Coherence of a non-equilibrium polariton condensate across the interaction-mediated phase transition

TL;DR

This study explores how interactions influence phase coherence and the Berezinskii-Kosterlitz-Thouless transition in a driven-dissipative polariton condensate, revealing algebraic decay of coherence and novel proportionality relations.

Contribution

It provides experimental and numerical evidence of BKT-like transition in a non-equilibrium polariton system with variable interactions, highlighting new coherence behavior.

Findings

First order coherence exhibits algebraic decay consistent with BKT transition.

The decay exponent is inversely proportional to polariton density, similar to equilibrium superfluids.

Interaction strength affects the phase transition and coherence properties.

Abstract

The emergence of spatial coherence in a confined two-dimensional Bose gas of exciton-polaritons with tuneable interactions offers a unique opportunity to explore the role of interactions in a phase transition in a driven-dissipative quantum system, where both the phase transition and thermalisation are mediated by interactions. We investigate, experimentally and numerically, the phase correlations and steady-state properties of the gas over a wide range of interaction strengths by varying the photonic/excitonic fraction of the polaritons and their density. We find that the first order spatial coherence function exhibits algebraic decay consistent with the Berezinskii-Kosterlitz-Thouless (BKT) phase transition. Surprisingly, the exponent of the algebraic decay is inversely proportional to the coherent density of polaritons, in analogy to equilibrium superfluids above the BKT transition, but with a different proportionality constant. Our work paves the way for future investigations of the phenomenon of phase transitions and superfluidity in a driven-dissipative setting