Better understanding the thermal capacity of Bose-Einstein photon condensates could enable the development of ultraprecise thermometers. The drop in the heat-storage capacity of liquid water as it changes phase to gas or solid is well-understood, but now physicists at the University of Bonn have observed similar behavior in a trapped 2D photon gas. Researchers at the Institute of Applied Physics of the University of Bonn have measured the temperature of a gas of light, when it condenses. The thermometer is figurative; the temperature was determined by recording the wavelengths of the light. Courtesy of Tobias Damm. If they are cooled sufficiently, photons can also condense, and many thousands of these light packets then fuse into a Bose-Einstein condensate — kind of super-photon with unusual characteristics. The Bonn researchers reported that photon gas at this phase transition behaves according to the theoretical predictions of Bose and Einstein: Similar to water, it abruptly changes its heat storage capacity. Atoms can also form a Bose-Einstein condensate, at which point they become indistinguishable from each other, behaving like a single giant atom. The change in heat capacity of atoms during the phase transition can be measured, but only imprecisely. Jan Klärs, formerly at Bonn and now at ETH Zurich, said the thermal capacity measurements of the photon condensate are substantially better. The heat capacity of a material is calculated from the energy needed to heat it by one degree and is generally determined by measuring the temperature of the substance before and after adding a defined amount of energy. However, the temperature of a gas of light cannot be measured with a thermometer; but that is also not necessary. "In order to determine the temperature of the gas, it is only necessary to know the different wavelengths of the light particles - the distribution of its colors,” said Klärs. At the measurement apparatus are Martin Weitz, Tobias Damm, David Dung, Julian Schmitt and Frank Vewinger of the Institute of Applied Physics of the University of Bonn. Courtesy of Volker Lannert/Uni Bonn. In the experiment, photons were captured inside a microcavity consisting of two spherically curved mirrors while repeatedly being absorbed and re-emitted by an embedded dye medium. The cavity length was of the same order as the wavelength itself, which caused a large frequency gap between the longitudinal resonator modes (free spectral range), comparable to the emission bandwidth of the dye molecules. Thermal equilibrium of the photon gas with the cavity environment at room temperature was achieved via repeated absorption and emission processes by the dye molecules, which established a thermal contact between photon gas and optical medium. Bose-Einstein condensation was triggered by increasing the photon number above the saturation level at a given temperature. The team reported that their experimental heat-capacity measurements at the transition from photon gas to Bose-Einstein condensate matched the theoretical predictions exactly. The heat content of the photon gas changed not only upon condensation to a super-photon but also continuously with the ambient temperature. The research was published in Nature Communications (doi: 10.1038/ncomms11340).