Entropic Fluctuations in Statistical Mechanics II. Quantum Dynamical Systems

TL;DR

This paper explores various extensions of classical fluctuation theorems to quantum systems, using measurement schemes, modular theory, and spectral analysis to deepen understanding of quantum entropy production.

Contribution

It introduces new quantum fluctuation theorem extensions using ancilla measurements and modular theory, linking entropy fluctuations to spectral properties of quantum operators.

Findings

Quantum fluctuation theorems can be extended via ancilla-based measurements.

Modular theory provides a quantum analogue of phase space contraction.

Spectral resonances of quantum transfer operators relate to entropy fluctuation properties.

Abstract

The celebrated Evans-Searles, respectively Gallavotti-Cohen, fluctuation theorem concerns certain universal statistical features of the entropy production rate of a classical system in a transient, respectively steady, state. In this paper, we consider and compare several possible extensions of these fluctuation theorems to quantum systems. In addition to the direct two-time measurement approach whose discussion is based on (LMP 114:32 (2024)), we discuss a variant where measurements are performed indirectly on an auxiliary system called ancilla, and which allows to retrieve non-trivial statistical information using ancilla state tomography. We also show that modular theory provides a way to extend the classical notion of phase space contraction rate to the quantum domain, which leads to a third extension of the fluctuation theorems. We further discuss the quantum version of the principle of regular entropic fluctuations, introduced in the classical context in (Nonlinearity 24, 699 (2011)). Finally, we relate the statistical properties of these various notions of entropy production to spectral resonances of quantum transfer operators. The obtained results shed a new light on the nature of entropic fluctuations in quantum statistical mechanics.