Observation of Attractive and Repulsive Polarons in a Bose-Einstein Condensate

TLDR

The motion of an impurity in a bosonic medium is a fundamental problem, yet the canonical case of a mobile impurity in a Bose‑Einstein condensate has not been realized. The study experimentally demonstrates a well‑defined quasiparticle state of an impurity in a BEC using radio‑frequency spectroscopy of ultracold K39 atoms. The authors measured impurity energies for attractive and repulsive interactions via spectroscopy, finding agreement with three‑body‑correlation theories across weak coupling and unitarity. The spectral response shows a well‑defined quasiparticle peak at weak coupling, broadens and is dominated by a many‑body continuum at stronger interactions, with no significant three‑body decay, opening prospects for studying mobile impurities and strongly interacting Bose systems.

Abstract

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 K39 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.