Compressibility and the Equation of State of an Optical Quantum Gas in a Box

TL;DR

This paper measures the compressibility and derives the equation of state of a 2D optical quantum gas in a box, revealing quantum degeneracy effects and signatures of Bose-Einstein condensation in a nanostructured microcavity.

Contribution

It presents the first measurement of the compressibility of an optical quantum gas in a box potential, linking optical properties to quantum many-body physics.

Findings

Observation of Bose-Einstein condensation signatures

Sharp increase in density response in the quantum degenerate regime

Evidence of infinite compressibility prediction for degenerate Bose gases

Abstract

The compressibility of a medium, quantifying its response to mechanical perturbations, is a fundamental property determined by the equation of state. For gases of material particles, studies of the mechanical response are well established, in fields from classical thermodynamics to cold atomic quantum gases. Here we demonstrate a measurement of the compressibility of a two-dimensional quantum gas of light in a box potential and obtain the equation of state for the optical medium. The experiment is carried out in a nanostructured dye-filled optical microcavity. We observe signatures of Bose-Einstein condensation at high phase-space densities in the finite-size system. Strikingly, upon entering the quantum degenerate regime, the measured density response to an external force sharply increases, hinting at the peculiar prediction of an infinite compressibility of the deeply degenerate Bose gas.