  We present observations of the high momentum tail in expanding Bose-Einstein
condensates of metastable Helium atoms released from a harmonic trap. The
far-field density profile exhibits features that support identification of the
tails of the momentum distribution as originating in the in-situ quantum
depletion prior to release. Thus, we corroborate recent observations of
slowly-decaying tails in the far-field beyond the thermal component. This
observation is in conflict with the hydrodynamic theory, which predicts that
the in-situ depletion does not survive when atoms are released from a trap.
Indeed, the depleted tails even appear stronger in the far-field than expected
before release, and we discuss the challenges of interpreting this in terms of
the Tan contact in the trapped gas. In complement to these observations, full
quantum simulations of the experiment show that, under the right conditions,
the depletion can persist into the far field after expansion. Moreover, the
simulations provide mechanisms for survival and for the the large-momentum
tails to appear stronger after expansion due to an acceleration of the depleted
atoms by the mean-field potential. However, while in qualitative agreement, the
final depletion observed in the experiment is much larger than in the
simulation.

Quantum Gases