**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.