Quantum Dynamics of Fast-moving Impurities Immersed in a Bose-Einstein Condensate

The interaction between a quantum impurity and its surrounding bath leads to exotic many-body phenomena, yet the non-equilibrium dynamics of impurities moving rapidly through a bosonic bath remains a challenging problem. Here, we demonstrate an experimental study of the dragged motion of fermionic ⁴⁰K impurities interacting with a finite-sized ²³Na Bose-Einstein condensate (BEC). Using a pair of counterpropagating Raman beams and Feshbach resonances, we initiate the relative motion and control the interaction strength between the impurities and the bath. We observe the deceleration of the impurities arising from interactions with the condensate and characterize the terminal speed as a function of the interaction strength. From their trajectories in various interaction regimes, we further extract the dependence of the drag force on the relative speed and the scattering length. Our experiment serves as a quantitative platform to explore non-equilibrium quantum dynamics of impurities in bosonic superfluid, contributing to the understanding of the formation and decoherence of quasiparticles in interacting quantum systems.