Dimensional Crossover in a Quantum Gas of Light

Many-body quantum systems exhibit different states of matter influenced by the dimensionality of the system. In low dimensional bosonic gases, Bose-Einstein condensation (BEC) depends on the confining geometry. For example, one dimensional system requires stronger confinement than in two dimensions. We experimentally study Bose-Einstein condensation along the dimensional crossover from one to two dimensions, where photons are trapped in an isotropic, anisotropic and highly elongated harmonic potential realized by nano-printed polymer structures in a dye microcavity. In this system, the thermalization of the photon gas is decoupled from its dimensionality, which allows us to cool down the photon gas in one dimensional trap. We report a softening of the second-order Bose-Einstein condensation phase transition to a crossover as we tune from two dimensions to one dimension, as determined by the caloric properties of the photon gas. This novel technique of nano-printing polymer structures in a dye microcavity systems opens up a wide range of physics in high tunneling rate lattice to high curvature potentials such as logarithmic or Coulomb-like trapping for photon gases.