Fluctuation-dissipation theorem in open quantum systems

We propose a fluctuation-dissipation theorem in open quantum systems from an information-theoretic perspective. We define the fidelity susceptibility that measures the sensitivity of the systems under perturbation and relate it to the fidelity correlator that characterizes the correlation behaviors for mixed quantum states. In particular, we determine the scaling behavior of the fidelity susceptibility in the strong-to-weak spontaneous symmetry breaking (SW-SSB) phase, strongly symmetric short-range correlated phase, and the quantum critical point between them. We then provide a geometric perspective of our construction using distance measures of density matrices. We further design a polynomial proxy that can in principle be used as an experimental probe for detecting the SW-SSB and phase transition through quantum metrology. In particular, we show that each term of the polynomial proxy is related to the Renyi versions of the fidelity correlators.