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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.

The photon chemical potential is zero for ordinary blackbody radiation in full thermal equilibrium with matter when photon emission and photon absorption freely adjust the number of photons. Its occupation law is thenA conserved-particle Bose-Einstein distribution instead contains $\hslash \omega -\mu$ in the exponential. The distinction concerns the allowed exchanges, rather than the fact that the particles are bosons. Scattering can redistribute photon energies without imposing the same number-changing equilibrium condition.

A thermalized photon gas in an optical cavity can have an effective nonzero chemical potential when an excitation reservoir controls its population. In a dye-filled curved-mirror optical cavity, repeated absorption and re-emission equilibrate transverse mode occupations, while pumped molecular excitations control the mean population. The reported Bose-Einstein condensation of photons therefore differs from cooling unconstrained blackbody radiation, where photons simply disappear into the walls. ^[ref]