je weiter ein Objekt von uns ist. Das Licht, das wir von Galaxien empfangen, stammt in erster Linie von ihren Sternen, aber auch von ihrem Gas und sogar von akkretierenden schwarzen Löchern. Die Spektrallinien und die Energieverteilung dieses Lichts verraten uns, wie stark die Sternentstehungsrate in einer Galaxie zu verschiedenen Zeitpunkten war und wie sich die Zusammensetzung und Masse von Galaxien mit der Zeit verändert haben. Bilder von Galaxien aus unterschiedlichen Epochen zeigen uns, wie sich die heutige Gestalt der Galaxien im Verlauf der Zeit entwickelt hat und welche Rolle beispielsweise Galaxienwechselwirkungen dabei gespielt haben. Anhand solcher Daten können wir mittlerweile bis in die Kinderstube der Galaxienentstehung zurückblicken, aber auch Rückschlüsse auf die zukünftige Entwicklung von Galaxien wie unserer Heimatgalaxis, der Milchstraße, ziehen.

**September 16 2015**

**21.09.2015:"Licht-Reflektionen" :Talk of J.-M. Raimond**

## Lecture series “Licht-Reflektionen” for the International Year of Light

**Jean-Michel Raimond**

Université Pierre et Marie Curie, Paris

Laboratoire Kastler Brossel, Collège de France ENS, CNRS, UPMC, Paris

**Montag, 21. September 2015, 17:30 Uhr
Campus, Gebäude C6 4, Großer Hörsaal**

*Quantenforschung mit Atomen und Licht in Resonatoren –
Exploring the quantum with atoms and light in cavities*

Abstract:

The fascinating properties of light have guided our first steps in the weird quantum

world. We have now acquired an in-depth understanding of the micro-world. It led to

many applications and to new experimental tools. In turn, these tools allow us to unveil

the most basic quantum phenomena, to realize the thought experiments repeatedly used

by the founding fathers to assess their interpretation of the newborn quantum theory.

We can also think of harnessing the quantum for radically new methods in information

transmission and processing.

Cavity quantum electrodynamics belongs to this very active trend. It deals with a single

atom interacting coherently with a few photons in a single field mode stored in a high

quality cavity, a modern equivalent of Einstein’s photon box. With circular Rydberg

atoms and superconducting cavities, we count the photons without losing them,

realizing an ideal measurement of the field intensity. We use this information in a

quantum feedback loop stabilizing the photon number against cavity losses. We also

prepare mesoscopic quantum superpositions, reminiscent of the famous Schrödinger

cat, and study their decoherence, shedding some light onto the quantum to classical

transition.

**July 20 2015**

**20.-22.07.2015: Visit of Nishant Dogra**

Nishant Dogra (Quantum Optics Group, ETH Zurich) is visiting us and will give a talk in our Ctrl-q seminar on Wednesday 22.07.2015 at 9.00 o'clock in E.04 about "Bose-Hubbard model with cavity-mediated long-range interactions: theory and experiment".

Abstract:

The Bose-Hubbard model has been a paramount example of quantum simulation of many-body

systems. It is realized by loading a quantum gas in a 3D optical lattice. This system undergoes a

quantum phase transition from a superfluid to a Mott-insulator phase due to the competition

between the kinetic energy and the short-range interactions. We study the effect of long-range

interactions on this system generated by strongly coupling the quantum gas to a high finesse cavity

and pumping it with a transverse laser field. This system can be mapped to an extended BoseHubbard

model. In the limit where the static lattices are commensurate with the cavity generated

dynamical lattice, we calculate the phase diagram of this system using a mean-field method. We find

that the cavity-mediated long-range interactions give rise to additional phases: charge density wave

and supersolid phase. We also calculate the excitation spectrum of these phases using a slave-Boson

approach and relate it to the nature of the transition between them. We also show the recent

experimental progress in observing the charge density wave and mapping the phase diagram of such

a system as a function of short-range and cavity-mediated long-range interactions.

**July 14 2015**

**14.07.2016: We congratulate M.Sc. Andreas Buchheit!**

We congratulate Andreas Buchheit to his Masters degree on "Critical Analysis of the Born-Markov Master Equation".

**July 09 2015**

**16.07.2015: Visit of Gabriele Ferrari**

Gabriele Ferrari is visiting us and will give a talk on Thursday 16th of July in the Physikalisches Kolloquium at 16.15.

**July 02 2015**

**09.07.2015: Visit of Sabrina Maniscalco**

Sabrina Maniscalco is visiting us and will give a talk on Thursday 9th of July in the Physikalisches Kolloquium at 16.15.

**June 23 2015**

** "Ion Traps for Tomorrow’s Applications" has been published!**

The book "Ion Traps for Tomorrow’s Applications" has been published by IOS. http://www.iospress.nl/book/ion-traps-for-tomorrows-applications/

**June 10 2015**

**17.-20.6.2015: Visit of Darrick Chang**

Darrick Chang (ICFO Barcelona) is visiting us and will give a talk on Thursday 18th of June in the Physikalisches Kolloquium at 16:15 about "Cold atoms coupled to photonic crystals: a platform for tunable long-range interactions".

**June 01 2015**

**08.-10.06.2015: Visit of Bruno Peaudecerf**

Bruno Peaudecerf (Strathclyde University, Glasgow) is visiting us and will give a talk about "Single-atom imaging of fermions in a quantum-gas microscope" in our group seminar on Tuesday 09.06.2015, 09.00 o'clock in room 4.18.

Abstract:

Ultracold atoms in optical lattices have become a key tool to simulate and test fundamental concepts of condensed matter physics, in particular to simulate electrons in solid crystals. A quantum-gas microscope with single-atom and single-site resolution furthermore allows for direct measurement of ordered quantum phases and out-of-equilibrium dynamics, with access to quantities ranging from spin-spin correlation functions to many-particle entanglement. Until recently, fluorescence imaging of individual atoms in a quantum-gas microscope had only been achieved for bosonic species with optical molasses cooling, while the detection of fermionic alkaline atoms in optical lattices proved more challenging. In this talk I will present our realisation of single-site- and single-atom-resolved fluorescence imaging of fermionic potassium-40 atoms in a quantum-gas microscope setup using electromagnetically-induced-transparency cooling [1]. We detected on average 1000 fluorescence photons from a single atom within 1.5 s, while keeping it close to the vibrational ground state of the optical lattice. Our results will enable the investigation of phenomena and properties of strongly correlated fermionic quantum systems, with direct probing at the single atom level, e.g., of the local entropy distribution and individual defects in fermionic Mott insulators, or of spin-spin correlations at the onset of antiferromagnetic ordering. [1] E. Haller et al., arXiv:1503:02005

**June 01 2015**

**01.06.2015: Sonderkolloquium John Martinis**

John Martinis is visiting and will give a talk in a Sonderkollquium on 01.06.2015 at 16ct in Building E2 5,Hörsaal I (E.1) about ""What's next after Moore's law: quantum computing".

**May 26 2015**

**26.05.2016: We congratulate M.Sc. Simon Jäger!**

We congratulate Simon Jäger to his Masters degree on "Molekularfeldmodell für die Selbstorganisation von Atomen im Resonator".

**May 20 2015**

**20.05.2015: Visit of Nikolai Lauk**

Nikolai Lauk (Kaiserslautern) is visiting us.

**April 27 2015**

**29.- 30.04.2015: Visit of Simone Montangero**

Simone Montangero is visiting us and will give a talk on Thursday 30th of April in the Physikalisches Kolloquium at 16.15 about "Simulation and control of complex quantum systems".

**April 06 2015**

**13.04. - 14.04.2015: Visit of Laurent de Forges de Parny **

Laurent de Forges de Parny (ENS Lyon) is visiting us and will give a talk in our group seminar about: "Multiple Transitions in a Coupled Atom-Molecule Mixture".

Abstract:

Since the pioneering experiments in 2002 by M. Greiner et al., ultracold bosons in optical lattices are ideal test bed for many condensed matter models. As you know, the Bose-Einstein condensation of pairs of particles is central in the theory of superconductivity and the control of such a mechanism is still a big challenge. Feshbach resonances - namely resonances between an unbound two-body state (atomic state) and a bound (molecular) state - allow the conversion of two unbound atoms into a molecule and vice versa.

My numerical studies focus on a 2D mixture of interacting bosonic atoms and molecules with conversion between these two species. This system involves many interesting phases and phase transitions:

- At zero temperature, the system exhibits molecular and atomic-molecular condensates and an insulating phase stabilized by the conversions - a "Feshbach insulator". First order, 3D XY and 3D Ising transitions appear.

- At finite temperature, this system also reveals interesting transitions: when increasing the temperature, the system evolves from an atomic-molecular superfluid to a molecular superfluid and then to a normal Bose liquid. The multiple transitions involved (2D Ising and KT transitions) are very well described by a coupled XY model and scaling analysis.

**March 27 2015**

**New Article: Phys. Rev. A 91, 033834 (2015)**

#### Interfacing microwave qubits and optical photons via spin ensembles

Susanne Blum, Christopher O'Brien, Nikolai Lauk, Pavel Bushev, Michael Fleischhauer, and Giovanna Morigi

Phys. Rev. A 91, 033834 (2015)

A protocol is discussed which allows one to realize a transducer for single photons between the optical and the microwave frequency range. The transducer is a spin ensemble, where the individual emitters possess both an optical and a magnetic-dipole transition. Reversible frequency conversion is realized by combining optical photon storage, by means of electromagnetically induced transparency, with the controlled switching of the coupling between the magnetic-dipole transition and a superconducting qubit, which is realized by means of a microwave cavity. The efficiency is quantified by the global fidelity for coherently transferring a qubit excitation between a single optical photon and the superconducting qubit. We test various strategies and show that the total efficiency is essentially limited by the optical quantum memory: It can exceed 80% for ensembles of nitrogen-vacancy centers and approaches 99% for cold atomic ensemble, assuming state-of-the-art experimental parameters. This protocol allows one to bridge the gap between the optical and the microwave regime in order to efficiently combine superconducting and optical components in quantum networks.

**March 23 2015**

**23.03. - 01.05.2015: Visit of Karl-Peter Marzlin**

Karl-Peter Marzlin (St Francis Xavier University,Canada) visits us.

**February 18 2015**

**20.02.2015: Visit of Eugene Demler**

Eugene Demler (Harvard University) visits us and will give a talk about 'Many Body Localization and Griffiths Effects' in the Ctrl-Q seminar starting at 09.30 .

Abstract:

I will discuss the nature of the transition between the metallic and Many Body Localized phases. I will point out the important role of rare fluctuations and Griffiths type phenomena at the transition.

**February 09 2015**

**13.02.2015: Visit of Haggai Landa**

Haggai Landa (LPTMS Paris) visits us.

**February 08 2015**

**12.-13.02.2015: Visit of Klemens Hammerer**

Klemens Hammerer (Leibniz University Hannover, Germany) visits us and will give a talk in the "Physikalisches Kolloquium" at 16.00 c.t. in building C6 4,HS II about "Entanglement of Matter and Continuous-Wave Light".

**January 15 2015**

**18.-20.01.2015: Visit of Cesare Nardini**

Cesare Nardini (University of Edinburgh, GB) visits us and will give a talk about "KINETIC THEORY FOR NON-EQUILIBRIUM LONG-RANGE INTERACTING SYSTEMS" on Tuesday @9.15 in room 4.18 in our group seminar.

Abstract:

Long-range interacting systems include gravitational systems, plasma in the low density limit, two-dimensional and geophysical fluid models. This talks describes our recent works on simple models of long-range interacting systems driven out of equilibrium by external forces.

In many physical context, long-range interacting systems are found to be out of equilibrium because of external drving. Examples come from climate dynamics, plasma physics, astrophysics and, recently, experimental setups with cold-atoms driven by laser light.

In order to address the description of driven long-range interacting systems in a theoretical way, we concentrate in this talk on models as simple as possible that still retain the following two main characteristics: non-local (i.e. long-range) nature of the interactions and broken detailed balance (i.e. non-equilibrium dynamics).

We present results both for particle systems and quasi two-dimensional flows and we show that their dynamics can be described very accurately in the limit where there is a separation of time-scales between the evolution of the mean state and the evolution of the fluctuations around it.

The main theoretical tool developed is kinetic theory: the accuracy of the results obtained will be compared to direct numerical simulations. Ongoing work and perspectives on a combination of kinetic theory and large deviations theory to describe bistability and rare events will also be described.

**January 13 2015**

**Physik-Nobelpreisträger üben scharfe Kritik an den Uni-Sparplänen**

Saarbrücker-Zeitung:Physik-Nobelpreisträger üben scharfe Kritik an den Uni-Sparplänen

**January 08 2015**

**12.-17.01.2015: Visit of Efrat Shimshoni **

Efrat Shimshoni (Bar-Ilan University, Israel) visits us and will give a "Introductory lecture on the XY model and the Kosterlitz-Thouless transition" on Tuesday @9.00 in room 4.18 in our group seminar.

**January 08 2015**

**12.-17.01.2015: Visit of Shmuel Fishman.**

Shmuel Fishman (Technion, Haifa, Israel) visits us and will give a talk about "Collapses and revivals of matter waves" on Wednesday @9.00 in room E.04 in the Ctrl-Q seminar.

Abstract:

Quantum collapses and revivals are fascinating manifestations of coherence. Of particular interest in recent years are macroscopic quantum interference effects in Bose-Einstein condensates. We studied such effects for the two site Bose-Hubbard model, and found an analytic expression for the difference in the occupation of the two sites. The result is valid for large number of particles and weak inter-particle interactions. This expression predicts the time scales of the dynamics and enables one to evaluate the shape of the revival peaks in absence of dissipation. The work reported was done with Hagar Veksler.

**December 16 2014**

**New Article: Phys. Rev. A 90, 063818 (2014)**

#### Quantum optical master equation for solid-state quantum emitters

Ralf Betzholz, Juan Mauricio Torres, and Marc Bienert

Phys. Rev. A 90, 063818 (2014)

We provide an elementary description of the dynamics of defect centers in crystals in terms of a quantum optical master equation which includes spontaneous decay and a simplified vibronic interaction with lattice phonons. We present the general solution of the dynamical equation by means of the eigensystem of the Liouville operator and exemplify the usage of this damping basis to calculate the dynamics of the electronic and vibrational degrees of freedom and to provide an analysis of the spectra of scattered light. The dynamics and spectral features are discussed with respect to the applicability for color centers, especially for negatively charged nitrogen-vacancy centers in diamond.

**November 24 2014**

** 04.12.2014: Visit of Christopher Monroe**

Christopher Monroe (University of Maryland and NIST Gaithersburg) is visiting us and will give a talk in the "Physikalisches Kolloquium" at 16.00 c.t. in building C6 4,HS II about "Quantum Networks of Trapped Ions".

**November 14 2014**

**New Article: Phys. Rev. Lett. 113, 203002 (2014)**

#### Prethermalization of Atoms Due to Photon-Mediated Long-Range Interactions.

Stefan Schütz and Giovanna Morigi

Phys. Rev. Lett. 113, 203002 (2014)

Atoms can spontaneously form spatially ordered structures in optical resonators when they are transversally driven by lasers. This occurs when the laser intensity exceeds a threshold value and results from the mechanical forces on the atoms associated with superradiant scattering into the cavity mode. We treat the atomic motion semiclassically and show that, while the onset of spatial ordering depends on the intracavity-photon number, the stationary momentum distribution is a Gaussian function whose width is determined by the rate of photon losses. Above threshold, the dynamics is characterized by two time scales: after a violent relaxation, the system slowly reaches the stationary state over time scales exceeding the cavity lifetime by several orders of magnitude. In this transient regime the atomic momenta form non-Gaussian metastable distributions, which emerge from the interplay between the long-range dispersive and dissipative mechanical forces of light. We argue that the dynamics of self-organization of atoms in cavities offers a test bed for studying the statistical mechanics of long-range interacting systems.

**November 06 2014**

**New Article: Phys. Rev. A 90, 053601 (2014)**

#### Structural transitions of nearly second order in classical dipolar gases.

Florian Cartarius, Giovanna Morigi, and Anna Minguzzi

Phys. Rev. A 90, 053601 (2014)

Particles with repulsive power-law interactions undergo a transition from a single to a double chain (zigzag) by decreasing the confinement in the transverse direction. We theoretically characterize this transition when the particles are classical dipoles, polarized perpendicularly to the plane in which the motion occurs, and argue that this transition is of first order, even though weakly. The nature of the transition is determined by the coupling between transverse and axial modes of the chain and contrasts with the behavior found in Coulomb systems, where the linear-zigzag transition is continuous and belongs to the universality class of the ferromagnetic transition. Our results hold for classical systems with power-law interactions 1/rα when α>2 and show that structural transitions in dipolar systems and Rydberg atoms can offer a test bed for simulating the critical behavior of magnets with lattice coupling.

**October 10 2014**

**12.-18.10.2014: Visit of Dmitriy Kupriyanov**

Dmitriy Kupriyanov (SPb state polytechnic university ) is visiting us and gives a talk in the Ctrl-Q Seminar on Wednesday at 9.15 in E04 and in our group seminar on Friday at 11.00 in 4.18.

**September 25 2014**

**25.-26.09.2014: Visit of Helmut Ritsch**

Helmut Ritsch (University of Innsbruck) is visiting us.

**August 15 2014**

**New Article: Phys. Rev. A 90, 023824 (2014).**

#### Arbitrary-quantum-state preparation of a harmonic oscillator via optimal control.

Katharina Rojan, Daniel M. Reich, Igor Dotsenko, Jean-Michel Raimond, Christiane P. Koch, and Giovanna Morigi

Phys. Rev. A. 90, 023824 (2014)

The efficient initialization of a quantum system is a prerequisite for quantum technological applications. Here we show that several classes of quantum states of a harmonic oscillator can be efficiently prepared by means of a Jaynes-Cummings interaction with a single two-level system. This is achieved by suitably tailoring external fields which drive the dipole and/or the oscillator. The time-dependent dynamics that leads to the target state is identified by means of optimal control theory (OCT) based on Krotov's method. Infidelities below 10−4 can be reached for the parameters of the experiment of Raimond, Haroche, Brune and co-workers, where the oscillator is a mode of a high-Q microwave cavity and the dipole is a Rydberg transition of an atom. For this specific situation we analyze the limitations on the fidelity due to parameter fluctuations and identify robust dynamics based on pulses found using ensemble OCT. Our analysis can be extended to quantum-state preparation of continuous-variable systems in other platforms, such as trapped ions and circuit QED.

**August 11 2014**

**New article: Phys. Rev. Lett. 113, 063603 **

#### Interfacing Superconducting Qubits and Telecom Photons via a Rare-Earth-Doped Crystal

Christopher O’Brien, Nikolai Lauk, Susanne Blum, Giovanna Morigi, and Michael Fleischhauer

Phys. Rev. Lett. 113, 063603

We propose a scheme to couple short single photon pulses to superconducting qubits. An optical photon is first absorbed into an inhomogeneously broadened rare-earth doped crystal using controlled reversible inhomogeneous broadening. The optical excitation is then mapped into a spin state using a series of π pulses and subsequently transferred to a superconducting qubit via a microwave cavity. To overcome the intrinsic and engineered inhomogeneous broadening of the optical and spin transitions in rare-earth doped crystals, we make use of a special transfer protocol using staggered π pulses. We predict total transfer efficiencies on the order of 90%.

**August 01 2014**

**30.07.2014: We congratulate B.Sc. Tim Keller!**

We congratulate Tim Keller for his Bachelor degree on "Theoretische Analyse der Dynamik zweier kalter gefangener Atome".

**July 30 2014**

**30.07.2014: We congratulate B.Sc.Rebecca Kraus!**

We congratulate Rebecca Kraus for her Bachelor degree on "Optische Kraft von Schwarzkörperstrahlung".

**July 17 2014**

**New article: Phys. Rev. A 90, 013617**

#### Quenching small quantum gases: Genesis of the orthogonality catastrophe

Steve Campbell, Miguel Ángel García-March, Thomás Fogarty, and Thomas Busch

Phys. Rev. A 90, 013617

We study the dynamics of two strongly interacting bosons with an additional impurity atom trapped in a harmonic potential. Using exact numerical diagonalization we are able to fully explore the dynamical evolution when the interaction between the two distinct species is suddenly switched on (quenched). We examine the behavior of the densities, the entanglement, the Loschmidt echo, and the spectral function for a large range of interspecies interactions and find that even in such small systems evidence of Anderson's orthogonality catastrophe can be witnessed.

**July 11 2014**

**New article: Phys. Rev. A 90, 012312**

#### Quantum reservoirs with ion chains

B. G. Taketani, T. Fogarty, E. Kajari, Th. Busch, and Giovanna Morigi

Phys. Rev. A 90, 012312

Ion chains are promising platforms for studying and simulating quantum reservoirs. One interesting feature is that their vibrational modes can mediate entanglement between two objects which are coupled through the vibrational modes of the chain. In this work we analyze entanglement between the transverse vibrations of two heavy impurity defects embedded in an ion chain, which is generated by the coupling with the chain vibrations. We verify general scaling properties of the defect dynamics and demonstrate that entanglement between the defects can be a stationary feature of these dynamics. We then analyze entanglement in chains composed of tens of ions and propose a measurement scheme which allows one to verify the existence of the predicted entangled state.

**July 11 2014**

**11.07.2014: We congratulate B.Sc. Oliver Köhn!**

We congratulate Oliver Köhn for his Bachelor degree on "Kalte Antiwasserstoffatome und Antineutronen gefangen in einem Gravitationspotential".

**July 09 2014**

**10.-11.07.2014: Visit of Markus Holzmann & Anna Minguzzi**

Markus Holzmann & Anna Minguzzi (LPMMC Grenoble) are visiting us.

**July 08 2014**

**"Theorie der Supraleitung, Supraflüssigkeit und BEC" entfällt heute.**

Das Seminar "Theorie der Supraleitung, Supraflüssigkeit und Bose-Einstein-Kondensation" entfällt heute am 08.07.2014.

Todays seminar (08.07.2014) on the 'Theory of supraconductivity, suprafluidity and Bose-Einstein-condensation' is cancelled.

**June 22 2014**

**25.-27.06.2014: Visit of Ignacio Wilson Rae**

Ignacio Wilson Rae (University of York) is visiting us and gives a talk in the Ctrl-Q on 26.06.2014 9.15 in E.04 about "Carbon-nanotube optomechanics" .

Abstract:

Carbon nanotubes (CNTs) constitute a promising system for quantum optomechanics. Here we propose two optomechanical schemes for demonstrating quantum effects in the motion of a suspended CNT subjected to an inhomogeneous electric field. The first one is based on deformation-potential exciton-phonon coupling. In this case the electric field generates an optically active quantum dot featuring a tunable parametric coupling between the confined excitons and the flexural phonons of the nanotube. For a small suspended length, this interaction enables efficient optical ground-state cooling of the fundamental mode and allows to realise a mechanical analogue of cavity-QED. In turn, in the limit of large suspended length, this system exhibits a collapse of the excitonic zero-phonon line that can be ascribed to the "localised" phase of a subohmic spin-boson model, and is associated to Non-Markovian dephasing leading to sideband spectra characterised by half-integer power-law decays. The second optomechanical scheme exploits geometric nonlinearity and a softening instability of the fundamental mode induced by the electric field. Tuning "below" the instability enhances the zero-point motion leading to a sizeable geometric nonlinearity per quanta. This allows to demonstrate the quantum fine structure of a "weakly anharmonic" mode and enables cavity-optomechanical setups where a classical optical input can induce a nonclassical motional steady-state of the CNT approaching a Fock state.

**June 12 2014**

**New article: Phys. Rev. B 89, 214408**

#### From classical to quantum criticality

Daniel Podolsky, Efrat Shimshoni, Pietro Silvi, Simone Montangero, Tommaso Calarco, Giovanna Morigi, and Shmuel Fishman

Phys. Rev. B 89, 214408

We study the crossover from classical to quantum phase transitions at zero temperature within the framework of ϕ4 theory. The classical transition at zero temperature can be described by the Landau theory, turning into a quantum Ising transition with the addition of quantum fluctuations. We perform a calculation of the transition line in the regime where the quantum fluctuations are weak. The calculation is based on a renormalization group analysis of the crossover between classical and quantum transitions, and is well controlled even for space-time dimensionality D below 4. In particular, for D=2 we obtain an analytic expression for the transition line which is valid for a wide range of parameters, as confirmed by numerical calculations based on the density matrix renormalization group. This behavior could be tested by measuring the phase diagram of the linear-zigzag instability in systems of trapped ions or repulsively interacting dipoles.

**June 06 2014**

**12.06.2014: Visit of Lazar Friedland**

Lazar Friedland (Racah institute) will give a talk about "Quantum phenomena in a chirped-driven anharmonic oscillator" at 11.00 in room 4.03, building C6 3.

**May 26 2014**

**02.-06.06.2014: Visit of Shmuel Fishman.**

Shmuel Fishman (Technion, Haifa, Israel) visits us and will give a talk about "Chaotic Analysis of the Nonlinear Schrödinger Equation (NLSE)" on Tuesday @9.15 in room 4.18 in our group seminar.

Abstract:

The Nonlinear Schrödinger Equation (NLSE) with a random potential

exhibits competition between Anderson localization resulting of

randomness and spreading enhanced by the non-linearity. In spite of

extensive research it is not clear if a wave-packet that is initially

localized, spreads to infinity for asymptotically long time. The

various conflicting results that were obtained will be discussed

critically.

**May 12 2014**

**15-16.05.2014: visit of Duncan O'Dell**

Duncan O'Dell (McMaster University) visits us and will give a talk on Friday in room E.11 at 11.00 o'clock about: "Using cavity QED for measurements of gravity"

Abstract:I will present a theoretical proposal for using cold atoms trapped inside a high finesse optical cavity to measure the local acceleration due to gravity. The atoms perform Bloch oscillations in the intracavity optical lattice inside a vertically oriented cavity that is pumped by a laser. In the regime of strong atom-light coupling achievable in cavities this leaves an imprint on the light leaking out of the cavity which can be used to make a continuous measurement of gravity. I will also discuss how atoms can be transported either with or against gravity depending on the detuning of the laser from the cavity resonance.

**April 29 2014**

**29.04.2014: We congratulate M.Sc. Tristan Tentrup!**

We congratulate Tristan Tentrup to his Masters degree on "Spektrale Eigenschaften einzelner Photonen von einzelnen Emittern".

**April 20 2014**

**22.-25.04.2014: visit of Pablo Barberis-Blostein**

Pablo Barberis-Blostein (Mexico) visits us and will give a talk on Tuesday 22.04.2014 in the group seminar @09.00 in 4.18 with the title "Quantum beats, quantum beats light shifts, and quantum beats manipulation using postselection".

Abstract:

Ground state quantum beats observed in the second order intensity

correlation from a continuously driven atomic ensemble inside a two

mode optical cavity are subject to a frequency shift and

decoherence. While driving the cavity with light of linear

polarization ($\pi$ transitions) the second order autocorrelation

function is measured in the undriven mode (orthogonal polarization):

a first photon detection prepares a superposition of atomic

ground-state Zeeman sublevels and the second measures the ground

state beats. Between these two detections, the atoms can become

excited and return to the ground state, emitting most of the photons

into modes other than the cavity modes. Depending on the drive

strength this process can happen several times. Each time there is a

relative phase advance between the Zeeman sublevels. The information

of this phase advance and its associated decoherence is then leaked

into the modes that are not the cavity modes, which form the

environment. It is possible to get information about the number of

photons leaked into the environment by monitoring the driven

mode. Here we propose a scheme to manipulate the loss of amplitude

of the beats (decoherence) and the beat frequency shift, by

postselecting on the basis of information gathered through

measurement of the driven cavity mode. This proposal is a new

strategy compared with controlling the decoherence and light shift

through turning off the driven field.

**April 10 2014**

**15.-17.04.2014: visit of Steve Campbell**

Steve Campbell (Queens University Belfast) visits us and will give a talk on Wednesday, 16.04.2014 at 13.00 in 4.18 about "Criticality and correlations: The advantages in studying the quantum discord".

**March 20 2014**

**New article: New J. Phys. 16 033021 (2014)**

#### Cavity cooling of an atomic array

O. S. Mishina

New J. Phys. 16 033021 (2014)

While cavity cooling of a single trapped emitter was demonstrated, cooling of many particles in an array of harmonic traps needs investigation and poses a question of scalability. This work investigates the cooling of a one dimensional atomic array to the ground state of motion via the interaction with the single mode field of a high-finesse cavity. The key factor ensuring the cooling is found to be the mechanical inhomogeneity of the traps. Furthermore it is shown that the pumped cavity mode does not only mediate the cooling but also provides the necessary inhomogeneity if its periodicity differs from the one of the array. This configuration results in the ground state cooling of several tens of atoms within a few milliseconds, a timescale compatible with current experimental conditions. Moreover, the cooling rate scaling with the atom number reveals a drastic change of the dynamics with the size of the array: atoms are either cooled independently, or via collective modes. In the latter case the cavity mediated atom interaction destructively slows down the cooling as well as increases the mean occupation number, quadratically with the atom number. Finally, an order of magnitude speed up of the cooling is predicted as an outcome the optimization scheme based on the adjustment of the array versus the cavity mode periodicity.

**March 11 2014**

**New article: Phys. Rev. B 89, 094103 (2014)**

#### Ab initio characterization of the quantum linear-zigzag transition using density matrix renormalization group calculations

Pietro Silvi, Tommaso Calarco, Giovanna Morigi, and Simone Montangero

Phys. Rev. B 89, 094103 (2014)

Ions of the same charge inside confining potentials can form crystalline structures which can be controlled by means of the ion density and of the external trap parameters. In particular, a linear chain of trapped ions exhibits a transition to a zigzag equilibrium configuration, which is controlled by the strength of the transverse confinement. Studying this phase transition in the quantum regime is a challenging problem, even when employing numerical methods to simulate microscopically quantum many-body systems. Here we present a compact analytical treatment to map the original long-range problem into a short-range quantum field theory on a lattice. We provide a complete numerical architecture, based on the density matrix renormalization group, to address the effective quantum ϕ4 model. This technique is instrumental in giving a complete characterization of the phase diagram, as well as pinpointing the universality class of the criticality.

**March 05 2014**

**New article: Phys. Rev. A 89, 033404 (2014)**

#### Electromagnetically-induced-transparency control of single-atom motion in an optical cavity

Tobias Kampschulte, Wolfgang Alt, Sebastian Manz, Miguel Martinez-Dorantes, René Reimann, Seokchan Yoon, Dieter Meschede, Marc Bienert, and Giovanna Morigi

Phys. Rev. A 89, 033404 (2014)

We demonstrate cooling of the motion of a single neutral atom confined by a dipole trap inside a high-finesse optical resonator. Cooling of the vibrational motion results from electromagnetically induced transparency (EIT)–like interference in an atomic Λ-type configuration, where one transition is strongly coupled to the cavity mode and the other is driven by an external control laser. Good qualitative agreement with the theoretical predictions is found for the explored parameter ranges. Further, we demonstrate EIT cooling of atoms in the dipole trap in free space, reaching the ground state of axial motion. By means of a direct comparison with the cooling inside the resonator, the role of the cavity becomes evident by an additional cooling resonance. These results pave the way towards a controlled interaction among atomic, photonic, and mechanical degrees of freedom.

**February 20 2014**

**20.02.2014: We congratulate B.Sc. Simon Jäger!**

We congratulate Simon Jäger for his Bachelor degree on "Strahlungsdämpfung in der Quantenelektrodynamik".

**February 19 2014**

**New article: Phys. Rev. A 89, 023832 (2014)**

#### Stationary entanglement of photons and atoms in a high-finesse resonator

Hessam Habibian, Stefano Zippilli, Fabrizio Illuminati, and Giovanna Morigi

Phys. Rev. A 89, 023832 (2014)

We predict that the collective excitations of an atomic array become entangled with the light of a high-finesse cavity mode when they are suitably coupled. This entanglement is of the Einstein-Podolsky-Rosen type, is robust against cavity losses, and is a stationary property of the coupled system. It is generated when the atomic array is aligned along the cavity axis and driven transversally by a laser, when coherent scattering of photons into the cavity mode is suppressed because of phase mismatching. We identify the parameter regimes under which entanglement is found and show that these are compatible with existing experimental setups.

**February 12 2014**

**17.-19.02.2014: Visit of Francesco Piazza**

Francesco Piazza (TU Munich) visits us and will give a talk on Tuesday 18.02.2014 in the group seminar @09.00 in 4.18 about "Umklapp Superradiance from a Collisionless Quantum Degenerate Fermi Gas".

Abstract:

The quantum dynamics of the electromagnetic light mode of an optical cavity filled with a coherently driven Fermi gas of ultracold atoms strongly depends on geometry of the Fermi surface. Superradiant light generation and self-organization of the atoms can be achieved at low pumping threshold due to resonant atom-photon Umklapp processes, where the fermions are scattered from one side of the Fermi surface to the other by exchanging photon momenta. The cavity spectrum exhibits sidebands, that, despite strong atom-light coupling and cavity decay, retain narrow linewidth, due to absorptionless transparency windows outside the atomic particle-hole continuum and the suppression of inhomogeneous broadening and thermal fluctuations in the collisionless Fermi gas.

**February 09 2014**

**13.02.2014: Visit of Haggai Landa**

Haggai Landa (LPTMS Paris) visits us.

**February 05 2014**

**05-07.02.2014: Visit of Mauricio Torres**

Mauricio Torres (TU Darmstadt) is visiting us.

**January 24 2014**

**27.01.2014: Talk of Giovanni Manfredi**

Giovanni Manfredi will give the third talk of his lecture series "Electron dynamics in thin metal films: from Wigner functions to quantum hydrodynamics" from 10:30-12:00 in E2.4 seminar room 7 (room 203).

**January 21 2014**

**21.01.2014: Visit of Christiane Koch**

Christiane Koch (Quantum Dynamics and Control Group in Kasse) is visiting us.

**December 20 2013**

**08.01-10.01.2014: Visit of Dylan Yost**

Dylan Yost (MPQ Munich) is visiting us.

**December 16 2013**

**18.-19.12.2013: Visit of Thomas Busch**

Thomas Busch (Okinawa Institute of Science and Technology) will visit us and give a talk at 13.30 in room E.04.

**December 16 2013**

**16-20.12.2013: Visit of Christian Arenz **

Christian Arenz (Aberystwyth University, Wales ) is visiting us and will give a talk in the Ctrl-q seminar about "Control of open quantum systems: Case study of the central spin model".

Abstract:

We study the controllability of a central spin guided by a classical field and interacting with a spin bath, showing that the central spin is fully controllable independently of the number of bath spins. Additionally we find that for unequal system-bath couplings even the bath becomes controllable by acting on the central spin alone. We then analyze numerically how the time to implement gates on the central spin scales with the number of bath spins and conjecture that for equal system-bath couplings it reaches a saturation value. We provide evidence that sometimes noise can be effectively suppressed through control.

**December 04 2013**

**04.12.2013: Visit of Daniel Reich**

Daniel Reich from the Quantum Dynamics and Control Group in Kassel is visiting us. RevModPhys.64_.1045_.pdf

**November 24 2013**

**28.11.2013 Talk of Luis Santos (Universität Hannover) in the Kolloquium**

Luis Santos (Universität Hannove) will give a talk in the physikalisches Kolloquium in C6.3 HS II at 16 ct. More information can be found at the Physics website.

**November 17 2013**

**22.11.2013: Visit of Cecilia Cormick**

Cecilia Cormick (Uni Ulm) will visit us and give a talk at 10.00 in SR9 in the maths building.

**November 08 2013**

**New article: Phys. Rev. A 88, 053807 (2013)**

#### Single-photon frequency conversion in nonlinear crystals

Susanne Blum, G. A. Olivares-Rentería, Carlo Ottaviani, Christoph Becher, and Giovanna Morigi

Phys. Rev. A 88, 053807 (2013)

Frequency conversion of single photons in a nonlinear crystal is theoretically discussed. Losses and noise are included within a Heisenberg-Langevin formalism for the propagating photon field. We calculate the first- and second-order correlation functions of the frequency-converted light when the input is a train of single-photon pulses. This model allows one to identify the requirements on the nonlinear device so that it can be integrated in a quantum network.

**November 07 2013**

**New article: J. Phys. B: At. Mol. Opt. Phys. 46 (2013) 224001**

#### Generation of two-mode entangled states by quantum reservoir engineering

Christian Arenz, Cecilia Cormick, David Vitali and Giovanna Morigi

J. Phys. B: At. Mol. Opt. Phys. 46 (2013) 224001

A method for generating entangled cat states of two modes of a microwave cavity field is proposed. Entanglement results from the interaction of the field with a beam of atoms crossing the microwave resonator, giving rise to non-unitary dynamics of which the target entangled state is a fixed point. We analyse the robustness of the generated two-mode photonic 'cat state' against dephasing and losses by means of numerical simulation. This proposal is an instance of quantum reservoir engineering of photonic systems.

**November 05 2013**

**05.11.2013: We congratulate B.Sc. Daniel Biehl!**

We congratulate Daniel Biehl for his Bachelor degree on "Quanten- und semiklassische Dynamik von einzelnen Atomen in Resonatoren hoher Güte".

**November 03 2013**

**07.11.2013 Talk of Kurt Busch (Humboldt Universität Berlin) in the Kolloquium**

Kurt Busch (Humboldt Universität Berlin) will give a talk in the physikalisches Kolloquium in C6.3 HS II at 16 ct. More information can be found at the Physics website.

**October 30 2013**

**New article: Annalen der Physik 525, 827 (2013) **

#### Full characterization of the quantum linear-zigzag transition in atomic chains

P. Silvi, G. De Chiara, T. Calarco, G. Morigi, and S. Montangero

Annalen der Physik 525, 827 (2013)

**October 23 2013**

**25.10.2013: Visit of Michael Fleischhauer, Christopher O'Brien and Nikolai Lauk**

Michael Fleischhauer, Christopher O'Brien and Nikolai Lauk (TU Kaiserslautern) visit us.

**October 23 2013**

**24.10.2013 Talk of Alice Sinatra (ENS Paris) in the Kolloquium**

Alice Sinatra (ENS Paris) will give a talk in the physikalisches Kolloquium in C6.3 HS II at 16 ct. The titel of her talk is "Spin squeezing with Bose-Einstein condensates: experimental achievements and ultimate limits". More information can be found at the Physics website.

**October 14 2013**

**New article: Phys. Rev. A 88, 043618 (2013)**

#### Quantum phases of incommensurate optical lattices due to cavity backaction

Hessam Habibian, André Winter, Simone Paganelli, Heiko Rieger, and Giovanna Morigi

Phys. Rev. A 88, 033427 (2013)

Ultracold bosonic atoms are confined by an optical lattice inside an optical resonator and interact with a cavity mode whose wavelength is incommensurate with the spatial periodicity of the confining potential. We predict that the intracavity photon number can be significantly different from zero when the atoms are driven by a transverse laser whose intensity exceeds a threshold value and whose frequency is suitably detuned from the cavity and the atomic transition frequency. In this parameter regime the atoms form clusters in which they emit in phase into the cavity. The clusters are phase locked, thereby maximizing the intracavity photon number. These predictions are based on a Bose-Hubbard model, whose derivation is reported here in detail. The Bose-Hubbard Hamiltonian has coefficients which are due to the cavity field and depend on the atomic density at all lattice sites. The corresponding phase diagram is evaluated using quantum Monte Carlo simulations in one dimension and mean-field calculations in two dimensions. Where the intracavity photon number is large, the ground state of the atomic gas lacks superfluidity and possesses finite compressibility, typical of a Bose glass.

**October 13 2013**

**17.10.2013 Talk of Britton Plourde (Syracuse University und UdS) in the Kolloquium**

Britton Plourde (Syracuse University und UdS) will give a talk in the physikalisches Kolloquium in C6.3 HS II at 16 ct. The titel of his talk is "Vortices in superconductors: microwaves and qubits". More information can be found at the Physics website.

**October 08 2013**

**08.10.2013: We congratulate M.Sc. Katharina Rojan!**

We congratulate Katharina Rojan to her Masters degree on "Optimal Control of the Quantum State of a Microwave Field".

**October 04 2013**

**09.10.2013 Masud Haque (MPI Dresden) is visiting us **

Masud Haque (Max-Planck Institute for Physics of Complex Systems in Dresden) is visiting us and will give a talk on Wednesday 09.10.2013 at 10.15 in E.04.

Title: "Non-equilibrium dynamics in isolated quantum systems"

Abstract:

Quantum many-body physics has traditionally been dominated by detailed

investigations of equilibrium properties and low-energy excitations. In

the last decade or so, motivated by novel experimental possibilities

such as cold atoms, the study of quantum matter far from equilibrium

has emerged as an important and exciting new field.

I will provide a selection of new questions and phenomena that have

appeared in the context of non-equilibrium evolution in isolated

systems. I will detail two examples, namely, (1) the dynamics of

composite objects in lattice systems, and (2) Bloch oscillations of

many-body systems in a tilted lattice.

**October 01 2013**

**New article: Phys. Rev. A 88, 033427 (2013)**

#### Cooling of atomic ensembles in optical cavities: Semiclassical limit

Stefan Schütz, Hessam Habibian, and Giovanna Morigi

Phys. Rev. A 88, 033427 (2013)

The semiclassical dynamics of atoms, when the atoms are confined inside a standing-wave high-finesse resonator, is theoretically studied. The atoms are cooled by scattering processes in which the photons of a transverse laser are coherently scattered into the cavity mode. We derive a Fokker-Planck equation for the atomic center-of-mass variables which allows us to determine the equations of motion in the semiclassical limit for any value of the intensity of the laser field. We extract its prediction for the dynamics when the resonator is essentially in the vacuum state and the atoms are cooled by scattering photons into the cavity mode, which then decays. Its predictions for the stationary atomic distribution are compared with the ones of the Fokker-Planck equation by Domokos et al. [ J. Phys. B 34 187 (2001)], which has been derived under different assumptions. We find full agreement in the considered parameter regime.

**October 01 2013**

**27.9.2013: We congratulate B.Sc. Andreas Buchheit!**

We congratulate Andreas Buchheit for his Bachelor degree on "Dynamik von lasergetriebenen Atomen".

**September 19 2013**

**New article: Phys. Rev. A 88, 033830 (2013)**

#### Seeding patterns for self-organization of photons and atoms

Wolfgang Niedenzu, Stefan Schütz, Hessam Habibian, Giovanna Morigi, and Helmut Ritsch

Phys. Rev. A 88, 033830 (2013)

When atoms scatter photons from a transverse laser into a high-finesse optical cavity, they form crystalline structures which maximize the intracavity light field and trap the atoms in the ordered array. Stable organization occurs when the laser field amplitude exceeds a certain threshold. For planar single-mode cavities there exist two equivalent possible atomic patterns, which determine the phase of the intracavity light field. Under these premises, we show that the effect of an additional laser pumping the cavity makes one pattern more favorable than the other and that it can dynamically force the system into a predetermined configuration. This is an instance of pattern formation and seeding in a nonlinear quantum-optical system.

**September 10 2013**

**Thomás Fogarty joined our group!**

We welcome our new group member Dr. Thomás Fogarty.

**July 15 2013**

**19.07.2013 Visit of Stefan Wißmann**

Stefan Wißmann visits us. He will give a presentation at 9:30, room 4.18 in building E2 6.

#### Quantum states with maximal memory effects and non-Markovian quantum probes

Perfect isolation of any quantum system is almost impossible to realize since it is usually influenced by the coupling to an environment. Typically, the interaction of the system with its environment cannot be neglected so that a modelling of a quantum physi- cal systems as a closed system fails to give a correct description of the system’s dynamics. The most prominent approach resorts to an approximation of the open system dynamics in terms of a so-called Markovian master equation which includes several rather drastic assumptions on the properties of the system and environment. It is not surprising that there exist complex systems for which this relatively simple description fails to give a faithful picture of the dynamics. Recently, there has been put much effort in developing a general theory and classifying non-Markovian open system dynamics. In my talk I summarize the basic principles of different approaches to quantify non-Markovian be- havior and present the latest results on the non-Markovianity measure proposed in [1] regarding the mathematical and physical properties of optimal pairs maximizing this quantity [2]. Moreover, I show some interesting applications how non-Markovian open system dynamics can be used as a probe for environmental properties [3, 4].

##### References

[1] Breuer, H.-P., Laine, E.-M. and Piilo, J. Measure for the degree of non-Markovian behavior of quantum processes in open systems. Phys. Rev. Lett. 103, 210401 (2009).

[2] Wißmann, S., Karlsson, A., Laine, E.-M., Piilo, J. and Breuer, H.-P. Optimal state pairs for non-Markovian quantum dynamics. Phys. Rev. A 86, 062108 (2012).

[3] Haikka, P., McEndoo, S. and Maniscalco, S. Non-Markovian probes in ultracold gases. Phys. Rev. A 87, 012127 (2013).

[4] Borrelli, M., Haikka, P., De Chiara, G. and Maniscalco, S. Non-markovian qubit dynamics induced by coulomb crystals. Phys. Rev. A 88, 010101 (2013).

**July 15 2013**

**18.07.2013 Talk of Giovanni Manfredi in the Kolloquium**

Giovanni Manfredi (Université de Strasbourg) will give a talk with the titel "Antimatter under extreme conditions: electronpositron clusters and positronium condensates" in the physikalisches Kolloquium in C6.3 HS II at 16 ct.

Abstract:

Recent years have witnessed a renewed spur of interest in the physics of

antimatter. For instance, the international collaboration GBAR [1], to which our

Strasbourg group contributes, aims at determining the gravitational acceleration of

anti-hydrogen atoms falling in the gravitational field of the Earth. These

investigations require the production of significant amounts of positrons, followed by

the formation of cold electron-positron plasmas and positronium gases. The

availability of such systems will open the possibility to perform experiments on

antimatter under extreme conditions of temperature and density.

In this talk, I will focus on two such systems: electron-positron clusters and Bose-

Einstein condensates of positronium atoms.

Previous research [2] suggested that electron-positron systems at sufficiently high

densities can display “metallic” properties very similar to those of ordinary metal

clusters (shell structure, magic numbers, etc…). Here, I will show results of a study

of electron-positron clusters using a quantum hydrodynamic model that includes

Coulomb and exchange interactions [3]. A variational method is used to determine

their stationary and dynamical properties, including the nonlinear response regime.

Secondly, I will discuss the possibility to produce a Bose-Einstein condensate of

positronium atoms in a porous silica material containing nanometric cavities [4].

Numerical simulations reveal that the condensation process takes place in around

ten nanoseconds and is thus compatible with the lifetime of ortho-positronium [5].

[1] P. Debu for the GBAR collaboration, Hyperfine Interact. 212, 51 (2012).

[2] A.V. Solov’yov, V.K. Ivanov, and R.G. Polozkov, Eur. Phys. J. D 40, 313 (2006).

[3] G. Manfredi, P.-A. Hervieux, and F. Haas, New J. Phys. 14 (2012) 075012.

[4] D. B. Cassidy, V. E. Meligne and A. P. Mills, Phys. Rev. Lett. 104, 173401 (2010).

[5] O. Morandi, P.-A. Hervieux, G. Manfredi, submitted to PRL.

**July 15 2013**

**17.07.2013 Visit of Pavel Bushev and Sebastian Probst.**

Pavel Bushev and Sebastian Probst visit us and will give a talk in the Ctrl-Q Seminar with the title "Towards a quantum interface between optical an microwave photons".

Abstract:

Future quantum networks will interconnect many quantum systems of diverse physical nature. Photons are ideal carriers of quantum information over long distances because they can be efficiently send through low loss optical fibers. On the other hand, fast and scalable quantum gates can be implemented in solid-state system, e.g. architectures involving superconducting circuits. Thus, interfacing photonic and solid-state qubits within a hybrid quantum architecture offers a promising route towards large scale distributed quantum computing.

Ensembles of optically active spins are promising candidates for realizing such a quantum media converter. Among these, spin ensembles consisting of rare earth (RE) erbium (Er3+) ions doped into a Y2SiO5 crystal matrix play a special role due to the 1.54 μm optical transition of Er3+, which exactly matches the low loss Telecom C-band of optical fiber communication.

We are working on realizing such a coherent quantum convertor. I will present our recent results on strong coherent coupling of superconducting resonators to erbium spins in Y2SiO5 and optical spectroscopy of theses crystals at millikelvin temperatures.

**July 11 2013**

**11.07.2013 Talk of Jean-François Roch 18.07.2013 in the Kolloquium**

More information at the Physics website.

**July 09 2013**

**11.07.2013 Christopher O'Brien is visiting us**

Christopher O'Brien (TU Kaiserslautern) is visiting us.

**July 04 2013**

**04.07.2013 We congratulate Dr. Hessam Habibian!**

We congratulate Dr. Hessam Habibian to his doctor's degree!

**July 03 2013**

**04.07.2013 PhD Defence of Hessam Habibian**

M. Sc. Hessam Habibian will give a colloquium with the title "Cavity Quantum Electrodynamics with Ultracold Atoms" at 10:00 o'clock in Campus E1 3, room 003.

**June 05 2013**

**05.-08.06.2013 Gianluca Giorgi is visiting us **

Gianluca Giorgi (INRIM (Istituto Nazionale di RIcerca Metrologica) in Turin) is visiting us.

**June 04 2013**

**05.06.2013 Talk of Tina Müller in the Ctrl-Q seminar**

Tina Müller, Cambridge University gives a talk in the Ctrl-Q seminar on "Optical signatures of a silicon-based spin in diamond" in room E.04 at 09:00.

**June 02 2013**

**06.06.13 Talk of David DiVincenzo (RWTH Aachen, FZ Juelich, Germany) in the Kolloquium**

David DiVincenzo (RWTH Aachen, FZ Juelich, Germany) will give a talk in the physikalisches Kolloquium in C6.3 HS II at 16 ct. David DiVincenzo is a guest of Frank Wilhelm-Mauch, an abstract can be found here.

**May 28 2013**

**New Article: Phys. Rev. A 87, 050304(R) (2013) **

#### Entangling two defects via a surrounding crystal

T. Fogarty, E. Kajari, B. G. Taketani, A. Wolf, Th. Busch, and Giovanna Morigi

Phys. Rev. A 87, 050304(R) (2013)

We theoretically show how two impurity defects in a crystalline structure can be entangled through coupling with the crystal. We demonstrate this with a harmonic chain of trapped ions in which two ions of a different species are embedded. Entanglement is found for sufficiently cold chains and for a certain class of initial, separable states of the defects. It results from the interplay between localized modes which involve the defects and the interposed ions, it is independent of the chain size, and it decays slowly with the distance between the impurities. These dynamics can be observed in systems exhibiting spatial order; viable realizations are optical lattices, optomechanical systems, or cavity arrays in circuit QED.

**May 08 2013**

**10.05.2013 Talk of Felix Rohde in the Ctrl-Q seminar**

Felix Rohde (PTB Braunschweig) gives a talk in the Ctrl-Q seminar on "Characterization of the tuning properties of single-frequency optical frequency synthesizers" in room E.11 at 09:30.

Abstract:

Precise and agile manipulation of frequency and phase of output

signals of single-frequency optical frequency synthesizers based on a novel

technique for frequency shifting of frequency combs is investigated by

relative measurements between identical synthesizers.

Optical frequency comb generators based on mode locked lasers, common in

precision-laser spectroscopy and metrology, are often referred to as

optical frequency synthesizers (OFS) [1]. They provide a broadband

frequency grid with fixed frequency and phase relations between the

individual comb lines. In analogy to an electrical synthesizer, a

single-frequency OFS provides a single-frequency optical field whose

frequency and phase can be arbitrarily adjusted within a certain tuning

range and resolution while they are related to a reference signal in a

phase-coherent fashion. The most advanced single-frequency OFS approaches

up to date are based on optical frequency combs combined with phase-locking

of a single-frequency "clean-up" laser to an individual comb line [2-4].

These implementations suffer from an ambiguity at those optical

frequencies, where the frequency of the beat-note from a specific comb line

becomes zero or coincides with that from an adjacent comb line, which poses

limitations on the agility of the OFS and in the worst case can lead to a

loss of the phase lock.

In this talk I will present the first characterization of a

single-frequency OFS based on a novel technique for frequency shifting

lines of an optical frequency comb [5]. The method is based on optical

carrier phase shifting between subsequent pulses emitted by the comb

generator, which corresponds to shifting of the carrier frequency. This

approach does not require any intrusion into the comb generator. Using a

single line selector, which selects one of the frequency-shifted comb lines

enables the implementation of a single-frequency OFS with outstanding

degree of agile tunability (10 THz/s), wide optical bandwidth (many THz)

and high resolution while it circumvents the previously mentioned

ambiguities.

I will discuss advantages and drawbacks of different types of single-line

selectors, such as phase locking of clean up lasers, selective

amplification by stimulated Brillouin scattering or feed forward schemes,

with respect to our frequency shifting method. Moreover, we demonstrate the

ability to precisely manipulate the frequency and phase of our

single-frequency OFS by comparison of two independent synthesizers sharing

the same frequency comb generator. Thus, frequency and phase fluctuations

of this ?master oscillator? are ruled out via common-mode rejection.

References

[1] R. Holzwarth, Th. Udem, T.W. Hänsch, J. C. Knight, W. J. Wadsworth, and

P.St.J. Russell, ?Optical Frequency Synthesizer for Precision

Spectroscopy,? Phys. Rev. Lett. 85, 2264-2267 (2000).

[2] J.D. Jost, J.L. Hall, and J. Ye, ?Continuously tunable, precise, single

frequency optical signal generator,? Opt. Expr. 10, 515-520 (2002).

[3] T.R. Schibli, K. Minoshima, F.-L. Hong, H. Inaba, Y. Bitou, A. Onae,

and H. Matsumoto, ?Phase-locked widely tunable optical single-frequency

generator based on a femtosecond comb,? Opt. Lett. 30, 2323-2325 (2005).

[4] V. Ahtee, M. Merimaa, and K. Nyholm, ?Single-frequency synthesis at

telecommunication wavelengths,? Opt. Expr. 17, 4890-4896 (2009).

[5] E. Benkler, F. Rohde, and H. R. Telle, "Endless frequency shifting of

optical frequency comb lines", Opt. Expr. 21, Issue 5, pp. 5793-5802 (2013)

**May 05 2013**

**07.-09.05.2013 visit of Tanja Mehlstäubler**

Tanja Mehlstäubler ( PTB Braunschweig) visits us and gives a talk about "Symmetry Breaking and Topological Defect Formation in Ion Coulomb Crystals" in the Ctrl-Q seminar on Wednesday 08.05.2013, 09.00 in E.04.

Abstract:

K. Pyka1, J. Keller1, H.L. Partner1, R. Nigmatullin2,3, T.

Burgermeister1, D.M. Meier1, K. Kuhlmann1, A. Retzker4, M.B.

Plenio2,3,5, W.H. Zurek6, A. del Campo6,7 & T.E. Mehlstäubler1

Structural defects in ion Coulomb crystals (kinks) have been proposed

for studies of quantum-mechanical effects with solitons and as

carriers of quantum information [1]. Defects form when a symmetry

breaking phase transition is crossed and the finite speed of

information prevents different regions from coordinating the choice of

the symmetry broken state. Where such local choices are incompatible,

defects will form with densities predicted to follow a power law

scaling in the rate of the transition. The importance of this

Kibble-Zurek mechanism (KZM) ranges from cosmology to condensed matter

[2-4]. In previous tests in homogeneous systems, defect formation was

seen, but weak dependence on the transition rate and limited control

of external parameters so far prevented tests of KZM scaling. As

recently predicted [5], in inhomogeneous systems propagation of the

critical front enhances the role of causality and steepens scaling of

defect density with the transition rate. We use ion Coulomb crystals

in a harmonic trap to demonstrate, for the first time, scaling of the

number of topological defects with the transition rate ? the central

prediction of KZM - in a well-controlled environment [6]. We will

detail on kink dynamics and stability in our system.

[1] Landa et al., Phys. Rev. Lett. 104, 043004 (2010)

[2] Kibble, T. W. B. Some implications of a cosmological phase

transition. Phys. Rep. 67, 183-199 (1980).

[3] Zurek, W. H. Cosmological experiments in superfluid helium? Nature

317, 505 (1985).

[4] Monaco, R., Mygind, J., Rivers, R. J., Koshelets, V. P.

Spontaneous fluxoid formation in superconducting

loops. Phys. Rev. B 80, 180501(R) (2009).

[5] del Campo, A., De Chiara, G., Morigi, G., Plenio, M. B., Retzker,

A. Structural Defects in Ion Chains by

Quenching the External Potential: The Inhomogeneous

Kibble-Zurek Mechanism. Phys. Rev. Lett. 105,

075701 (2010).

[6] Pyka et al., arXiv:1211.7005 (2012)

**April 29 2013**

Thomas Udem (Max-Planck-Institut für Quantenoptik, Garching, Germany) will give a talk in the physikalisches Kolloquium in C6.3 HS II at 16 ct. The titel of his talk is "Calibrating Astronomical Data with a Laser Frequency Comb".

Abstract:

The past years have seen the birth of precision spectroscopy in astrophysics. Today,

the best spectrographs are limited in stability by their calibration light source. A laser

frequency comb (LFC) would be an ideal calibrator for this purpose. It generates a

spectrum of lines that are equidistantly spaced in frequency with an accuracy given

by the atomic clock that controls the LFC. The main challenge is to cover a large

optical bandwidth with a line spacing that is adapted to the spectrographs resolution.

The short term repeatability demonstrated so far corresponds to a Doppler shift of

2.5cm/s. With this applications such as the detection of Earth-like planets or even

measuring the cosmic acceleration in real time come into reach.

**April 22 2013**

**New article: Phys. Rev. A 87, 044301 (2013)**

Entanglement detection by Bragg scattering

C. Macchiavello and G. Morigi

Phys. Rev. A 87, 044301 (2013) .

We show how to measure the structural witnesses proposed previously [P. Krammer et al., Phys. Rev. Lett. 103 100502 (2009)] for detecting entanglement in a spin chain using photon scattering. The procedure allows one to measure the two-point correlation function of the spin array. This proposal could be performed in existing experimental platforms, realizing ion chains in Paul traps or atomic arrays in optical lattices.

**April 21 2013**

PhD Day of the Universität des Saarlandes.

**April 19 2013**

**21.-24.4.2013 Tracy Northup (Innsbruck) visits us**

Tracy Northup (Innsbruck) will give a talk on Wednesday 24.04 at 09:00 about "An ion-cavity quantum interface".

Abstract:

In order to link remote ion-trap systems in a quantum network, we

require a coherent interface between ions and photons. In our

experimental work, an optical cavity provides this interface. After

describing how we implement the ion-cavity interaction, I will present

the mapping of a quantum state from an ion to a photon. A second

result, entanglement between an ion and a cavity photon, then provides

the groundwork for a demonstration of heralded ion-ion entanglement.

Finally, I discuss routes towards the cavity-based simulation of quantum

field theories.

**April 14 2013**

**18.04.13 Talk of Johanna Stachel (Universität Heidelberg , Germany) in the Kolloquium**

Johanna Stachel (Universität Heidelberg , Germany) will give a talk in the physikalisches Kolloquium in C6.3 HS II at 16 ct. Johanna Stachel is a guest of Karin Jacobs, an abstract can be found here.

**March 28 2013**

**New article: Phys. Rev. A 87, 032116 (2013) **

Quantum quenches of ion Coulomb crystals across structural instabilities II: thermal effects

Jens D. Baltrusch, Cecilia Cormick, Giovanna Morigi

Phys. Rev. A 87, 032116 (2013) .

We theoretically analyze the efficiency of a protocol for creating mesoscopic superpositions of ion chains, described in Baltrusch et al. [ Phys. Rev. A 84 063821 (2011)], as a function of the temperature of the crystal. The protocol makes use of state-dependent forces, so that a coherent superposition of the electronic states of one ion evolves into an entangled state between the chain's internal and external degrees of freedom. Ion Coulomb crystals are well isolated from the external environment and should therefore experience a coherent, unitary evolution, which follows the quench and generates structural Schrödinger-cat-like states. The temperature of the chain, however, introduces a statistical uncertainty in the final state. We characterize the quantum state of the crystal by means of the visibility of Ramsey interferometry performed on one ion of the chain and determine its decay as a function of the crystal's initial temperature. This analysis allows one to determine the conditions on the chain's initial state in order to efficiently perform the protocol.

**March 25 2013**

**25.-27.3.2013 visit of Pietro Silvi**

Pietro Silvi (Uni Ulm) is visiting us.

**March 25 2013**

**24.-27.3.2013 visit of Shmuel Fishman**

Shmuel Fishman (Technion, Haifa, Israel) is visiting us.

**February 28 2013**

**06.-08.03.2013 visit of Monika Schleier-Smith **

Monika Schleier-Smith (Max Planck Institute of Quantum Optics,Ludwig-Maximilians-Universität, Munich) is visiting us and will give a talk on Thursday 07.03.2013 in the Ctrl-Q meeting in room E.04 at 10:30.

She will talk about "Engineering Entanglement from the Top Down: Cavity Spin Squeezing and Beyond"

The collective interaction of many atoms with a single mode of light--as realized in an optical cavity--constitutes a powerful tool for engineering many-body entangled states. I will describe experiments in which such an interaction produced squeezed spin states of an atomic ensemble, permitting the first demonstration of a quantum enhancement in the stability of an atomic clock. I will also touch on future prospects in cavity-based quantum-state control and detection, ranging from interfacing spin and motional degrees of freedom to probing entanglement near a quantum critical point.

**February 27 2013**

**New article: Phys. Rev. Lett. 110, 093602 (2013)**

Interferometry with Bose-Einstein Condensates in Microgravity

H. Müntinga, H. Ahlers, M. Krutzik, A. Wenzlawski, S. Arnold, D. Becker, K. Bongs, H. Dittus, H. Duncker, N. Gaaloul, C. Gherasim, E. Giese, C. Grzeschik, T. W. Hänsch, O. Hellmig, W. Herr, S. Herrmann, E. Kajari, S. Kleinert, C. Lämmerzahl, W. Lewoczko-Adamczyk, J. Malcolm, N. Meyer, R. Nolte, A. Peters, M. Popp, J. Reichel, A. Roura, J. Rudolph, M. Schiemangk, M. Schneider, S. T. Seidel, K. Sengstock, V. Tamma, T. Valenzuela, A. Vogel, R. Walser, T. Wendrich, P. Windpassinger, W. Zeller, T. van Zoest, W. Ertmer, W. P. Schleich, and E. M. Rasel

Phys. Rev. Lett. 110, 093602 (2013).

Atom interferometers covering macroscopic domains of space-time are a spectacular manifestation of the wave nature of matter. Because of their unique coherence properties, Bose-Einstein condensates are ideal sources for an atom interferometer in extended free fall. In this Letter we report on the realization of an asymmetric Mach-Zehnder interferometer operated with a Bose-Einstein condensate in microgravity. The resulting interference pattern is similar to the one in the far field of a double slit and shows a linear scaling with the time the wave packets expand. We employ delta-kick cooling in order to enhance the signal and extend our atom interferometer. Our experiments demonstrate the high potential of interferometers operated with quantum gases for probing the fundamental concepts of quantum mechanics and general relativity.

**February 13 2013**

**New article: Phys. Rev. Lett. 110, 075304 (2013)**

Bose-Glass Phases of Ultracold Atoms due to Cavity Backaction

Hessam Habibian, André Winter, Simone Paganelli, Heiko Rieger, Giovanna Morigi

Phys. Rev. Lett. 110, 075304 (2013).

We determine the quantum ground-state properties of ultracold bosonic atoms interacting with the mode of a high-finesse resonator. The atoms are confined by an external optical lattice, whose period is incommensurate with the cavity mode wavelength, and are driven by a transverse laser, which is resonant with the cavity mode. While for pointlike atoms photon scattering into the cavity is suppressed, for sufficiently strong lasers quantum fluctuations can support the buildup of an intracavity field, which in turn amplifies quantum fluctuations. The dynamics is described by a Bose-Hubbard model where the coefficients due to the cavity field depend on the atomic density at all lattice sites. Quantum Monte Carlo simulations and mean-field calculations show that, for large parameter regions, cavity backaction forces the atoms into clusters with a checkerboard density distribution. Here, the ground state lacks superfluidity and possesses finite compressibility, typical of a Bose glass. This system constitutes a novel setting where quantum fluctuations give rise to effects usually associated with disorder.

**February 07 2013**

**11.2-15.2.2013: visit of Haggai Landa (Tel Aviv University)**

Haggai Landa (Tel Aviv University) will visit us. He will give a talk in the Ctrl-Q Seminar on thursday 14. February at 09.30.

**February 06 2013**

**08.02.2013 Giulia Gualdi from Kassel is visiting us**

Giulia Gualdi (Kassel, Koch group) is visiting us and will give a talk about "renormalization approach to open quantum system dynamics" on 08.02.2013 11.00 in room E.12.

**February 05 2013**

**New article: Phys. Rev. A 87, 013823 (2013).**

Experimental investigation of the transition between Autler-Townes splitting and electromagnetically-induced-transparency models

L. Giner, L. Veissier, B. Sparkes, A. S. Sheremet, A. Nicolas, O. S. Mishina, M. Scherman, S. Burks, I. Shomroni, D. V. Kupriyanov, P. K. Lam, E. Giacobino, J. Laurat

Phys. Rev. A 87, 013425 (2013).

Two phenomena can affect the transmission of a probe field through an absorbing medium in the presence of an additional field: electromagnetically induced transparency (EIT) and Autler-Townes splitting (ATS). Being able to discriminate between the two is important for various practical applications. Here we present an experimental investigation into a method that allows for such a disambiguation as proposed by Anisimov, Dowling, and Sanders in Phys. Rev. Lett. 107 163604 (2011). We apply the proposed test based on Akaike's information criterion to a coherently driven ensemble of cold cesium atoms and find a good agreement with theoretical predictions, therefore demonstrating the suitability of the method. Beyond the applicability of the test, our results demonstrate that the transition features are highly sensitive to the properties of the medium under study, potentially providing a practical characterizing tool for complex systems.

**January 27 2013**

**31.01.2013 Talk of Malte Henkel (Université de Lorraine, Nancy, France) in the kolloquium**

Malte Henkel (Université de Lorraine, Nancy, France) will give a talk in the physikalisches Kolloquium on "Physikalisches Altern" in C6.3 HS II at 16 ct. Malte Henkel is a guest of Christian Wagner, an abstract can be found here

**January 25 2013**

**New article: Phys. Rev. A 87, 013425 (2013).**

Stability and dynamics of ion rings in linear multipole traps

Florian Cartarius, Cecilia Cormick, Giovanna Morigi

Phys. Rev. A 87, 013425 (2013).

Trapped singly charged ions can crystallize as a result of laser cooling. The emerging structure depends on the number of particles and on the geometry of the trapping potential. In linear multipole radio frequency traps, the geometry of the radial potential can lead to the formation of single-ring structures. We analyze the conditions and stability of single rings as a function of the number of poles. The rings form tubes for a large number of ions and sufficiently small trap aspect ratios. In these structures the arrangement of the ions corresponds to a triangular lattice folded onto a cylinder. The stability of the tubular structures is numerically studied for different lattice constants and their normal-mode spectrum is determined.

**January 25 2013**

**New article: Phys. Rev. A 87, 013829 (2013).**

Ion chains in high-finesse cavities

Cecilia Cormick and Giovanna Morigi

Phys. Rev. A 87, 013829 (2013)..

We analyze the dynamics of a chain of singly charged ions confined in a linear Paul trap and which couple with the mode of a high-finesse optical resonator. In these settings the ions interact via the Coulomb repulsion and are subject to the mechanical forces due to scattering of cavity photons. We show that the interplay of these interactions can give rise to bistable equilibrium configurations, into which the chain can be cooled by cavity-enhanced photon scattering. We characterize the resulting equilibrium structures by determining the stationary state in the semiclassical limit for both cavity field and crystal motion. The mean occupation of the vibrational modes at steady state is evaluated, showing that the vibrational modes coupled to the cavity can be simultaneously cooled to low occupation numbers. It is also found that at steady state the vibrations are entangled with the cavity field fluctuations. The entanglement is quantified by means of the logarithmic negativity. The spectrum of the light at the cavity output is evaluated and the features signaling entanglement are identified.

**January 16 2013**

**16.01.2013 Heute ist Welt-Nichts-Tag**

Heute ist Welt-Nichts-Tag, dazu hat die Saarbrücker Zeitung ein Interview mit Dr. Marc Bienert über das Nichts veröffentlicht. Das vollständige Interview finden Sie hier.

**January 07 2013**

**10.01.13 Talk of Alexander Brinkman (MESA, University of Twente, Netherlands) in the Kolloquium**

Alexander Brinkman (MESA, University of Twente, Netherlands) will give a talk in the physikalisches Kolloquium on "Topological insulator - superconductor hybrids Living on the edge" in C6.3 HS II at 16 ct. Alexander Brinkman is a guest of Frank Wilhelm-Mauch, an abstract can be found here

**December 17 2012**

** 20.12.12 Talk of Daniel Esteve (Quantronics, CEA-Saclay)**

Daniel Esteve (Quantronics, CEA-Saclay) will give a talk in the physikalisches Kolloquium on "Quantum physics with electrical circuit" in C6.3 HS II at 16 ct. Daniel Esteve is a guest of Frank Wilhelm-Mauch, an abstract can be found here

**December 10 2012**

**13.12.2012 Talk of Ralf Röhlsberger (DESY Hamburg) in the Kolloquium**

Ralf Röhlsberger (DESY Hamburg) will give a talk in the physikalisches Kolloquium on "Quantum optics with atomic nuclei: How to make iron transparent" in C6.3 HS II at 16 ct. Ralf Röhlsberger is a guest of Jürgen Eschner, an abstract can be found here

**December 03 2012**

**06.12.2012 Talk of Jürgen Eschner in the Kolloquium**

Jürgen Eschner will give a talk about the work of this years Nobel prize winners. The talk with the title “Dompteure der Quanten – Die Physik des Nobelpreises 2012“ will be given at 16.00c.t. in C6.3 HS II. Further information can be found here.

**November 29 2012**

### Melanie Rolles

has been awarded the "Excellence Prize" of the "Deutsch-Französischen Hochschule" for her Master Thesis. Congratulations!

For more information please have a look at the DFH-UFA website.

**November 06 2012**

**New article:Phys. Rev. A 86, 053402 (2012).**

Cooling the motion of a trapped atom with a cavity field

Marc Bienert and Giovanna Morigi

Phys. Rev. A 86, 053402 (2012).

We theoretically analyze the cooling dynamics of an atom which is tightly trapped inside a high-finesse optical resonator. Cooling is achieved by suitably tailored scattering processes, in which the atomic dipole transition either scatters a cavity photon into the electromagnetic field external to the resonator, or performs a stimulated emission into the cavity mode, which then dissipates via the cavity mirrors. We identify the parameter regimes in which the atom center-of-mass motion can be cooled into the ground state of the external trap. We predict that, in particular, for high cooperativities, interference effects mediated by the atomic transition may lead to higher efficiencies. The dynamics is compared with the cooling dynamics of a trapped atom inside a resonator studied by Zippilli and Morigi [ Phys. Rev. Lett. 95 143001 (2005)] where the atom, instead of the cavity, is driven by a laser field.

**October 31 2012**

**27.11.2012 Talk of Maxim Efremov (Ulm) on "Novel resonant states in three-body problem."**

Maxim Efremov from the group of Prof. Schleich will give a talk on ” Novel resonant states in three-body problem” in our group seminar in room 4.18 at 10:30 on 27.11.2012.

Abstract:

We consider the bound states of the three-body system consisting of a light particle and two heavy ones when the heavy-light short-range interaction potential has a resonance corresponding to the non-zero orbital moment. Within the Born-Oppenheimer approach we suggest the novel method to find the effective potential between the heavy particles by self-consistent scattering of a light particle by two heavy ones. In the case of the exact resonance in the p-wave scattering the effective potential is shown to be attractive and long-range, namely decreases as the third power of inter-atomic distance. Moreover, the range and power of the potential, as well as the number of the bound states are determined mainly by the mass ratio of the heavy and light particles and the parameters of the heavy-light short-range potential.

**October 31 2012**

**21.-22.11.2012 Sabrina Maniscalco (Edinburgh) visits us**

21.-22.11.2012 Sabrina Maniscalco (Edinburgh) visits us and will give a talk in the Ctrl-Q Seminar on Wednesday 21.11.2012 at 09.15 in E.04. Further information can be found here.

TITLE:

Non-Markovianity as a quantum probe for complex systems and quantum simulators

ABSTRACT:

Quantum probes interacting with complex system can be used, under

certain conditions, to extract information on the system they are

interacting with, ideally in a nondestructive way.

In an open quantum system approach, the interaction of the qubit probe

with the complex environment gives rise to decoherence phenomena

affecting the qubit dynamics. Due to the nontrivial structure of the

environment, the qubit time evolution is usually characterised by

partial information backflow, i.e. non-Markovianity [1]. It has been

shown that non-Markovianity of a qubit probe is a useful

characterisation of the complex system dynamics in the context of

ultracold gases [2].

In the first part of the talk I will introduce a non-Markovianity

measure and, focussing on purely dephasing environments, I will

present a necessary and sufficient condition linking information

backflow with the form of the spectrum of the environment [3].

I will then use this result in the second part of the talk to discuss

two examples of non-Markovianity as a quantum probe. In the case of an

Ising model in a transverse filed coupled to a central spin

(Heppman-Coleman model), I will show that the non-Markovianity measure

vanishes exactly only at criticality, for any value of the number of

spins in the chain [3]. For the case of an ion crystal undergoing a

second order phase transition from a linear to a zigzag structure, I

will show that the non-Markovianity measure decreases monotonically as

we move towards the critical value of the trap frequency.

[1] J. Piilo, S. Maniscalco, K. Härkönen, and K. -A. Suominen, Phys.

Rev. Lett. 100, 180402 (2008); H.-P. Breuer, E.-M. Laine, and J.

Piilo, Phys. Rev. Lett. 103, 210401 (2009).

[2] P. Haikka, S. McEndoo, G. De Chiara, G. M. Palma and S.

Maniscalco, Phys. Rev. A 84, 031602 (R) (2011).

[3] P. Haikka, T. H. Johnson, and S. Maniscalco, ?Time Invariant

Discord and Non-Markovianity? arXiv:1203.6469v2.

[4] P. Haikka, J. Goold, S. McEndoo, F. Plastina, and S. Maniscalco,

Phys. Rev. A 85, 060101 (R) (2012).

**October 31 2012**

**15.11.2012 Talk of Peter Schmelcher (Uni Hamburg) in the Kolloquium**

Peter Schmelcher (Uni Hamburg) will give a talk in the physikalisches Kolloquium on "Mesoscopic physics with ultracold atoms: from correlated tunneling to confinement-induced transparency" in C6.3 HS II at 16 ct.

**October 30 2012**

**13.11.2012 Talk of Herwig Ott (Kaiserslautern) in the Ctrl-Q Seminar**

Prof. Herwig Ott from the University of Kaiserslautern will give a talk in the Ctrl-Q Seminar. The talk is at 14.00 in Sr. 3 in the maths building (U.11). Prof. Ott is talking about "Dissipation in Ultracold Quantum Gases".

**October 30 2012**

**08.11.2012 Talk of Selim Jochim (Uni Heidelberg) in the Kolloquium**

Selim Jochim (Uni Heidelberg) will give a talk in the physikalisches Kolloquium on "One, two, many: From few- to many body phsics with ultracold atoms" in C6.3 HS II at 16 ct. Selim Jochim is a guest of Jürgen Eschner, an abstract can be found here

**October 30 2012**

**05.11.2012 Talk of Peter Young (Uni California at Santa Cruz) in the Graduiertenkolleg 1276**

Peter Young will give a talk in the Seminar of the Graduiertenkolleg for Strukturbildung und Transport in komplexen Systemen on Monday, 05.11.2012, 11.00 c.t. in E.04 about -"Mind the gap", solving optimization problems on quantum computers-. Peter Young is a guest of Heiko Rieger, the abstract can be found here.

**October 22 2012**

**23.10.2012 Talk of Sandro Wimberger (Heidelberg) on "Many body open system dynamics"**

Sandro Wimberger will be visiting our group on October 23rd. He will give a talk about "Many body open system dynamics" in 4.18 at 09:30.

**October 15 2012**

**Pietro Silvi visited us from October 17th to 19th**

Pietro Silvi will be visiting our group from October, 17th to October 19th. On Friday 19th he will give a talk in E.12 at 10:00.

**October 09 2012**

**Congratulations! Serge Haroche and David Wineland win Nobel Prize in physics.**

Serge Haroche and David Wineland won the Nobel Prize in Physics 2012 for their "for ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems". For more information please have a look at the Nobel Prize webpage.

**September 07 2012**

**New article:Phys. Rev. A 86, 032104 (2012).**

Quantum quenches of ion Coulomb crystals across structural instabilities

Jens D. Baltrusch, Cecilia Cormick and Giovanna Morigi

Phys. Rev. A 86, 032104 (2012).

Quenches in an ion chain can create coherent superpositions of motional states across the linear-zigzag structural transition. The procedure has been described in Baltrusch et al. [ Phys. Rev. A 84 063821 (2011)] and makes use of spin-dependent forces, so that a coherent superposition of the electronic states of one ion evolves into an entangled state between the chain's internal and external degrees of freedom. The properties of the crystalline state so generated are theoretically studied by means of Ramsey interferometry on one ion of the chain. An analytical expression for the visibility of the interferometric measurement is obtained for a chain of an arbitrary number of ions and as a function of the time elapsed after the quench. Sufficiently close to the linear-zigzag instability the visibility decays very fast, but exhibits revivals at the period of oscillation of the mode that drives the structural instability. These revivals have a periodicity that is independent of the crystal size, and they signal the creation of entanglement by the quantum quench.

**August 03 2012**

**New article: Phys. Rev. Lett. 109, 053003 (2012)**

Structural Transitions of Ion Strings in Quantum Potentials

Cecilia Cormick and Giovanna Morigi

Phys. Rev. Lett. 109, 053003 (2012).

We analyze the stability and dynamics of an ion chain confined inside a high-finesse optical resonator. When the dipolar transition of the ions strongly couples to one cavity mode, the mechanical effects of light modify the chain properties close to a structural transition. We focus on the linear chain close to the zigzag instability and show that linear and zigzag arrays are bistable for certain strengths of the laser pumping the cavity. For these regimes the chain is cooled into one of the configurations by cavity-enhanced photon scattering. The excitations of these structures mix photonic and vibrational fluctuations, which can be entangled at steady state. These features are signaled by Fano-like resonances in the spectrum of light at the cavity output.

**August 03 2012**

"How to publish your work in the Physical Review".

Dr. Thomas Pattard (APS Editorial Office, Ridge, NY)

August, Friday 3rd at 11:00. Room: E04.

Abstract:

When you try to publish your work in a peer-reviewed journal such as the Physical Review, it
often appears that authors, referees and editors all speak different languages and pursue
different goals (a "me versus them" feeling). Indeed, the pressure to "publish or perish"
in the modern academic world sometimes seems incompatible with the idea of conveying
actual and useful information to the reader of a journal, who wants to stay up-to-date with
current developments in her/his field of interest. On the other hand, a significant part of
this feeling is rooted in miscommunication that could easily be avoided by a better mutual
understanding of the needs of authors, referees, editors and readers.

In this talk, I will try to give some insight into the publishing process from the
perspective of an editor. I will give a short overview of APS publishing, and the workflow
associated with the handling of a typical manuscript. This will lead us to the question of
what we are looking for in a paper / referee report / cover letter, and how the
communication between all involved parties can be improved. Using examples from actual
communication we receive, I will try to give a few hints that might actually make you a
better author and/or referee. In the last few minutes of the talk, I will briefly discuss
various recent APS initiatives.

**July 25 2012**

**2nd Internal PICC Review Meeting**

The 2nd Internal Scientific Review Meeting takes place at July 25 in Saarbrücken. For more information see the PICC website.

**July 12 2012**

**Physics colloquium: Klaas Bergmann - July 12th**

"Optisch induzierter Besetzungstransfer zwischen Quantenzuständen - nahezu perfekt"

Klaas Bergmann, TU Kaiserslautern.

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**July 03 2012**

**Tobias Kampschulte will visit us**

Dr. Tobias Kampschulte (Institut für Angewandte Physik, Universität Bonn) will be visiting our group from July, 9th to July, 11th. On Wednesday 11th he will give a talk on "EIT-control of single-atom motion in an optical cavity" (at 9:15, room E.04).

**July 03 2012**

**Wolfgang Niedenzu is visiting us**

Wolfgang Niedenzu (Innsbruck University) is visiting us during the month of July.

**June 27 2012**

**27.6.2012: Talk of Stephan Ritter (MPQ)**

Stephan Ritter, from the Max-Planck-Institute of Quantum Optics, gives a talk on "An elementary quantum network of single atoms in optical cavities" The talk takes place on Wednesday, June 27th, in our Ctrl-Q seminar (at 9:15 in Room E.04).

**June 25 2012**

**Meeting with Fleischhauer's group**

On June, Monday 25th the research group of Prof. Michael Fleischhauer (Kaiserslautern University) will be visiting us. The program of the meeting is:

Talks:

Location: E2.6 Room E04

10:10 - 10:50 Susanne Blum: "Frequency down-conversion of single photons"

10:55 - 11:35 Fabian Grusdt: "Fractional Quantum Hall Physics with Rydberg Dark State Polaritons"

11:35 - 11:50 Coffee break

11:50 - 12:30 Jens Baltrusch: "Probing Quantum Many-Body States of Ion Coulomb Crystals by Ramsey
Interferometry"

12:30 - 13:30 Lunch break

13:30 - 14:10 Michael Höning: "Steady-state crystallization of Rydberg excitations in optically driven lattice gases"

14:15 - 14:55 Matthias Moos: "Criticality in nonequilibrium steady states of 1D lattice systems"

14:55 - 15:10 Coffee break

15:10 - 15:50 Florian Cartarius: "Crystalline structures in multipole traps"

16:00 Lab tour to the laboratories of AG Eschner and AG Becher

**June 21 2012**

PhD students' Day 2012 will take place on June, 28th and 29th. More information can be found in the webpage: http://jacobs.physik.uni-saarland.de/phd2012/

**June 20 2012**

**Simone Montangero will visit us**

Simone Montangero (Ulm University) will be visiting us on June 20h and will give a talk at 9:00am, on "Control of correlated quantum dynamics"

Abstract:

"Recent developments in the field of optimal control of many body quantum systems are reviewed. We introduce the concept of complexity of the optimization task and we present different theoretical and experimental applications of optimal control to correlated quantum systems dynamics as the optimal production of stable and robust-against-noise entangled states."

**June 12 2012**

**Physics colloquia: Astakhov, Brenner, Bushev**

Georgy Astakhov (Universität Würzburg), "Optical control of silicon vacancy qubits in silicon carbide".

June 13th, 10:15 h.

Thomas Brenner (MPI Erlangen), "Photonen losschicken und einfangen - Organische Optoelektronik aus der Nanoskala".

June 13th, 14:15 h.

Pavel Bushev (KIT Karlsruhe), "Interconnecting of solid-state qubits over long distances".

June 14th, 10:15 h.

All three talks will take place in building D2.2, HS A.

Complete information (including abstracts for these and future talks) can be found in the Physics Colloquium Schedule.

**June 09 2012**

Prof. Geza Giedke (Max-Planck-Institut für Quantenoptik and TU München, Garching, Germany) will be visiting us on June 14th and 15th and will give a talk on Friday 15th at 10 am, on "Spin Physics in Open and Driven Quantum Dots".

Abstract: The electron spin in a quantum dot interacts with a large number of nuclear spins, realizing a central spin system. The fast control of the central electronic spin and the long nuclear spin lifetimes suggests the use as a versatile quantum register and enables the study of many-body quantum effects. We discuss some QIP applications such as dissipative state preparation, but focus on the steady state properties of the driven central spin system. We show that it features first and second order dissipative phase transitions and present methods to analyze them.

**May 24 2012**

**New article: Phys. Rev. A 85, 053635**

Homodyne detection of matter-wave fields

Stefan Rist and Giovanna Morigi

Phys. Rev. A 85, 053635 (2012).

A scheme is discussed that allows for performing homodyne detection of the matter-wave field of ultracold bosonic atoms. It is based on a pump-probe laser setup, which both illuminates a Bose-Einstein condensate, acting as reference system, and a second ultracold gas, composed by the same atoms but in a quantum phase to determine. Photon scattering outcouples atoms from both systems, which then propagate freely. Under appropriate conditions, when the same photon can either be scattered by the Bose-Einstein condensate or by the other quantum gas, both flux of outcoupled atoms and scattered photons exhibit oscillations, the amplitude of which is proportional to the condensate fraction of the quantum gas. The setup can be extended to measure the first-order correlation function of a quantum gas. The dynamics discussed here makes use of the entanglement between atoms and photons, which is established by the scattering process in order to access detailed information on the quantum state of matter.

**May 18 2012**

Prof. Thomas H. Seligman (Instituto de Ciencias Físicas, UNAM) will be visiting us on May, Monday 21st.

**April 23 2012**

**Physics colloquium: Jörg Schmiedmayer - May 3rd**

"Relaxation dynamics and pre-thermalization in an isolated quantum system"

Jörg Schmiedmayer, Universität Vienna.

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**April 17 2012**

**New article: Phys. Rev. A 85, 042318**

Statistical mechanics of entanglement mediated by a thermal reservoir

Endre Kajari, Alexander Wolf, Eric Lutz and Giovanna Morigi

Phys. Rev. A 85, 042318 (2012).

Two defect particles that couple to a harmonic chain, acting as common reservoir, can become entangled even when the two defects do not directly interact and the harmonic chain is effectively a thermal reservoir for each individual defect. This dynamics is encountered for sufficiently low temperatures of the chain and depends on the initial state of the two oscillators. In particular, when each defect is prepared in a squeezed state, entanglement can be found at time scales at which the steady state of a single defect is reached. We provide a microscopic description of the coupled quantum dynamics of chain and defects. By means of numerical simulations, we explore the parameter regimes for which entanglement is found under the specific assumption that both particles couple to the same ion of the chain. This model provides the microscopic setting where bath-induced entanglement can be observed.

**April 17 2012**

Igor Lesanovsky (University of Nottingham) will be visiting us from April 23th to 25th, and on Tuesday 24th he will give a talk on "Many-body physics with Rydberg atoms" (10 am, in room E.11).

**April 02 2012**

**Physics colloquium: Anna Minguzzi - April 19th**

"Strongly correlated one dimensional quantum gases"

Anna Minguzzi, Universität Grenoble.

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**March 21 2012**

Dr. Johannes Otterbach (Harvard University) is visiting us and will give a talk on Quantum interfaces for the Quantum Journal Club on Wednesday, March 21 at 9:15 in E.04.

**March 19 2012**

Dr. Pietro Silvi (Ulm University) is visiting us from March 19 (Monday) until 23 (Friday). On Friday he will give a talk on ``Tree Tensor Network Ansatz States: efficiency, criticality, and boundary phenomena.'' (11.00h, room 4.18).

**February 28 2012**

**Talk by Shmuel Fishman on transport in random potentials**

Thursday March 1st, 14:00, room 4.18.

Transport in potentials random in space and time: From Anderson
localization to super-ballistic motion

Shmuel Fishman

Technion, Haifa (Israel)

**February 02 2012**

**New article: New J. Phys. 14, 023002**

Cavity cooling of a trapped atom using electromagnetically induced transparency

Marc Bienert and Giovanna Morigi

New J. Phys. 14, 023002 (2012).

A cooling scheme for trapped atoms is proposed, which combines cavity-enhanced scattering and electromagnetically induced transparency. The cooling dynamics exploits a three-photon resonance, which combines laser and cavity excitations. It is shown that relatively fast ground-state cooling can be achieved in the Lamb–Dicke regime and for large cooperativity. Efficient ground-state cooling is found for parameters of ongoing experiments.

**January 16 2012**

On March 1st, Shmuel Fishman (Technion, Haifa, Israel) will come to visit us until Friday, March 2nd. On March 1st he will give a talk on:

``Transport in potentials random in space and time: From Anderson localization to super-ballistic motion''.

**December 08 2011**

**New article: Phys. Rev. A 84, 063821**

Quantum superpositions of crystalline structures

Jens D. Baltrusch, Cecilia Cormick, Gabriele De Chiara, Tommaso Calarco and Giovanna Morigi

Phys. Rev. A 84, 063821 (2011).

See accompanying Physics Synopsis

A procedure is discussed for creating coherent superpositions of motional states of ion strings. The motional states are across the structural transition linear-zigzag, and their coherent superposition is achieved by means of spin-dependent forces, such that a coherent superposition of the electronic states of one ion evolves into an entangled state between the chain's internal and external degrees of freedom. It is shown that the creation of such an entangled state can be revealed by performing Ramsey interferometry with one ion of the chain.

**December 08 2011**

**New article: J. Phys.: Conf. Ser. 327, 012050**

The Space Atom Interferometer project: status and prospects

F. Sorrentino, K. Bongs, P. Bouyer, L. Cacciapuoti, M. de Angelis, H. Dittus, W. Ertmer, J. Hartwig, M. Hauth, S. Herrmann, K. Huang, M. Inguscio, E. Kajari, T. Könemann, C.Lämmerzahl, A. Landragin, G. Modugno, F. Pereira dos Santos, A. Peters, M. Prevedelli, E. M. Rasel, W. P. Schleich, M. Schmidt, A. Senger, K. Sengstock, G. Stern, G. M. Tino, T. Valenzuela, R. Walser, and P. Windpassinger,

J. Phys.: Conf. Ser. 327, 012050 (2011).

This paper presents the current status and future prospects of the Space Atom Interferometer project (SAI), funded by the European Space Agency. Atom interferometry provides extremely sensitive and accurate tools for the measurement of inertial forces. Operation of atom interferometers in microgravity is expected to enhance the performance of such sensors. Main goal of SAI is to demonstrate the possibility of placing atom interferometers in space. The resulting drop-tower compatible atom interferometry acceleration sensor prototype is described. Expected performance limits and potential scientific applications in a micro-gravity environment are also discussed.

**December 06 2011**

**Physics colloquium: Dieter Meschede - December 15th**

"Bottom up to Discrete Quantum Simulation with Neutral Atoms"

Dieter Meschede, Universität Bonn.

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**December 02 2011**

Dr. Stefan Rist (Scuola Normale Superiore, Pisa) will be visiting our group from December 13th to 16th. On Wednesday 14th December at 9:15 in Room E.04, he will give a talk for the Quantum Journal Club. Title: "ARPES spectra of massless Dirac fermions on the verge of exciton condensation (an introduction)"

**November 28 2011**

**Stefano Zippilli is visiting us**

Dr. Stefano Zippilli (University of Salerno) is visiting us from November 28th until December 2nd.

**October 24 2011**

**Physics colloquium: Silke Ospelkaus - November 3rd**

"Ultrakalte polare Moleküle"

Silke Ospelkaus, Universität Hannover.

Exceptionally, in the context of the anual Deutsche Physikerinnen Tagung, the talk takes place in Hörsaal (AudiMO) Bioinformatik, building E 2.2, at 17 h.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**October 18 2011**

**Physics colloquium: Piet Schmidt - October 27th**

"Vom Quantencomputer zur optischen Uhr"

Piet Schmidt, PTB Braunschweig and Leibniz-Universität Hannover

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**October 17 2011**

**Lecture: Conformal Field Theory by Prof. Henkel**

Tuesday 18th October starts the lecture on "Conformal Field Theory", held by Prof. Malte Henkel of Nancy University. The course takes place on

Tuesdays, from 15h30 till 16h30, Room E11.

The language of the course will be decided (between english and german) on the first day of the course, depending on the attendees.

**October 07 2011**

**Physics colloquium: Andreas Buchleitner - October 20th**

"Transport, disorder, and entanglement"

Andreas Buchleitner, Universität Freiburg

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**September 15 2011**

**New article: Phys. Rev. A 84, 033829**

Quantum light by atomic arrays in optical resonators

Hessam Habibian, Stefano Zippilli, and Giovanna Morigi.

Phys. Rev. A 84, 033829 (2011).

Light scattering by a periodic atomic array is studied when the atoms couple with the mode of a high-finesse optical resonator and are driven by a laser. When the von-Laue condition is not satisfied, there is no coherent emission into the cavity mode, and the latter is pumped via inelastic scattering processes. We consider this situation and identify conditions for which different nonlinear optical processes can occur. We show that these processes can be controlled by suitably tuning the strength of laser and cavity coupling, the angle between laser and cavity axis, and the array periodicity. We characterize the coherence properties of the light when the system can either operate as degenerate parametric amplifier or as a source of antibunched light. Our study permits us to identify the individual multiphoton components of the nonlinear optical response of the atomic array and the corresponding parameter regimes, thereby in principle allowing one to control the nonlinear optical response of the medium.

**September 13 2011**

**Power cut programmed for the end of September**

From Friday, September 23th in the morning until Monday, September 26th at 4am there will be no electric supply in the university, and access to the building will not be permitted.

**September 13 2011**

**New article: Europhys. Lett. 95, 60008**

Entangling two distant oscillators with a quantum reservoir

A. Wolf, G. De Chiara, E. Kajari, E. Lutz and G. Morigi.

Europhys. Lett. 95, 60008 (2011).

The generation of entanglement between two oscillators that interact via a common reservoir is theoretically studied. The reservoir is modeled by a one-dimensional harmonic crystal initially in thermal equilibrium. Starting from a separable state, the oscillators can become entangled after a transient time, that is of the order of the thermalization time scale. This behaviour is observed at finite temperature even when the oscillators are at a distance significantly larger than the crystal's interparticle spacing. The underlying physical mechanisms can be explained by the dynamical properties of the collective variables of the two oscillators which may decouple from or be squeezed by the reservoir. Our predictions can be tested with an ion chain in a linear Paul trap.

**September 12 2011**

**New article: Phys. Rev. A 84, 033408**

Cavity sideband cooling of trapped molecules

Markus Kowalewski, Giovanna Morigi, Pepijn W. H. Pinkse and Regina de Vivie-Riedle.

Phys. Rev. A 84, 033408 (2011).

The efficiency of cavity sideband cooling of trapped molecules is theoretically investigated for the case in which the infrared transition between two rovibrational states is used as a cycling transition. The molecules are assumed to be trapped either by a radiofrequency or optical trapping potential, depending on whether they are charged or neutral, and confined inside a high-finesse optical resonator that enhances radiative emission into the cavity mode. Using realistic experimental parameters and COS as a representative molecular example, we show that in this setup, cooling to the trap ground state is feasible.

**September 12 2011**

Friday, 16/09/2011, 10.00 s.t., seminar room E.04 in building E2.6

Multiple entangled laser beams

Hans Bachor

Australian National University, Canberra

Laser beams are an excellent medium for the communication of information. They can carry modulation with very high speed and in many independent parallel channels. Technology is now reaching the regime where the rate of information transfer approaches physical limits - set by the statistics of the photons in the beam. To reach beyond these quantum limits, entanglement between different modes of light can be used. This talks summarises the state of the art and reports recent experimental progress.

**August 18 2011**

**New article: Phys. Rev. A 84, 022321**

Tripartite nonlocality and continuous-variable entanglement in thermal states of trapped ions

Jie Li, Thomás Fogarty, Cecilia Cormick, John Goold, Thomas Busch, and Mauro Paternostro.

Phys. Rev. A 84, 022321 (2011).

We study a system of three trapped ions in an anisotropic bidimensional trap. By focusing on the transverse modes of the ions, we show that the mutual ion-ion Coulomb interactions set entanglement of a genuine tripartite nature, to some extent persistent to the thermal nature of the vibronic modes. We tackle this issue by addressing a nonlocality test in the phase space of the ionic system and quantifying the genuine residual tripartite entanglement in the continuous variable state of the transverse modes.

**July 19 2011**

**Physics colloquium: Benni Reznik - July 21th**

"Trans-Planckian physics in Laboratory black holes"

Benni Reznik, Tel Aviv University

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**July 07 2011**

**Thomas Fogarty will be visiting us**

Thomas Fogarty (University of Cork, group of Prof. Thomas Busch) will be visiting us next week and will give a talk in the group meeting on Tuesday 12th.

**July 04 2011**

**Physics colloquium: Thomas Pohl - July 7th**

"Dressed-up and excited: Nonlinear Optics in Rydberg-dressed Bose-Einstein Condensates"

Thomas Pohl, MPI-PKS Dresden

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**July 04 2011**

Prof. Tommaso Calarco (University of Ulm) will be visiting our group on Tuesday, July 5th.

**June 22 2011**

**Physics colloquium: Marc Bienert - June 30th**

"Photonische Werkzeuge: Hammer und Metermaß für Objekte im Quantenregime"

Marc Bienert, Universität des Saarlandes

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**June 06 2011**

**Physics colloquium: David Vitali - June 16th**

"Quantum dynamics of a vibrational mode of a thin membrane within an optical cavity"

David Vitali, University of Camerino (Italy)

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**June 03 2011**

Prof. Jonathan Home (ETH Zurich) will give a talk in our group meeting on Tuesday, June 7th, on "Mixed-species ion chains for investigations of open quantum systems".

**May 30 2011**

David Petrosyan (Technische Universität Kaiserslautern and Institute of Electronic Structure & Laser, Crete)

"Electromagnetically induced transparency with strongly interacting Rydberg atoms".

Tuesday, May 31st, 10am (room 4.18).

**May 23 2011**

Prof. Thomas Busch (University of Cork) gives a talk in our group seminar on Tuesday, May 17th, at 9.30am (room 4.18).

Title: "Qubit embedding in a Fermi gas".

**May 17 2011**

**Meeting with Fleischhauer's group**

Meeting with the Theoretical Quantum Optics Group of Prof. Dr. Michael Fleischhauer, Technische Universität Kaiserslautern

Kaiserslautern, May 17th

Program

A) Start at 9:30

1. Dr. Cecilia Cormick, "Trapping ions with lasers".

2. Hessam Habibian, "Ultracold atoms in an optical cavity potential".

3. Stefan Schütz, "Self-organization of atoms in optical cavities".

B) Lunch ~13:00

C) After lunch session (ends at 15:00)

4. Dominik Muth "Dynamics of repulsively and attractively interacting Bosons in 1D after an interaction quench".

5. Johannes Otterbach "Strongly correlated photons using Rydberg interactions".

6. David Dzsotjan "Collective behaviour of quantum emitters coupled to plasmon modes of nanowires".

**May 09 2011**

Rafael Mottl (ETH, Zurich) gives a talk in the group meeting on May 10th at 10:00, room 4.18.

Symmetry breaking and fluctuations at the Dicke quantum phase transition

Abstract:

The quantized motion of ultracold atoms coupled to the light field of an optical cavity realizes an effective version of the Dicke Hamiltonian. This system exhibits a zero-temperature phase transition from a normal phase to a superradiant phase which can be controlled by the coupling strength between the atoms and the light field. As the cavity field actively takes part in the coupled system dynamics, the photons leaking out of the cavity provide time-resolved information about the system. This allows us to study the symmetry breaking process at the phase transition and to characterize the fluctuations below the phase transition. We observe temporal correlations of the fluctuations with a characteristic and diverging time scale.

**May 06 2011**

**New article: Phys. Rev. A 83, 051602(R)**

Quantum-noise quenching in atomic tweezers

S. Zippilli, B. Mohring, E. Lutz, G. Morigi, W. Schleich

Phys. Rev. A 83, 051602(R) (2011)

The efficiency of extracting single atoms or molecules from an ultracold bosonic reservoir is theoretically investigated for a protocol based on lasers, coupling the hyperfine state in which the atoms form a condensate to another stable state, in which the atom experiences a tight potential in the regime of collisional blockade, the quantum tweezers. The transfer efficiency into the single-atom ground state of the tight trap is fundamentally limited by the collective modes of the condensate, which are thermally and dynamically excited. The noise due to these excitations can be quenched for sufficiently long laser pulses, thereby achieving high efficiencies. These results show that this protocol can be applied to initializing a quantum register based on tweezer traps for neutral atoms.

**April 15 2011**

**Workshop on Engineering and Control of Quantum Systems**

October 10 - 14, 2011. Dresden (Germany).

Scientific Coordinators: Christiane Koch (Universität Kassel), Giovanna Morigi.

More information at http://www.mpipks-dresden.mpg.de/~ecoqas11/

**April 15 2011**

**New article: Phys. Rev. A 83, 042319**

Fast and robust quantum computation with ionic Wigner crystals

J. D. Baltrusch, A. Negretti, J. M. Taylor, T. Calarco.

Phys. Rev. A 83, 042319 (2011).

We present a detailed analysis of the modulated-carrier quantum phase gate implemented with Wigner crystals of ions confined in Penning traps. We elaborate on a recent scheme, proposed by two of the authors, to engineer two-body interactions between ions in such crystals. We analyze the situation in which the cyclotron (ωc) and the crystal rotation (ωr) frequencies do not fulfill the condition ωc=2ωr. It is shown that even in the presence of the magnetic field in the rotating frame the many-body (classical) Hamiltonian describing small oscillations from the ion equilibrium positions can be recast in canonical form. As a consequence, we are able to demonstrate that fast and robust two-qubit gates are achievable within the current experimental limitations. Moreover, we describe a realization of the state-dependent sign-changing dipole forces needed to realize the investigated quantum computing scheme.

**April 14 2011**

**New article: N. J. Phys. 13, 043019 (2011)**

Trapping ions with lasers

Cecilia Cormick, Tobias Schaetz and Giovanna Morigi

New Journal of Physics 13, 043019 (2011)

This work theoretically addresses the trapping of an ionized atom with a single valence electron by means of lasers, analyzing qualitatively and quantitatively the consequences of the net charge of the particle. In our model, the coupling between the ion and the electromagnetic field includes the charge monopole and the internal dipole, within a multipolar expansion of the interaction Hamiltonian. Specifically, we perform a Power–Zienau–Woolley transformation, taking into account the motion of the center of mass. The net charge produces a correction in the atomic dipole that is of order me/M, with me the electron mass and M the total mass of the ion. With respect to neutral atoms, there is also an extra coupling to the laser field that can be approximated by that of the monopole located at the position of the center of mass. These additional effects, however, are shown to be very small compared to the dominant dipolar trapping term.

**April 11 2011**

The meeting will take place on April 11th from 10 a.m. on in seminar room 4.18.

Talks will be given by Dragi Karevski, Jean-Yves Fortin and Malte Henkel from Nancy, and by Marc Bienert, Endre Kajari and Jens Baltrusch from Saarbrücken.

**February 21 2011**

**QIPC 2011 - Conference at ETH Zurich in September**

International conference on Quantum Information Processing and Communication (QIPC) 2011 at ETH Zurich from September 5 - 9, 2011.

The conference program will cover a broad range of topics: in

- Quantum Information and Quantum Communication
- Physical Realizations for Information Technology e.g. Photons, Single Atoms, Ions, Molecules, Nuclear and Electron Spins, Superconducting Circuits, Micro- and Nano-Mechanics, Hybrid Quantum Systems
- Cavity QED, Optical Lattices, Quantum Memories, Foundations of Quantum Information, and Many-Body Systems

Read more at the website of the QIPC 2011 Conference

**February 21 2011**

Oxana Mishina, postdoc at Laboratory Kastler-Brossel, University of Pierre and Marie Curie (Paris, France) will be visiting us on Feb. 23rd to 25th.

**February 21 2011**

Pérola Milman (ISMO-Orsay et CNRS) is visiting us on Feb. 21st and 22nd and will give a talk in our group seminar on Tuesday 22nd.

**February 17 2011**

**Workshop on Quantum Information and Quantum Dynamics in Ion Traps**

This workshop is intended to bring experts in the field of quantum physics of trapped ions, to interchange ideas and present the latest advances in the field.

Registration Deadline: February 28th, 2011

**February 10 2011**

**Alex Retzker will visit us on February 14-16**

Alex Retzker (University of Ulm) will be visiting us on February 14th to 16th, and will give a talk in the group meeting on Tuesday 15th.

**February 07 2011**

**Physics colloquium: Stefano Ruffo - February 10th**

"Statistical mechanics and dynamics of systems with long-range interactions"

Stefano Ruffo, Università di Firenze, Italy

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**January 19 2011**

**New article: Phys. Rev. Lett. 106, 010401**

Quantum Zigzag Transition in Ion Chains

Efrat Shimshoni, Giovanna Morigi and Shmuel Fishman.

Phys. Rev. Lett. 106, 010401 (2011).

A string of trapped ions at zero temperature exhibits a structural phase transition to a zigzag structure, tuned by reducing the transverse trap potential or the interparticle distance. The transition is driven by transverse, short wavelength vibrational modes. We argue that this is a quantum phase transition, which can be experimentally realized and probed. Indeed, by means of a mapping to the Ising model in a transverse field, we estimate the quantum critical point in terms of the system parameters, and find a finite, measurable deviation from the critical point predicted by the classical theory. A measurement procedure is suggested which can probe the effects of quantum fluctuations at criticality. These results can be extended to describe the transverse instability of ultracold polar molecules in a one-dimensional optical lattice.

**January 18 2011**

**Dr. Axel Kreuter gives a talk on January 25th**

Dr. Axel Kreuter (Berkeley) will be visiting us and will give a talk in the group meeting on Tuesday 25th, at 10:30.

**January 17 2011**

**Physics colloquium: Martin Plenio - January 27th**

"Taming the quanta"

Martin Plenio, Universität Ulm

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**January 07 2011**

**Prof. Susana Huelga will visit us**

Prof. Susana Huelga (Institut für Theoretische Physik, Universität Ulm) will visit the group on Jan. 10th and 11th, and will give a talk in the group meeting on Jan. 11th.

**December 10 2010**

**Physics colloquium: Adrian Bachtold - December 16th**

"NEMSs based on Nanotubes: From Thermal Motors to Resonators"

Adrian Bachtold, CIN2 (ICN-CSIC) Barcelona

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including abstracts for this and future talks) can be found in the Physics Colloquium Schedule.

**December 06 2010**

**Physics colloquium: Ennio Arimondo - December 9th**

"Bose-Einstein condensates: a quantum-mechanics playground for tests and applications"

Ennio Arimondo (INO-CNR, Università di Pisa)

Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including schedule for future talks, and abstracts) can be found in the Physics Colloquium Schedule.

**December 06 2010**

Professor Jakob Reichel (École Normale Supérieure, Paris) will be visiting our group on 9-10 December.

**December 05 2010**

**Talk by John Goold on Monday 6th**

John Goold (Oxford) will be visiting us and will give a talk on "Dynamics of a Tonks-Girardeau gas" on Monday 6th, 9:30, at room 4.18.

**December 02 2010**

**New article: New J. Phys. 12, 115003**

Spontaneous nucleation of structural defects in inhomogeneous ion chains.

Gabriele De Chiara, Adolfo del Campo, Giovanna Morigi, Martin B. Plenio and Alex Retzker.

New Journal of Physics 12, 115003 (2010).

Structural defects in ion crystals can be formed during a linear quench of the transverse trapping frequency across the mechanical instability from a linear chain to a zigzag structure. The density of defects after the sweep can be conveniently described by the Kibble–Zurek mechanism (KZM). In particular, the number of kinks in the zigzag ordering can be derived from a time-dependent Ginzburg–Landau equation for the order parameter, here the zigzag transverse size, under the assumption that the ions are continuously laser cooled. In a linear Paul trap, the transition becomes inhomogeneous, since the charge density is larger in the center and more rarefied at the edges. During the linear quench, the mechanical instability is first crossed in the center of the chain, and a front, at which the mechanical instability is crossed during the quench, is identified that propagates along the chain from the center to the edges. If the velocity of this front is smaller than the sound velocity, the dynamics become adiabatic even in the thermodynamic limit and no defect is produced. Otherwise, the nucleation of kinks is reduced with respect to the case in which the charges are homogeneously distributed, leading to a new scaling of the density of kinks with the quenching rate. The analytical predictions are verified numerically by integrating the Langevin equations of motion of the ions, in the presence of a time-dependent transverse confinement. We argue that the non-equilibrium dynamics of an ion chain in a Paul trap constitutes an ideal scenario to test the inhomogeneous extension of the KZM, which lacks experimental evidence to date.

**November 30 2010**

**DPG Meeting (Dresden, March 2011)**

The 75th Annual Meeting of the DPG and DPG Spring Meeting will take place in Dresden, 13th - 18th March 2011. More information in the DPG Meeting website.

**November 12 2010**

**Physics colloquium: Christian Ast - Thursday 18th**

"Graphene: Nobel Material"

Christian Ast (MPI für Festkörperforschung, Stuttgart)

November 18th. Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including schedule for future talks, and abstracts) can be found in the Physics Colloquium Schedule.

**November 11 2010**

**Talk by Maryam Roghani on Monday 15th**

Maryam Roghani (Cold Atoms group, University of Freiburg) will give a talk in our group meeting, exceptionally on Monday Nov. 15th, at 11:00.

**November 02 2010**

**Physics colloquium: Philipp Treutlein - Thursday 4th**

"Atom chips - a quantum laboratory on a microchip"

Philipp Treutlein (Universität Basel)

November 4th. Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including schedule for future talks, and abstracts) can be found in the Physics Colloquium Schedule.

**October 26 2010**

**Physics colloquium: Artur Zrenner - Thursday 28th**

"Kohärente Optoelektronik mit Quantenpunkten"

Artur Zrenner (Universität Paderborn und CeOPP)

October 28th. Hörsaal II, Building C6.3. As usual, the talk begins at 16:15, from 16:00 there will be coffee and cookies.

Complete information (including schedule for future talks, and abstracts) can be found in the Physics Colloquium Schedule.

**October 25 2010**

**Talk by Caroline Champenois on Tuesday 26th**

Caroline Champenois (CNRS-Université de Provence, Marseille) is visiting us and will give a talk for the Quantum Journal Club of the groups Morigi, Eschner & Becher, on Tuesday, October 26th at 10:30 in room E.11.

**September 06 2010**

**Wolfgang Niedenzu is visiting us**

Wolfgang Niedenzu (PhD student, University of Innsbruck) is visiting the group and will stay for five weeks here. On Wednesday 8th September he will give a talk on microscopic dynamics of ultracold particles in a ring-cavity optical lattice.

**September 04 2010**

**Upcoming events: EMALI (Barcelona)**

International Meeting on Engineering, Manipulation, and Characterization of Quantum States of Matter and Light.

September 23rd to 25th, Barcelona.

http://www.quantumoptics.es/EMALI2010

**September 03 2010**

**Upcoming events: ECTI 2010 (Durham)**

2010 European Conference on Trapped Ions (ECTI 2010)

19-24 September 2010, Durham, UK.

http://www.sussex.ac.uk/physics/iqt/ECTI/

**August 11 2010**

**New article: Phys. Rev. Lett. 105, 075701 (2010)**

**Structural Defects in Ion Chains by Quenching the External Potential: The Inhomogeneous Kibble-Zurek Mechanism**

A. del Campo, G. De Chiara, Giovanna Morigi, M. B. Plenio, and A. Retzker

The nonequilibrium dynamics of an ion chain in a highly anisotropic trap is studied when the transverse trap frequency is quenched across the value at which the chain undergoes a continuous phase transition from a linear to a zigzag structure. Within Landau theory, an equation for the order parameter, corresponding to the transverse size of the zigzag structure, is determined when the vibrational motion is damped via laser cooling. The number of structural defects produced during a linear quench of the transverse trapping frequency is predicted and verified numerically. It is shown to obey the scaling predicted by the Kibble-Zurek mechanism, when extended to take into account the spatial inhomogeneities of the ion chain in a linear Paul trap.

Link to the article: Phys. Rev. Lett. 105, 075701 (2010). Also available as arXiv:1002.2524.

**July 30 2010**

**Mikayel Khanbekyan is visiting our group in August**

Dr. Mikayel Khanbekyan (Friedrich-Schiller-Universität Jena, Germany) is visiting our group during August 2-3. As a guest of Prof. Morigi, he will be giving a talk on Tuesday August 3rd on “Cavity-assisted spontaneous emission of a single three-level atom as a single-photon source”. (11am, Seminarraum 4.18 at E2.6)

**July 07 2010**

**Chiara Macchiavello is visiting our group in July**

Chiara Macchiavello (Universita' di Pavia, Italy) is visiting our group in July. As a guest of Prof. Morigi, she will be giving a talk (the Physics Colloquium on Thursday July 15) on her work: "Detecting entanglement via structure factors".

**July 06 2010**

**Cecilia Cormick has been awarded the Humboldt Fellowship!**

Cecilia Cormick has been awarded the Humboldt Fellowship. Congratulations, Ceci! :-)

**June 30 2010**

**Efrat Shimshoni is visiting our group in July**

Prof. Dr. Efrat Shimshoni (Bar-Ilan University, Israel) is visiting our group during July 7-9. As a guest of Prof. Morigi, she will be giving a talk (the Physics Colloquium on Thursday July 8st) on her work: "Large Magneto-Thermal Effects in Quasi One-dimensional Spin Systems".

**June 28 2010**

**Marcus Aspelmeyer visits Physik@UdS**

Prof. Marcus Aspelmeyer (Universität Wien, Austria) is visiting the Physics Department as a guest of Prof. Eschner. He will be giving a talk (the Physics Colloquium on Thursday July 1st) on his group's work on cooling nanomechanical resonators.

**May 03 2010**

**Diego Porras is visiting our group in May**

Dr. Diego Porras (Universidad Complutense de Madrid, Spain) will be visiting our group during May 24-28. He will be working with us on different topics on Ion traps. He will also give an informal talk on his work during that week's group seminar.

**April 20 2010**

**Rosario Fazio is visiting our group in May**

Prof. Dr. Rosario Fazio (Scuola Normale Superiore, Pisa, Italy) is visiting our group during May 17-19. Also he will be giving an informal talk on his work during that week's group meeting.

**March 30 2010**

**Monique Combescot is visiting our group in April**

Monique Combescot (CNRS, Université Pierre et Marie Curie, Paris, France) is visiting and working with our group during April 12-16. Also, as a guest of Prof. Morigi, she will be giving a talk (the Physics Colloquium on Thursday April 15) about her new work on superconductivity theory: "A striking understanding of Cooper pairs".

**February 16 2010**

**Alejandro Saenz is visiting our group in March**

Prof. Dr. Alejandro Saenz (Humboldt-Universität Zu Berlin, Germany) is visiting our group during March 23-24. He will be giving a talk on his work on the Journal Club time.