Its bosonic and ideal (interaction-free) nature should make a photon gas an obvious candidate for a Bose-Einstein condensation. However, the thermodynamic behavior of photon gases usually does not include a condensation process. For blackbody radiation, the most omnipresent Bose gas, the number of photons follows the available thermal energy. At low temperatures, the photon number simply decreases and no macroscopic occupation of the cavity ground state occurs. In contrast to a three-dimensional thermal photon gas as Planck’s blackbody radiation, photons can exhibit Bose-Einstein condensation, if the thermalization process is restricted to two motional degrees of freedom. Experimentally, this situation has been realized in a microcavity enclosing a dye medium, designated as a room temperature heat bath for the photon gas. Detailed experimental studies of the thermalization and condensation process, as well as the quantum statistics of the photon condensate, have revealed the signatures of this unusual ‘superfluid’.
- D. Dung, C. Kurtscheid, T. Damm, J. Schmitt, F. Vewinger, M. Weitz, and J. Klaers, “Variable potentials for thermalized light and coupled condensates”, Nature Photonics 11, 565 (2017). link
- J. Klaers, J. Schmitt, F. Vewinger, and M. Weitz, ”Bose-Einstein condensation of photons in an optical microcavity”, Nature 468, 545 (2010). link
- J. Klaers, F. Vewinger, and M. Weitz, “Thermalization of a two-dimensional photonic gas in a ‘white wall’ photon box”, Nature Physics 6, 512 (2010). link