Hanbury Brown—Twiss interferometry

For ultracold atoms. Nearly 50 years ago, the HBT effect demonstrated that photons emitted from a thermal source unexpectedly showed intensity correlations, often called photon bunching. Unraveling the correlations allowed astronomers to measure the sizes of stars. In the coherent light emitted by lasers, such photon bunching is absent. (For more, see page 19 of this issue.) Two groups have now demonstrated the HBT effect for bosonic atoms rather than for photons. At the Institute of Optics in Orsay, France, a team of physicists used a 0.55-μK gas of metastable helium atoms. The scientists released a tiny cloud of the atoms from a magnetic trap; some of the atoms became insensitive to magnetic fields and fell 47 cm to a microchannel plate detector where their individual positions and arrival times were recorded. By accumulating and analyzing data from about 1000 repetitions of the experiment, the physicists found the expected correlations. At higher temperatures, the cloud was larger and the correlation length too small to resolve with their detector. At colder temperatures, the helium gas became a Bose—Einstein condensate and the correlations abruptly vanished, also as expected. The other group, at ETH Zürich, used an atom laser from a BEC of rubidium-87 atoms. The atoms fell 36 mm into a high-finesse optical cavity where single atoms were recorded and the absence of bunching was observed, just like in an optical laser. When the Swiss group made the beam quasi-thermal, HBT correlations showed up in their data. (M. Schellekens et al., Science 310 , 648, 2005 http://dx.doi.org/10.1126/science.1118024 ; A. Öttl et al., Phys. Rev. Lett. 95 , 090404, 2005 .)