Quantum gases of light, such as photon or polariton condensates in optical microcavities, are collective quantum systems enabling a tailoring of dissipation from, for example, cavity loss. This characteristic makes them a tool to study dissipative phases, an emerging subject in quantum many-body physics. We experimentally demonstrate a non-Hermitian phase transition of a photon Bose-Einstein condensate to a dissipative phase characterized by a biexponential decay of the condensate’s second-order coherence. The phase transition occurs because of the emergence of an exceptional point in the quantum gas. Although Bose-Einstein condensation is usually connected to lasing by a smooth crossover, the observed phase transition separates the biexponential phase from both lasing and an intermediate, oscillatory condensate regime. Our approach can be used to study a wide class of dissipative quantum phases in topological or lattice systems.

光的量子气体（例如光学微腔中的光子或极化子冷凝物）是集体量子系统，能够对例如腔体损耗的耗散进行调整。这一特性使它们成为研究耗散相的工具，耗散相是量子多体物理学中的新兴课题。我们实验证明了光子玻色-爱因斯坦凝聚物的非厄密相变到以凝聚物的二阶相干性的双指数衰减为特征的耗散相。由于量子气体中出现异常点而发生相变。尽管Bose-Einstein凝聚通常通过平滑的交换与激射相关，但是观察到的相变将双指数相与激射和中间的振荡凝结区分开。