Quantum dynamics of fast-moving impurities in a Bose-Einstein Condensate

The interaction between a quantum impurity and its surrounding medium gives rise to rich many-body phenomena, including the emergence of phonon-dressed polarons. Although polaronic properties in its equilibrium have been widely studied, understanding the nonequilibrium dynamics of fast impurities driven through an interacting Bose-Einstein condensate (BEC) remains an open experimental challenge.

In this poster, we investigate the dragged motion of fermionic impurities interacting with a ²³Na BEC. We initiate controlled relative motion by applying a counterpropagating two-photon Raman pulse to the Na atoms, which simultaneously transfers the BEC from a weakly interacting initial state to a strongly interacting final state and imprints a well-defined momentum kick, producing a bath velocity that can exceed the condensate’s Landau critical velocity. A dilute cloud of ⁴⁰K impurities interacts with the moving condensate bath, with the interspecies interaction strength tuned via a Feshbach resonance.

By tracking the impurity velocity as a function of time, we extract the impurity acceleration and infer a dissipative drag force exerted by the condensate. We discuss how the drag depends on relative velocity and interaction strength, and connect our results to nonequilibrium quasiparticle formation process in the presence of dissipative motion.