Observation of $1/k^4$-tails after expansion of Bose-Einstein Condensates with impurities

TL;DR

This study observes $1/k^4$ momentum tails in a Bose-Einstein condensate with impurities after expansion, revealing impurity-induced effects on momentum distribution not explained by equilibrium interactions.

Contribution

It demonstrates that impurity-bath interactions cause significant algebraic tails in momentum density during expansion, a novel insight into impurity effects in BEC dynamics.

Findings

$1/k^4$ tails observed in BEC with impurities

Tails are much larger than equilibrium predictions

No tails found in pure BEC without impurities

Abstract

We measure the momentum density in a Bose-Einstein condensate (BEC) with dilute spin impurities after an expansion in the presence of interactions. We observe tails decaying as $1/k^4$ at large momentum $k$ in the condensate and in the impurity cloud. These algebraic tails originate from the impurity-BEC interaction, but their amplitudes greatly exceed those expected from two-body contact interactions at equilibrium in the trap. Furthermore, in the absence of impurities, such algebraic tails are not found in the BEC density measured after the interaction-driven expansion. These results highlight the key role played by impurities when present, a possibility that had not been considered in our previous work [Phys. Rev. Lett. 117, 235303 (2016)]. Our measurements suggest that these unexpected algebraic tails originate from the non-trivial dynamics of the expansion in the presence of impurity-bath interactions.