Hanbury Brown and Twiss correlation paper now published in Nature Physics:

Hanbury Brown and Twiss correlations across the Bose–Einstein condensation threshold
A. Perrin, R. Bücker, S. Manz, T. Betz, C. Koller, T. Plisson, T. Schumm & J. Schmiedmayer
Nature Physics (2012)

We have performed Hanbury-Brown-Twiss measurements while a thermal Rubidium gas is successively cooled to Bose-Einstein condensation. In the thermal regime we observe bosonic bunching. At the condensation threshold we observe divergence of the radial coherence length as expected for an atom laser. In the longitudinal direction we observe a persistent population of several quantum modes. Probing momentum correlations we perform a heterodyning of these modes which allows us to detect excitations out of the ground state wave function down to the percent level. These measurements reveal that the commonly stated analogy between a BEC wave function and a photon laser fails at the condensation threshold. Although the qualitative observations are well described by an ideal gas theory, our results call for a more comprehensive theoretical description of particle correlations, including the effects of interactions and finite temperature.

Press release at Vienna University of Technology