title: Calorimetry of a Bose-Einstein-condensed photon gas.
abstract: 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.
authors.affiliation: Institut für Angewandte Physik, Atominstitut, Institute of Quantum Electronics, Universität Bonn, Wegelerstrasse 8, 53115 Bonn, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Damm
authors.affiliation: Institut für Angewandte Physik, Atominstitut, Institute of Quantum Electronics, Universität Bonn, Wegelerstrasse 8, 53115 Bonn, Germany.
authors.forename: Julian
authors.initials: J
authors.lastname: Schmitt
authors.affiliation: Institut für Angewandte Physik, Atominstitut, Institute of Quantum Electronics, Universität Bonn, Wegelerstrasse 8, 53115 Bonn, Germany.
authors.forename: Qi
authors.initials: Q
authors.lastname: Liang
authors.affiliation: Institut für Angewandte Physik, Atominstitut, Institute of Quantum Electronics, Universität Bonn, Wegelerstrasse 8, 53115 Bonn, Germany.
authors.forename: David
authors.initials: D
authors.lastname: Dung
authors.affiliation: Institut für Angewandte Physik, Atominstitut, Institute of Quantum Electronics, Universität Bonn, Wegelerstrasse 8, 53115 Bonn, Germany.
authors.forename: Frank
authors.initials: F
authors.lastname: Vewinger
authors.affiliation: Institut für Angewandte Physik, Atominstitut, Institute of Quantum Electronics, Universität Bonn, Wegelerstrasse 8, 53115 Bonn, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Weitz
authors.affiliation: Institut für Angewandte Physik, Atominstitut, Institute of Quantum Electronics, Universität Bonn, Wegelerstrasse 8, 53115 Bonn, Germany.
authors.forename: Jan
authors.initials: J
authors.lastname: Klaers