First-order spatial coherence measurements in a thermalized two-dimensional photonic quantum gas

TL;DR

This study measures first-order spatial correlations in a two-dimensional photon gas across the Bose-Einstein condensation threshold, revealing the emergence of long-range order and quantum statistical effects, aligning with equilibrium Bose gas theory.

Contribution

It provides the first detailed experimental observation of spatial coherence transitions in a 2D photon gas near Bose-Einstein condensation, confirming theoretical predictions.

Findings

Coherence length scales with thermal de Broglie wavelength in uncondensed phase

Long-range order appears at condensation threshold

Quantum statistical effects observed in overlapping thermal wave packets

Abstract

Phase transitions between different states of matter can profoundly modify the order in physical systems, with the emergence of ferromagnetic or topological order constituting important examples. Correlations allow to quantify the degree of order and classify different phases. Here we report measurements of first-order spatial correlations in a harmonically trapped two-dimensional photon gas below, at, and above the critical particle number for Bose-Einstein condensation, using interferometric measurements of the emission of a dye-filled optical microcavity. For the uncondensed gas, the transverse coherence decays on a length scale determined by the thermal de Broglie wavelength of the photons, which shows the expected scaling with temperature. At the onset of Bose-Einstein condensation true long-range order emerges, and we observe quantum statistical effects as the thermal wave packets overlap. The excellent agreement with equilibrium Bose gas theory prompts microcavity photons as promising candidates for studies of critical scaling and universality in optical quantum gases.