Motional and Spin Dynamics of Individual Neutral Impurities in an Ultracold Gas

Recent studies on the dynamics of single, neutral impurities immersed in an ultracold gas are reviewed. This paradigmatic model system is realized by the controlled doping of single Caesium atoms into an ultracold Rubidium gas. Interaction between the impurity and the gas in both the motional and internal degrees of freedom is studied for a broad range of bath temperatures in the thermal and quantum‐degenerate regime. Tracing single‐atom diffusion it is found that, even for a granular bath, where the assumption of a continuous medium is not fulfilled, a modified Langevin equation yields excellent predictions for tracer diffusion. Numerical modeling of impurity dynamics in a three‐dimensional superfluid bath shows the emergence of a prethermalized state for intermediate times due to the existence of a superfluid critical momentum, while for lower dimensions the effect of thermal phonons dominates and obstructs the formation of such nonthermal states. Finally, unleashing the quasi‐spin degree of freedom, the authors observe and control spin‐exchange dynamics between individual impurities and the bath. Moreover, a regime of sympathetic impurity cooling is realized, where the internal‐state coherence of the impurity is preserved. In the future, control over the motional and spin‐degree of freedom will enable using individual impurities as thermalized, localized, minimally invasive quantum probes for a quantum fluid. Schmidt et al. review the paradigmatic model system of a single neutral impurity immersed in an ultracold gas. The interactions of impurity and bath are studied both, in the motional and internal degrees of freedom. Experimental investigations of the motional degrees of freedom for a granular bath and numerical studies of cooling in the superfluid regime shed light on the elastic part of this interaction. Employing the internal quasi‐spin of the impurity atoms, inelastic interaction which drives spin‐exchange dynamics is observed and coherence‐preserving sympathetic cooling is realized. These findings pave the way to use individual impurities as minimally invasive quantum probes.