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

TL;DR

This paper investigates whether quantum depletion in a Bose-Einstein condensate persists after release from a trap, combining experimental observations of high-momentum tails with quantum simulations, challenging existing hydrodynamic predictions.

Contribution

It provides experimental evidence and quantum simulations showing quantum depletion can survive expansion, contradicting hydrodynamic theory predictions.

Findings

High-momentum tails originate from in-situ quantum depletion

Depletion persists into the far field after expansion

Simulations suggest mean-field acceleration enhances tails

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.