Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - DOQS (Many-Body Physics with Driven Open Quantum Systems of Atoms, Light and Solids)

Teaser

Understanding the quantum many-particle problem is one of the grand challenges of modern physics. Tremendous progress has been made over the past decades in thermodynamic equilibrium, but non- equilibrium many-body quantum physics is still in its infancy. This project has the...

Summary

Understanding the quantum many-particle problem is one of the grand challenges of modern physics. Tremendous progress has been made over the past decades in thermodynamic equilibrium, but non- equilibrium many-body quantum physics is still in its infancy. This project has the goal of pioneering our understanding of an important, uprising class of dynamical non-equilibrium phases of quantum matter, which emerge in driven open quantum systems – systems where a Hamiltonian is not the only resource of many- body dynamics. This draws strong motivation from recent experimental surges in diverse areas, ranging from cold atomic gases over light-driven semiconductors to microcavity arrays. Here systems move into the focus, which define a novel interface of the grand disciplines quantum optics, many-body physics and statistical mechanics. They create scenarios without counterpart in traditional condensed matter, and call for a new conceptual framework for their understanding.

Our approach is structured around three key challenges: (i) We will identify novel universal macroscopic phenomena, which are uniquely tied to the driven microscopic nature of dynamics. This concerns non- thermal stationary states: we will construct a notion of driven quantum criticality, and shape an understanding of new, genuine non-equilibrium phases and phase transitions. But it also encompasses emergent universal regimes in open system time evolution. And we will push the concept of topological order to a broader non-equilibrium context, unleashing its potential for quantum information processing. (ii) We will create an efficient theoretical machinery, in particular advancing an innovative Keldysh dynamical quantum field theory for open systems. (iii) We will harness a broad spectrum of cutting edge experimental platforms to further explore our theoretical scenarios; with an emphasis on cold atomic gases, this program also comprises exciton-polariton condensates and coupled circuit QED architectures.

Work performed

Research highlights obtained so far:
(i) The establishment of a new universality class for bosons, in which quantum coherent effects are crucial even at criticality, in this way realizing an analogue of a quantum critical point. At the same time, detailed balance is manifestly absent, highlighting the importance of non-equilibrium conditions.
(ii) The identification of a new first order phase transition which occurs as function of the strength of the violation of detailed balance. This phase transition could be a representative of a new class of phase transitions between and dissipative fixed point and a non-equilibrium chaotic phase.
(iii) We discover that – contrary to common wisdom – topological order persists in mixed many-body quantum states of fermions in one dimension, provided suitable many-body correlators are considered. We establish the mechanism behind this scenario and provide a prescription how to measure these correlators using many-body interferometry.

Final results

(i) Analytical understanding of the phase transition mechanism driven by the strength of non-equilibrium conditions.
(ii) Creating and understanding a scenario of driven criticality of fermions
(iii) Understanding more broadly the status of topology in general mixed quantum states.

Website & more info

More info: http://www.thp.uni-koeln.de/diehl/researchgroup.html.