Observation of critical scaling in the Bose gas universality class

TL;DR

This study experimentally observes critical scaling in a nearly noninteracting photon gas, confirming long-standing theoretical predictions for the Bose gas universality class through measurements of correlation length near the condensation transition.

Contribution

First experimental verification of critical scaling in the Bose gas universality class using a photon-based quantum gas system.

Findings

Critical exponent for correlation length is measured as ν = 0.52(3).

Observation of critical scaling behavior in a noninteracting photon gas.

Experimental confirmation of theoretical predictions for the Bose gas universality class.

Abstract

Critical exponents characterize the divergent scaling of thermodynamic quantities near phase transitions and allow for the classification of physical systems into universality classes. While quantum gases thermalizing by interparticle interactions fall into the XY model universality class, the ideal Bose gas has been predicted to form a distinct universality class whose signatures have not yet been revealed experimentally. Here, we report the observation of critical scaling in a two-dimensional quantum gas of essentially noninteracting photons, which thermalize by radiative contact to a reservoir of molecules inside a microcavity. By measuring the spatial correlations near the condensation transition, we determine the critical exponent for the correlation length to be $\nu = 0.52(3)$. Our results constitute a first experimental test of the long-standing scaling predictions for the Bose gas universality class.