# Fluctuation dynamics of an open photon Bose-Einstein condensate

## TL;DR

This paper investigates the fluctuation dynamics of an open photon Bose-Einstein condensate, revealing temporal oscillations in coherence functions due to nonlinear, weakly driven-dissipative effects, supported by both theory and experiment.

## Contribution

It provides a combined theoretical and experimental analysis of photon BEC dynamics, highlighting a novel oscillatory regime in coherence functions caused by weak driving and dissipation.

## Key findings

- Observation of temporal oscillations in $g^{(2)}(\tau)$
- Identification of a regime with large particle-number fluctuations
- Confirmation of weak light-matter coupling effects

## Abstract

Bosonic gases coupled to a particle reservoir have proven to support a regime of operation where Bose-Einstein condensation coexists with unusually large particle-number fluctuations. Experimentally, this situation has been realized with two-dimensional photon gases in a dye-filled optical microcavity. Here, we investigate theoretically and experimentally the open-system dynamics of a grand canonical Bose-Einstein condensate of photons. We identify a regime with temporal oscillations of the second-order coherence function $g^{(2)}(\tau)$, even though the energy spectrum closely matches the predictions for an equilibrium Bose-Einstein distribution and the system is operated deeply in the regime of weak light-matter coupling. The observed temporal oscillations are attributed to the nonlinear, weakly driven-dissipative nature of the system which leads to time-reversal symmetry breaking.