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Microscopic signatures of the quantum depletion of a Bose-Einstein condensate


Quantum fluctuations play a fundamental role in condensed matter physics, determining the nature of the ground state of a many-body system.  For an ensemble of bosons at zero temperature, these fluctuations deplete the condensate mode while populating higher energy states [1].   The resulting many-body state exhibits strong correlations, such as pairing between particles of opposite momenta and characteristic tails in the momentum-space distribution [2].  In this talk, I will present our recent progress towards the observation of these signatures of quantum depletion.  Our experiments are performed with an ultracold gas of metastable Helium-4 atoms for which electronic detection of individual particles is possible [3], providing direct access to the three-dimensional, momentum-space distribution of the gas.  We find characteristic power-law scaling of the depletion population with momentum, in agreement with Bogoliubov's microscopic theory.  Importantly, our measurements are able to distinguish between quantum and thermal effects, allowing for an unambiguous identification of quantum depletion.

[1] N. N. Bogoliubov, J. Phys. (Moscow) 11, 23 (1947)
[2] L. Mathey et al, PRA 79, 013609 (2009)
[3] T. Jeltes et al., Nature 445, 402-405 (2007)