Fluctuation-dissipation relation for a Bose-Einstein condensate of photons

TL;DR

This paper experimentally verifies the fluctuation-dissipation relation in a photon Bose-Einstein condensate, confirming its thermal nature and demonstrating the theorem's applicability to a quantum light system.

Contribution

First experimental demonstration of the fluctuation-dissipation relation in a photon Bose-Einstein condensate, linking fluctuations to response functions in a quantum light system.

Findings

Confirmed the fluctuation-dissipation relation in a photon BEC

Measured the scale factor consistent with environment temperature

Validated the thermal nature of the optical condensate

Abstract

For equilibrium systems, the magnitude of thermal fluctuations is closely linked to the dissipative response to external perturbations. This fluctuation-dissipation relation has been described for material particles in a wide range of fields. Here we experimentally probe the relation between the number fluctuations and the response function for a Bose-Einstein condensate of photons coupled to a dye reservoir, demonstrating the fluctuation-dissipation relation for a quantum gas of light. The observed agreement of the scale factor with the environment temperature both directly confirms the thermal nature of the optical condensate and demonstrates the validity of the fluctuation-dissipation theorem for a Bose-Einstein condensate.