The quantum theory has introduced physicists to two major counter-intuitive concepts. On the one hand, there is wave-particle duality, expressing the idea that objects normally described as particles can also behave as waves, while entities primarily described as waves, such as light, can behave as particles. This revolutionary idea nev- ertheless relies on concepts borrowed from classical physics, either waves or particles evolving in an ordinary space-time. On the other hand, entanglement can lead to interferences between the amplitudes of multi-particle states, which are described via Hilbert space and have no classical counter-part. This fundamental feature has of course been strikingly demonstrated by the violation of Bell’s inequalities. There is however, a conceptually simpler situation in which the interference between two-particle amplitudes entails a behavior impossible to describe by any classical model. This is the celebrated Hong Ou and Mandel experiment, in which two photons arriving simultaneously in the input channels of a beam-splitter always emerge together in one of the output channels. This effect has been extensively used to characterize the quality of non- classical light sources. Our group in Palaiseau has recently realized a Hong-Ou-Mandel experiment using metastable helium atoms [1]. A schematic diagram of the apparatus is shown in Fig. 1. We monitor the cross correlation signal between the two output ports ⟨nˆcnˆd⟩ as a function of the time t3 of application of the beam splitter. At this time we observe a 65% reduction in the cross correlation. This degree of supression cannot be explained by either a wavelike or particlelike description of the atoms.