## Hanbury Brown and Twiss

Hanbury Brown and Twiss correlations—correlations in far- field intensity fluctuations—yield fundamental information on the quantum statistics of light sources, as demonstrated 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. We used two-point density correlations to study how coherence is gradually established when crossing the Bose–Einstein condensation 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, and for all probed temperatures. The qualitative features of our observations are captured by ideal Bose gas theory, whereas the quantitative differences illustrate the role of particle interactions.

_{c}0.85 T

_{c}, 0.95 T

_{c}, respectively. The aspect ratio λ of the atomic source is 13.5. d–f, Circles: radial cuts of a–c. Dashed line: radial cuts of the corresponding autocorrelation of the mean density profile. Solid line: radial cuts of predictions of ideal Bose gas theory for the second-order correlation function g2(0,Δy). g–i, Circles: radially averaged axial cuts of a–c over 160 µm, where the shot noise peak s is excluded. The width of the exclusion region is 32 µ m. Dashed line: radially averaged axial cuts of the corresponding autocorrelation of the mean density profile. Solid line: radially averaged axial cuts of the predictions of ideal Bose gas theory for the second-order correlation function

A. Perrin, R. Bücker, S. Manz, T. Betz, C. Koller, T. Plisson, T. Schumm, and Jörg Schmiedmayer.

*Hanbury Brown and Twiss Correlations Across the Bose–Einstein Condensation Threshold*.

*Nature Physics* 8 (2012): 195