Calorimetry of a Bose-Einstein condensed photon gas

TL;DR

This paper investigates the thermodynamic properties of a two-dimensional photon Bose-Einstein condensate, revealing a clear phase transition through specific heat measurements and confirming quantum statistical mechanics predictions.

Contribution

It provides the first spectroscopic measurement of specific heat and entropy across the Bose-Einstein transition in a photon gas, demonstrating critical behavior and theoretical agreement.

Findings

Observation of a cusp singularity in specific heat at the phase transition

Quantitative agreement with quantum statistical mechanics predictions

Spectroscopic determination of thermodynamic quantities in a photon gas

Abstract

Phase transitions, as the condensation of a gas to a liquid, are often revealed by a discontinuous behavior of thermodynamic quantities. For liquid Helium, for example, a divergence of the specific heat signals the transition from the normal fluid to the superfluid state. Apart from liquid helium, determining the specific heat of a Bose gas has proven to be a challenging task, for example for ultracold atomic Bose gases. Here we examine the thermodynamic behavior of a trapped two-dimensional photon gas, a system that allows us to spectroscopically determine the specific heat and the entropy of a nearly ideal Bose gas from the classical high temperature to the Bose-condensed quantum regime. The critical behavior at the phase transition is clearly revealed by a cusp singularity of the specific heat. Regarded as a test of quantum statistical mechanics, our results demonstrate a quantitative agreement with its predictions at the microscopic level.