Allen Liu

A central challenge of quantum physics is to understand the structural properties of many-body systems, both in equilibrium and out of equilibrium. For classical systems, we have a unified perspective which connects structural properties of systems at thermal equilibrium to the Markov chain dynamics which mix to them. We lack such a perspective for quantum systems: many of the most fundamental ideas of the modern classical theory are notably absent from our quantum toolkit. We develop a theory which brings the broad scope and flexibility of the classical theory to quantum Gibbs states at high temperature. At its core is a natural quantum analogue of Dobrushin’s condition; whenever this condition holds, a concise path-coupling argument proves rapid mixing for the corresponding Markovian evolution. The same machinery bridges dynamic and structural properties: rapid mixing yields exponential decay of CMI without restrictions on the size of the probed subsystems, resolving a central question in the theory of open quantum systems. Our key technical insight is an optimal transport viewpoint which couples the quantum dynamics to a linear differential equation, enabling precise control over how local deviations from equilibrium propagate to distant sites.