Hanbury Brown and Twiss correlations across the Bose-Einstein condensation threshold

TL;DR

This paper investigates how coherence develops in ultracold Bose gases crossing the BEC threshold by analyzing two-point density correlations, revealing persistent multimode features and the influence of interactions.

Contribution

It provides experimental insights into the gradual establishment of coherence across the BEC transition using HBT correlations in various geometries and temperatures.

Findings

Persistent multimode character of the emerging matter-wave

Correlation functions show non-trivial spatial shapes across geometries

Ideal Bose gas theory captures qualitative features, quantitative differences highlight interactions

Abstract

Hanbury Brown and Twiss (HBT) correlations, i.e. correlations in far-field intensity fluctuations, yield fundamental information on the quantum statistics of light sources, as highlighted after the discovery of photon bunching. Drawing on the analogy between photons and atoms, similar measurements have been performed for matter-wave sources, probing density fluctuations of expanding ultracold Bose gases. Here we use two-point density correlations to study how coherence is gradually established when crossing the Bose-Einstein condensation (BEC) threshold. Our experiments reveal a persistent multimode character of the emerging matter-wave as seen in the non-trivial spatial shape of the correlation functions for all probed source geometries from nearly isotropic to quasi-one-dimensional (quasi-1D), and for all probed temperatures. The qualitative features of our observations are captured by ideal Bose gas theory, the quantitative differences illustrate the role of particle interactions.