A universe of mixed-state quantum phases

Title：A universe of mixed-state quantum phases
Time：February 11, 2025 09:00（Beijing）
Organizer：新加坡国立大学理学院物理系

Reporter ：Jian-Hao Zhang （University of Colorado Boulder）
Abstract:
Recently the exploration of phases of matter in open quantum systems has garnered substantial interest across various disciplines including condensed matter physics and quantum information science, especially those phenomena that are intrinsically mixed without analogues in closed systems. In particular, the energy gap is a crucial concept in physics, particularly in the study of solid-state physics and quantum mechanics.
In this talk, based on the correlation behaviors of density matrix, we first generalize the concept of “gap” to mixed quantum states. For gapped mixed states, we introduce the concept of “average symmetry-protected topological (ASPT) phases”, together with a systematic construction and classification paradigm. In particular, a plethora of ASPT states that cannot be realized in pure states are provided, dubbed “intrinsic ASPT” phases.
For “gapless” mixed states, we systematically introduce a new concept of “strong-to-weak spontaneous symmetry breaking” (SWSSB) in mixed quantum states. As a universal mixed-state quantum phase, we demonstrate the universality of SWSSB through the stability against symmetric finite-depth local channel perturbation, and the spontaneity implies that a local perturbation may lead to a global change of the whole density matrix.