“Fluctuation-dissipation theorem in open quantum systems”
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, spontaneous symmetry breaking is celebrated as one of the organizing principles in condensed matter physics. It was recently recognized that different from pure states, there are two types of symmetry defined for mixed quantum states: strong symmetry for every single state in the ensemble and weak symmetry only for the entire ensemble.
In this talk, 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. To quantify this phenomenon, we introduce the “fidelity susceptibility” to characterize the sensitivity of the mixed quantum states against external channel perturbation and relate the susceptibility to an average of the fidelity correlators. We call it the “fluctuation-dissipation theorem” in open quantum systems. We further propose an experimental proposal to probe the SW-SSB by quantum metrology.
Host: Meng Cheng