Comment on "Observation of Kardar--Parisi--Zhang universal scaling in two dimensions"

Abstract

In their paper published in Science 392, 221 (2026), Widmann and collaborators reported interferometry experiments to explore the emission coherence decay of a two-dimensional polariton condensate generated in an array of coupled resonators. The authors claim evidence of Kardar--Parisi--Zhang (KPZ) universal scaling in the measured spatio-temporal coherence decay. We argue in the following that the data were not properly analyzed. We re-analyze the experimental data acquired both with the square and the triangular lattices for various values of the excitation power. Instead of stretched exponential decays, in the space (time) windows considered in the paper we find that the measured $|g^{(1)}(\delta {\bf r}, \delta t)|$ at $\delta t=0$ ($|\delta {\bf r}| $ close to $0$) rather show Gaussian (exponential) decay for all excitation powers. As a result, using as temporal and spatial exponents $\beta=0.5$ and $\chi=1$, the data for all pump powers are found to collapse onto a single curve, which is not the KPZ scaling function. In particular, we show that the data collapse onto the KPZ scaling function presented in the paper is an artifact stemming from incorrect data normalization. We thus conclude that the main claim of the paper is not justified as the spatio-temporal decays of the coherence over the space-time windows analyzed in the paper are not well described by the KPZ universal behavior.