Non-equilibrium pre-thermal states in a two-dimensional photon fluid

TL;DR

This paper reports the experimental observation of a non-equilibrium pre-thermal state in a 2D photon fluid, showing algebraic correlations and a crossover akin to the Kosterlitz-Thouless transition, indicating non-equilibrium phase transition precursors.

Contribution

First experimental demonstration of pre-thermalization and algebraic correlations in a 2D photon fluid after an interaction quench, linking non-equilibrium dynamics to equilibrium phase transitions.

Findings

Observation of long-range algebraic correlations in a 2D photon fluid

Demonstration of a crossover from algebraic to exponential correlations

Partial memory of initial conditions consistent with theoretical predictions

Abstract

Thermalization is the dynamical process by which a many-body system evolves toward a thermal equilibrium state that maximizes its entropy. In certain cases, however, the establishment of thermal equilibrium is significantly slowed down and a phenomenon of pre-thermalization can emerge. It describes the initial relaxation toward a quasi-steady state after a perturbation. While having similar properties to their thermal counterparts, pre-thermal states exhibit a partial memory of initial conditions. Here, we observe the dynamical formation of a pre-thermal state in a non-equilibrium, two-dimensional (2D) fluid of light after an interaction quench. Direct measurements of the fluid's first-order correlation function reveal the spontaneous emergence of long-range algebraic correlations spreading within a light-cone, providing a clear signature of a quasi steady-state strongly similar to a 2D thermal superfluid. Detailed experimental characterization of the algebraic order is presented and a partial memory of the initial conditions is demonstrated, in agreement with recent theoretical predictions. Furthermore, by a controlled increase of the fluid fluctuations, we unveil a cross-over from algebraic to short-range (exponential) correlations, analogous to the celebrated Kosterlitz-Thouless transition observed at thermal equilibrium. These results suggest the existence of non-equilibrium precursors for thermodynamic phase transitions.