Observation of Kardar-Parisi-Zhang universal scaling in two dimensions

Equilibrium and nonequilibrium states of matter can exhibit fundamentally different behavior. A key example is the Kardar-Parisi-Zhang universality class in two spatial dimensions (2D KPZ), where microscopic deviations from equilibrium give rise to macroscopic scaling laws without equilibrium counterparts. Although extensively studied theoretically, experimental evidence of 2D KPZ scaling has remained limited to interface growth. Here, we report the observation of KPZ universal scaling in 2D exciton-polariton condensates-quantum fluids of light that inherently break equilibrium conditions. Using spectroscopy and Michelson interferometry, we probed the phase correlations across microscopically different systems. Our analysis revealed correlation dynamics and scaling exponents in excellent agreement with 2D KPZ predictions. These results establish exciton-polariton condensates as an experimental platform for exploring 2D nonequilibrium universality.