Kadanoff Seminar: Universality in driven open quantum matter. - Sebastian Diehl, University of Cologne

Universality in driven open quantum matter.

Recent experimental developments in platforms from cold atomic gases over light driven quantum materials to NISQ devices move systems into the focus, which realize instances of driven open quantum matter: Coherent quantum dynamics, drive, and dissipation occur on an equal footing, and they are operated in the thermodynamic limit. They are thus located on the interface of quantum optics, condensed matter physics and statistical mechanics.

We will develop the tools to understand such systems based on a Keldysh field theory approach to the many-body Lindblad equation, suited to perform the transition from microscopic physics to macroscopic observables, and thus to distill universal phenomena in such setups. 

In this framework, we then describe how to delimit equilibrium vs. non-equilibrium systems based on symmetry. As applications, we will discuss two cases, where a weak microscopic breaking of equilibrium conditions leads to strong macrocscopic non-equilibrium effects: (i) the emergence of the Kardar-Parisi-Zhang equation in exciton-polariton systems, (ii) a novel non-equilibrium universality class at the onset of time crystalline order. 

Finally, we will argue that drive and dissipation need not to act destructively on quantum mechanical correlations such as phase coherence, entanglement or topological order, but can even be used to create them. We will concentrate on topology, and apply the field theory framework to demonstrate (i) the universality of topological response, being identical in ground- and dissipatively induced topological states, and (ii) a topological characterization of symmetry protected fermion matter in mixed quantum states.