Calorimetry of a Bose–Einstein-condensed photon gas

• Published in: Nature communications Vol. 7; no. 1; p. 11340
• Main Authors: Damm, Tobias, Schmitt, Julian, Liang, Qi, Dung, David, Vewinger, Frank, Weitz, Martin, Klaers, Jan
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 19-04-2016; Nature Publishing Group; Nature Portfolio
• Subjects: 639/766/36; 639/766/530/951; Humanities and Social Sciences; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms11340

Phase transitions, as the condensation of a gas to a liquid, are often revealed by a discontinuous behaviour 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 behaviour 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 behaviour 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.
Phase transitions are often revealed by a discontinuous behaviour of thermodynamic quantities. Here, the authors study the thermodynamic behaviour of a trapped 2D photon gas, revealing critical behaviour at the phase transition through a cusp singularity of the specific heat.