Nature 520, 66–68 (2015)

In 1987, C. K. Hong, Z. Y. Ou and L. Mandel experimentally demonstrated that two photons arriving at a beamsplitter from two different sides are bound to exit together if they are indistinguishable, that is, with identical spectral, spatial and polarization properties. In their two-photon interference scheme, the two detectors monitoring the two exit ports of the beamsplitter recorded zero coincidences for zero time delay between them, the so-called Hong–Ou–Mandel (HOM) dip. This is a purely quantum effect that has been extensively investigated with photons. Raphaël Lopes and colleagues now report similar observations using freely propagating twin beams of metastable 4He atoms. They observe a HOM dip approaching 0.33, well below the value of 0.5, which represents the limit for classical correlations for single distinguishable particles. Indistinguishability is considered a key property for the implementation of quantum information systems; these results open up the possibility to study quantum physics using massive particles, which, contrary to photons, can interact with each other.