Quantum physics: Two-atom bunching.

Abstract

The Hong–Ou–Mandel effect, whereby two identical quantum particles launched into the two input ports of a 'beam-splitter' always bunch together in the same output port, has now been demonstrated for helium-4 atoms. See Letter p.66 The Hong-Ou-Mandel effect, in which two indistinguishable photons entering a 50:50 beam splitter lead to a dip in the coincidence rate of the detectors, demonstrates basic features of the theory of quantum mechanics and has no classical analogue. It is the basis of many other experiments in quantum information and quantum optics. Here Marc Cheneau and colleagues realize the Hong-Ou-Mandel experiments with helium-4 atoms instead of photons, building on previous developments on atom pair production. Since atoms, as opposed to photons, are massive particles, this experiment opens possibilities to perform basic quantum physics experiments, like Bell tests, with observables of massive particles. In the long run, this could contribute to research on the effect of mass and gravity in quantum mechanics, especially in relation to the quantum-to-classical transition.