Observation of Attractive and Repulsive Polarons in a Bose Einstein Condensate.
The problem of an impurity particle moving through a bosonic medium plays a fundamental role in physics. However, the canonical scenario of a mobile impurity immersed in a Bose Einstein condensate (BEC) has not yet been realized. Here, we use radio frequency spectroscopy of ultracold bosonic {39}K atoms to experimentally demonstrate the existence of a well defined quasiparticle state of an impurity interacting with a BEC. We measure the energy of the impurity both for attractive and repulsive interactions, and find excellent agreement with theories that incorporate three body correlations, both in the weak coupling limits and across unitarity. The spectral response consists of a well defined quasiparticle peak at weak coupling, while for increasing interaction strength, the spectrum is strongly broadened and becomes dominated by the many body continuum of excited states. Crucially, no significant effects of three body decay are observed. Our results open up exciting prospects for studying mobile impurities in a bosonic environment and strongly interacting Bose systems in general.
Physical review letters