QUROPE Quantum Information Processing and Communication in Europe

Hanbury Brown and Twiss correlations, i.e. correlations in far-field intensity fluctuations, yield fundamental information on the nature of light sources, as highlighted after the discovery of photon bunching. Drawing on the analogy between photons and atoms, comparable observations have been made studying expanding Bose gases. We have used two-point density correlations to study how matter-wave coherence is established when crossing the Bose-Einstein condensation threshold. Our experiments reveal a persistent multimode character of the source, also significantly below the condensation threshold temperature. Complex quantum correlations are observed for a variety of source geometries, from quasi-isotropic to highly elongated. Ideal Bose gas theory reproduces most of the qualitative features of our measurements, however a quantitative analysis highlights the need for a more comprehensive theoretical description including particle interactions.