Polaronic Response of a Supersonic Impurity Strongly Coupled to a Bose Condensate
How a mobile impurity exchanges momentum and energy with a many-body environment is a central question in nonequilibrium quantum physics. This exchange becomes particularly complex when the impurity moves rapidly and is strongly coupled to its bath. Here, we realize an interaction-tunable cold-atom collider with fermionic $^{40}$K impurities immersed in a $^{23}$Na Bose--Einstein condensate. A species-selective Raman pulse simultaneously launches the condensate and quenches the interspecies scattering length, producing an initial relative speed 36 times the condensate speed of sound. We track the ensuing relative motion as the interspecies interaction is tuned from weak coupling to resonance. At weak and intermediate coupling, the impurity dynamics are well described by a finite-energy two-body collision model. Near resonance, however, the early-time impurity acceleration is markedly suppressed relative to the two-body prediction, corresponding to a strong enhancement of the apparent dynamical inertia. The enhancement is observed near two distinct Feshbach resonances. These observations provide evidence for a polaronic response in the strongly coupled, supersonic regime of a degenerate Bose--Fermi mixture.