# Fluctuation theorem in cavity quantum electrodynamics systems

## TL;DR

This paper derives a fluctuation theorem for cavity quantum electrodynamics systems, addressing divergence issues at zero temperature and linking the theorem to photon emission, with implications for quantum heat engines.

## Contribution

It introduces a novel integral fluctuation theorem for cavity QED systems, accounting for absolute irreversibility and providing simplified relations under certain conditions.

## Key findings

- Two types of absolute irreversibility affect the fluctuation theorem.
- The contributions relate to the average photon number emitted.
- The theorem simplifies under stationary, low-loss conditions.

## Abstract

We derive an integral fluctuation theorem (FT) in a general setup of cavity quantum electrodynamics systems. In the derivation, a key difficulty lies in a diverging behavior of entropy change arising from the zero-temperature limit of an external bath, which is required to describe the cavity loss. We solve this difficulty from the viewpoint of absolute irreversibility and find that two types of absolute irreversibility contribute to the integral FT. Furthermore, we show that, in a stationary and small cavity-loss condition, these contributions have simple relationships to the average number of photons emitted out of the cavity, and the integral FT yields an approximate form independent of the setup details. We illustrate the general results with a numerical simulation in a model of quantum heat engine.