Radiofrequency Spectroscopy of Bose Polarons with Strong Final State Interactions

The fate of impurities immersed in a quantum bath is a fundamental problem in many-body physics. In experiments on Bose-Fermi mixtures of Na-40K we have previously observed the formation of Bose polarons, impurity atoms strongly coupled to a Bose-Einstein condensate [1]. The nature of these polarons and their excitations in the case where a two-body bound state is supported by the interatomic potential is not well understood. We study this energy landscape by starting with a more weakly coupled Bose polaron and exploring the final states it can be coupled to via radiofrequency spectroscopy . We choose the final spin of the impurity such that the impurity-boson scattering length is positive, and thus supports two-body bound states (i.e. heteronuclear Feshbach molecules). We find that two-body physics is insufficient to explain our spectra, and instead reveal many-particle correlations between the impurity and the host bosons.

[1] Z. Z. Yan, Y. Ni, C. Robens, and M. W. Zwierlein, Science 368, 190 (2020).