Abstract
Two-particle interference is a fundamental feature of quantum mechanics, and is even less intuitive than wave–particle duality for a single particle. In this duality, classical concepts—wave or particle—are still referred to, and interference happens in ordinary space-time. On the other hand, two-particle interference takes place in a mathematical space that has no classical counterpart. Entanglement lies at the heart of this interference, as it does in the fundamental tests of quantum mechanics involving the violation of Bell's inequalities1,2,3,4. The Hong, Ou and Mandel experiment5 is a conceptually simpler situation, in which the interference between two-photon amplitudes also leads to behaviour impossible to describe using a simple classical model. Here we report the realization of the Hong, Ou and Mandel experiment using atoms instead of photons. We create a source that emits pairs of atoms, and cause one atom of each pair to enter one of the two input channels of a beam-splitter, and the other atom to enter the other input channel. When the atoms are spatially overlapped so that the two inputs are indistinguishable, the atoms always emerge together in one of the output channels. This result opens the way to testing Bell's inequalities involving mechanical observables of massive particles, such as momentum, using methods inspired by quantum optics6,7, and to testing theories of the quantum-to-classical transition8,9,10,11. Our work also demonstrates a new way to benchmark non-classical atom sources12,13 that may be of interest for quantum information processing14 and quantum simulation15.
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Acknowledgements
We thank J. Ruaudel and M. Bonneau for contributions to the early steps of the experiment. We also thank K. Kheruntsyan, J. Chwedenczuk and P. Deuar for discussions. We acknowledge funding by IFRAF, Triangle de la Physique, Labex PALM, ANR (PROQUP, QEAGE), FCT (scholarship SFRH/BD/74352/2010 co-financed by ESF, POPH/QREN and EU to R.L.) and EU (ERC grant 267775, QUANTATOP, and Marie Curie CIG 618760, CORENT).
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Extended data figures and tables
Extended Data Figure 1 HOM dip visibility as a function of the integration volumes.
a, Visibility V as a function of the longitudinal integration interval Δvz. The transverse integration interval is kept constant at Δv⊥ = 0.48 cm s−1. b, Visibility as a function of the transverse integration interval Δv⊥. The longitudinal integration interval is kept constant at Δvz = 0.28 cm s−1. The red points mark the values discussed in the main text. Error bars denote the standard deviation of the statistical ensemble.
Extended Data Figure 2 Averaged number of incident atoms over the HOM dip.
a, Averaged atom number detected in 
, nc, as a function of the propagation time τ. The mean value of nc(τ) is 0.20 with a standard deviation of 0.01. b, Averaged atom number detected in 
, nd, as a function of the propagation time τ. The mean value of nd(τ) is 0.19 with a standard deviation of 0.01. c, The cross-correlation between the output ports c and d (solid blue circles), displaying the HOM dip, is compared to 〈nc〉 · 〈nd〉 (open grey circles). Error bars denote the standard deviation of the statistical ensemble.
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Lopes, R., Imanaliev, A., Aspect, A. et al. Atomic Hong–Ou–Mandel experiment. Nature 520, 66–68 (2015). https://doi.org/10.1038/nature14331
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DOI: https://doi.org/10.1038/nature14331
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