**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).