Demonstrating Quantum Microscopic Reversibility Using Coherent States of Light

TL;DR

This paper experimentally demonstrates a quantum generalization of microscopic reversibility using optical coherent and thermal states, highlighting quantum effects' impact on thermodynamic principles at low temperatures.

Contribution

It introduces and experimentally tests a quantum extension of microscopic reversibility, emphasizing the role of quantum coherence in thermodynamic processes.

Findings

Quantum effects reduce the likelihood of backward processes with coherence.

Quantum-to-classical transition observed as temperature increases.

Quantum modification is critical at low temperatures.

Abstract

The principle of microscopic reversibility lies at the core of fluctuation theorems, which have extended our understanding of the second law of thermodynamics to the statistical level. In the quantum regime, however, this elementary principle should be amended as the system energy cannot be sharply determined at a given quantum phase space point. In this Letter, we propose and experimentally test a quantum generalization of the microscopic reversibility when a quantum system interacts with a heat bath through energy-preserving unitary dynamics. Quantum effects can be identified by noting that the backward process is less likely to happen in the existence of quantum coherence between the system's energy eigenstates. The experimental demonstration has been realized by mixing coherent and thermal states in a beam-splitter, followed by heterodyne detection in an optical setup. We verify that the quantum modification for the principle of microscopic reversibility is critical in the low-temperature limit, while the quantum-to-classical transition is observed as the temperature of the thermal field gets higher.