Dimensional crossover in a quantum gas of light

The dimensionality of a system profoundly influences its physical behaviour, leading to the emergence of different states of matter in many-body quantum systems. In lower dimensions, fluctuations increase and lead to the suppression of long-range order. For example, in bosonic gases, Bose–Einstein condensation in one dimension requires stronger confinement than in two dimensions. Here we observe the dimensional crossover from one to two dimensions in a harmonically trapped photon gas and study its properties. The photons are trapped in a dye microcavity where polymer nanostructures provide the trapping potential for the photon gas. By varying the aspect ratio of the harmonic trap, we tune from isotropic two-dimensional confinement to an anisotropic, highly elongated one-dimensional trapping potential. Along this transition, we determine the caloric properties of the photon gas and find a softening of the second-order Bose–Einstein condensation phase transition observed in two dimensions to a crossover behaviour in one dimension.