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Jan Klaers and collaborators observed Bose-Einstein condensation of photons in a dye-filled optical microcavity. Repeated absorption and emission by the dye thermalized the confined light near room temperature, while cavity geometry supplied an effective two-dimensional trapping environment.^[ref]

Increasing the photon population produced a strongly occupied ground mode above a thermal distribution. This differs from simply cooling ordinary blackbody radiation, where photons can disappear into the walls and their number is not independently constrained. The experimental signatures included a threshold and its dependence on cavity geometry.