Jong Yeon Lee, "Open Quantum System and Spontaneous Symmetry Breaking"

Video recording for "Focus Week on Non-equilibrium Quantum Dynamics," September 30 - October 4, Kavli IPMU, University of Tokyo, Japan.
[workshop webpage] indico.ipmu.jp...
[speaker] Jong Yeon Lee (University of Illinois at Urbana-Champaign)
[title] Understanding Phases and Transitions in Mixed States
[abstract] With the rapid development of quantum simulator platforms, understanding the stability of quantum phases in the presence of environmental coupling has become increasingly important. As a pure quantum state evolves into a mixed state due to decoherence, traditional notions of quantum phases need to be reconsidered.
In this talk, I will present recent progress on mixed-state phases from an information-theoretic perspective, which contrasts sharply with conventional descriptions that rely on order parameters and fail to capture mixed-state phases. The presentation will be divided into three parts: (i) spontaneous symmetry breaking in mixed states, (ii) symmetry-protected topological states under decoherence and the extraction of long-range entangled states, and (iii) decohered topological orders and intrinsic error thresholds. If time permits, I will discuss how fault tolerance can be incorporated into this framework using a newly developed spacetime SPT (symmetry-protected topological) picture, highlighting the deep connection between fault tolerance and phase stability in open quantum systems.