Topological order and thermal equilibrium in polariton condensates

Abstract

The Berezinskii-Kosterlitz-Thouless phase transition from a disordered to a quasi-ordered state, mediated by the proliferation of topological defects in two dimensions, governs seemingly remote physical systems ranging from liquid helium, ultracold atoms and superconducting thin films to ensembles of spins. Here we observe such a transition in a short-lived gas of exciton-polaritons, bosonic light-matter particles in semiconductor microcavities. The observed quasi-ordered phase, characteristic for an equilibrium two-dimensional 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. This is made possible thanks to long polariton lifetimes in high-quality samples and in a reservoir-free region. Our results show that the joint measurement of coherence both in space and time is required to characterize driven-dissipative phase transitions and enable the investigation of topological ordering in open systems.

Photonic-band-gap architectures for long-lifetime room-temperature polariton condensation in GaAs quantum wells

“…Several works in the literature, for example Refs. [30][31][32][33][34], associate polariton condensates with this transition. In our work, we use the word condensate to refer exclusively to Bose-Einstein condensation.…”

A Yellow Polariton Condensate in a Dye Filled Microcavity

“…This technique has been utilised to realise experimentally the long range phase coherence of the condensate in many previous organic condensate studies. [19][20][21][22] The interference fringes across the image are plotted in Fig. 4(b) and are fitted with a convoluted Gaussian (red curve).…”

Phase ordering kinetics of a nonequilibrium exciton-polariton condensate

“…The latter have been intensively investigated in both recent experimental [38][39][40] and theoretical [36,54] works. It has been claimed that polariton systems display a kind of dissipative Berezinskii-Kosterlitz-Thouless (BKT) transition, while critical exponents may differ from the ones obtained in thermal equilibrium [36,[38][39][40]. Here, we assume that the system is sufficiently far away from the critical point on the ordered side of it, so the system converges to an approximately defect-free phase.…”