The interplay of quantum fluctuations with interactions lies at the core of quantum many-body physics, and generates specific particle correlations that could be revealed in experiments. A conceptually simple and emblematic example is the quantum depletion, present in the ground state of an ensemble of weakly-interacting bosons. It is composed of virtual pairs of bosons at opposite momenta, continuously coming in and out of the condensate, and should manifest in momentum-space correlations. I will report our recent observation of pairs of atoms with opposite momenta in an interacting Bose gas at equilibrium [1]. 

This experiment exploits the single-atom detection of metastable Helium in the momentum space [2] and the adiabatic preparation of interacting lattice gases [3]. We have studied the properties of the pair correlations (i.e. amplitude and width) and contrast them with those of Hanbury-Brown and Twiss type of correlations [4], to highlight the quantum coherences of the virtual pairs in the many-body ground state. I will discuss these observations in the textbook description of the Bogoliubov theory [5].

[1] A. Tenart et al., in preparation (2021).
[2] H. Cayla et al., Phys. Rev. A 97, 061609(R) (2018).
[3] C. Carcy et al., Phys. Rev. Lett. 126, 045301 (2021). 
[4] H. Cayla et al., Phys. Rev. Lett. 125, 165301 (2020).
[5] S. Butera et al., Phys. Rev. A 103, 013302 (2021).

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