Observation of pairs of atoms at opposite momenta in an equilibrium interacting Bose gas

Quantum fluctuations play a central role in the properties of quantum matter. In non-interacting ensembles, they manifest as fluctuations of non-commuting observables, quantified by Heisenberg inequalities(1). In the presence of interactions, additional quantum fluctuations appear, from which many-body correlations and entanglement arise(2). Weak interactions are predicted to deplete Bose-Einstein condensates by the formation of correlated pairs of bosons with opposite momenta(3,4). Here we report the observation of these atom pairs in the depletion of an equilibrium interacting Bose gas(5). Our measurements of atom-atom correlations, both at opposite and close-by momenta(6,7), allow us to characterize the equilibrium many-body state. We also show that the atom pairs share the properties of two-mode squeezed states(8,9), including relative number squeezing(10-12). Our results illustrate how interacting systems acquire non-trivial quantum correlations as a result of the interplay between quantum fluctuations and interactions(13).

Automated summary

Quantum fluctuations play a crucial role in the properties of quantum matter, leading to many-body correlations and entanglement. The observation of atom pairs in interacting Bose gas depletion allows for the characterization of equilibrium many-body states. Interacting systems acquire non-trivial quantum correlations as a result of the interplay between quantum fluctuations and interactions.