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Survival of the quantum depletion of a Bose-Einstein condensate after release from a magnetic trap

Jacob A. Ross, Piotr Deuar, D. K. Shin, Kieran F. Thomas, B. M. Henson, Sean S. Hodgman, Andrew G. Truscott

Published Mar 29, 2021• 0 citations
OpenAlex

Abstract

We present observations of quantum depletion in expanding condensates released from a harmonic trap. We confirm experimental observations of slowly-decaying tails in the far-field beyond the thermal component, consistent with the survival of the quantum depletion. Our measurements support the hypothesis that the depletion survives the expansion, and even appears stronger in the far-field than expected before release based on the Bogoliubov theory. This result is in conflict with the hydrodynamic theory which predicts that the in-situ depletion does not survive when atoms are released from a trap. Simulations of our experiment show that the depletion should indeed survive into the far field and become stronger. However, while in qualitative agreement, the final depletion observed in the experiment is much larger than in the simulation. In light of the predicted power-law decay of the momentum density, we discuss general issues inherent in characterizing power laws.

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We present observations of quantum depletion in expanding condensates released from a harmonic trap. We confirm experimental observations of slowly-decaying tails in the far-field beyond the thermal component, consistent with the survival of the quantum depletion. Our measurements support the hypothesis that the depletion survives the expansion, and even appears stronger in the far-field than expected before release based on the Bogoliubov theory. This result is in conflict with the hydrodynamic theory which predicts that the in-situ depletion does not survive when atoms are released from a trap. Simulations of our experiment show that the depletion should indeed survive into the far field and become stronger. However, while in qualitative agreement, the final depletion observed in the experiment is much larger than in the simulation. In light of the predicted power-law decay of the momentum density, we discuss general issues inherent in characterizing power laws.
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.
Measurements of Tan's contact in expanding condensate conflict with predictions of the in situ quantum depletion. It is unclear how the depletion survives without a condensate, and even appears stronger in the far-field than before the trap release. We confirm experimental observations of a power-law decay in the far-field, consistent with the survival of the quantum depletion. Simulations of our experiment support this conclusion and provide a partial explanation. A gap between the numerical and empirical results is an open question obstructing studies of quantum depletion in the far field.
We report on the single-atom-resolved measurement of the distribution of momenta ℏk in a weakly interacting Bose gas after a 330 ms time of flight. We investigate it for various temperatures and clearly separate two contributions to the depletion of the condensate by their k dependence. The first one is the thermal depletion. The second contribution falls off as k^{-4}, and its magnitude increases with the in-trap condensate density as predicted by the Bogoliubov theory at zero temperature. These observations suggest associating it with the quantum depletion. How this contribution can survive the expansion of the released interacting condensate is an intriguing open question.
He, is closely related to Bose-Einstein condensation (BEC) phenomenon. However, even at zero temperature, a fraction of the quantum liquid is excited out of the condensate into higher momentum states via interaction-induced fluctuations-the phenomenon of quantum depletion. Quantum depletion of atomic BECs in thermal equilibrium is well understood theoretically but is difficult to measure. This measurement is even more challenging in driven-dissipative exciton-polariton condensates, since their non-equilibrium nature is predicted to suppress quantum depletion. Here, we observe quantum depletion of a high-density exciton-polariton condensate by detecting the spectral branch of elementary excitations populated by this process. Analysis of this excitation branch shows that quantum depletion of exciton-polariton condensates can closely follow or strongly deviate from the equilibrium Bogoliubov theory, depending on the exciton fraction in an exciton polariton. Our results reveal beyond mean-field effects of exciton-polariton interactions and call for a deeper understanding of the relationship between equilibrium and non-equilibrium BECs.