Microcanonical and resource-theoretic derivations of the thermal state of a quantum system with noncommuting charges

TL;DR

This paper explores how quantum systems with noncommuting charges reach thermal equilibrium, proposing an approximate microcanonical ensemble and a resource-theoretic framework to understand their thermal states.

Contribution

It introduces a novel microcanonical ensemble and a resource-theoretic approach to characterize thermal states in quantum systems with noncommuting charges.

Findings

The approximate microcanonical ensemble predicts the system's thermal state upon tracing out the bath.

Complete passivity constrains the form of the thermal state in noncommuting charge systems.

The framework advances understanding of quantum thermodynamics with noncommuting observables.

Abstract

The grand canonical ensemble lies at the core of quantum and classical statistical mechanics. A small system thermalizes to this ensemble while exchanging heat and particles with a bath. A quantum system may exchange quantities represented by operators that fail to commute. Whether such a system thermalizes and what form the thermal state has are questions about truly quantum thermodynamics. Here we investigate this thermal state from three perspectives. First, we introduce an approximate microcanonical ensemble. If this ensemble characterizes the system-and-bath composite, tracing out the bath yields the system's thermal state. This state is expected to be the equilibrium point, we argue, of typical dynamics. Finally, we define a resource-theory model for thermodynamic exchanges of noncommuting observables. Complete passivity---the inability to extract work from equilibrium states---implies the thermal state's form, too. Our work opens new avenues into equilibrium in the presence of quantum noncommutation.