Topological order and equilibrium in a condensate of exciton-polaritons

TL;DR

This paper demonstrates the observation of a Berezinskii-Kosterlitz-Thouless transition in a 2D exciton-polariton gas, providing evidence of equilibrium phase transition behavior in a driven/dissipative system with implications for quantum degeneracy.

Contribution

It provides experimental evidence of a BKT transition in exciton-polariton condensates and shows that these systems can exhibit true equilibrium-like topological order.

Findings

Observation of BKT transition in exciton-polariton gas

Evidence of algebraic decay of coherence in space and time

Confirmation of polariton condensates as genuine quantum degenerate states

Abstract

We report the observation of the Berezinskii-Kosterlitz-Thouless transition for a 2D gas of exciton-polaritons, and through the joint measurement of the first-order coherence both in space and time we bring compelling evidence of a thermodynamic equilibrium phase transition in an otherwise open driven/dissipative system. This is made possible thanks to long polariton lifetimes in high-quality samples with small disorder and in a reservoir-free region far away from the excitation spot, that allow topological ordering to prevail. The observed quasi-ordered phase, characteristic for an equilibrium 2D bosonic gas, with a decay of coherence in both spatial and temporal domains with the same algebraic exponent, is reproduced with numerical solutions of stochastic dynamics, proving that the mechanism of pairing of the topological defects (vortices) is responsible for the transition to the algebraic order. Finally, measurements in the weak-coupling regime confirm that polariton condensates are fundamentally different from photon lasers and constitute genuine quantum degenerate macroscopic states.