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RESEARCH PRODUCT

Spatial quantum noise interferometry in expanding ultracold atom clouds

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It is ten years since the exotic form of matter known as a Bose–Einstein condensate was first created. It was the birth of ultra-low-temperature physics, and practitioners gathered last month in Banff, Canada, to celebrate and discuss the latest news, as Karen Fox reports. And this week a new development that could have a major impact in the field is announced. In the 1950s, Hanbury Brown and Twiss showed that it is possible to measure angular sizes of astronomical radio sources from correlations of signal intensities in independent detectors. ‘HBT interferometry’ later became a key technique in quantum optics, and now it has been harnessed to identify a quantum phase of ultracold bosonic atoms. In a pioneering experiment1, Hanbury Brown and Twiss (HBT) demonstrated that noise correlations could be used to probe the properties of a (bosonic) particle source through quantum statistics; the effect relies on quantum interference between possible detection paths for two indistinguishable particles. HBT correlations—together with their fermionic counterparts2,3,4—find numerous applications, ranging from quantum optics5 to nuclear and elementary particle physics6. Spatial HBT interferometry has been suggested7 as a means to probe hidden order in strongly correlated phases of ultracold atoms. Here we report such a measurement on the Mott insulator8,9,10 phase of a rubidium Bose gas as it is released from an optical lattice trap. We show that strong periodic quantum correlations exist between density fluctuations in the expanding atom cloud. These spatial correlations reflect the underlying ordering in the lattice, and find a natural interpretation in terms of a multiple-wave HBT interference effect. The method should provide a useful tool for identifying complex quantum phases of ultracold bosonic and fermionic atoms11,12,13,14,15.