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.

系统的维度深刻地影响其物理行为，导致多体量子系统中不同物质状态的出现。在较低维度中，波动增加并导致长程有序的抑制。例如，在玻色子气体中，一维的玻色-爱因斯坦凝聚需要比二维更强的限制。在这里，我们观察了谐波捕获光子气体中从一维到二维的维度交叉并研究了其特性。光子被捕获在染料微腔中，其中聚合物纳米结构为光子气体提供捕获潜力。通过改变谐波陷阱的纵横比，我们从各向同性的二维限制调整为各向异性的、高度伸长的一维陷阱势。沿着这个转变，我们确定了光子气体的热量特性，并发现二维观察到的二阶玻色-爱因斯坦凝聚相变软化到一维交叉行为。