Bose-Einstein condensation of photons in an optical microcavity

Understand

Bose-Einstein condensation, the macroscopic ground state accumulation of particles with integer spin (bosons) at low temperature and high density, has been observed in several physical systems, including cold atomic gases and solid state physics quasiparticles.

• However, the most omnipresent Bose gas, blackbody radiation (radiation in thermal equilibrium with the cavity walls) does not show this phase transition, because the chemical potential of photons vanishes and, when the temperature is reduced, photons disappear in the cavity walls.
• Theoretical works have considered photon number conserving thermalization processes, a prerequisite for Bose-Einstein condensation, using Compton scattering with a gas of thermal electrons, or using photon-photon scattering in a nonlinear resonator configuration.
• In a recent experiment, we have observed number conserving thermalization of a two-dimensional photon gas in a dye-filled optical microcavity, acting as a 'white-wall' box for photons.