Understanding and characterizing phase transitions in driven-dissipative systems constitutes a new frontier for many-body physics^{1,2,3,4,5,6,7,8}. A generic feature of dissipative phase transitions is a vanishing gap in the Liouvillian spectrum⁹, which leads to long-lived deviations from the steady state as the system is driven towards the transition. Here, we show that photon correlation measurements can be used to characterize the corresponding critical slowing down of non-equilibrium dynamics. We focus on the extensively studied phenomenon of optical bistability in GaAs cavity polaritons^10,11, which can be described as a first-order dissipative phase transition^12,13,14. Increasing the excitation strength towards the bistable range results in an increasing photon-bunching signal along with a decay time that is prolonged by more than nine orders of magnitude as compared with that of single polaritons. In the limit of strong polariton interactions leading to pronounced quantum fluctuations, the mean-field bistability threshold is washed out. Nevertheless, the functional form with which the Liouvillian gap closes as the thermodynamic limit is approached provides a signature of the emerging dissipative phase transition. Our results establish photon correlation measurements as an invaluable tool for studying dynamical properties of dissipative phase transitions without requiring phase-sensitive interferometric measurements.

理解和表征驱动耗散系统中的相变构成了多体物理学^{1,2,3,4,5,6,7,8}的新领域。耗散相变的一般特征是Liouvillian谱^9中消失的间隙，随着系统向相变的驱动，这会导致与稳态的长期偏离。在这里，我们表明光子相关性测量可用于表征非平衡动力学的相应临界减慢。我们专注于广泛研究的GaAs腔极化子^10,11中的光学双稳态现象，可以将其描述为一阶耗散相变^12,13,14。与单极化子相比，将激发强度增加到双稳态范围会导致光子聚集信号增加，并且衰减时间延长了九个数量级。在导致明显的量子波动的强极化子相互作用的极限下，平均场双稳态阈值被冲掉。然而，随着接近热力学极限，Liouvillian间隙关闭的功能形式为新兴的耗散相变提供了标志。我们的结果将光子相关性测量确定为研究耗散相变动力学特性而无需相位敏感干涉测量的宝贵工具。