On the survival of the quantum depletion of a condensate after release from a magnetic trap

Abstract

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.

Observation of 1/k4 -Tails after Expansion of Bose-Einstein Condensates with Impurities

“…Our observations confirm the scenario predicted theoretically in [35], i.e., that the 1=k 4 -tails associated with the quantum depletion of a spin-polarized BEC decrease adiabatically during an expansion in the presence of interactions. This conclusion differs from that of a recent work [38] which studied magnetically trapped 4 He à atoms. So far, unambiguous signals of the quantum depletion in TOF experiments have been found only when interactions do not affect the expansion [46,47].…”

“…Here we show unambiguously that the tails disappear when the impurities are removed from the BEC, and we confirm the scenario predicted theoretically [35]. This conclusion differs from that of recent work [38] where similar 1=k 4 -tails were found in expanding BECs without impurities.…”

“…Note that the observed decay does not span a large enough dynamical range to precisely determine the power-law exponent (see Ref. [38] and Fig. 4).…”