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

TL;DR

This study experimentally observes and characterizes attractive and repulsive polarons in a Bose-Einstein condensate using radio frequency spectroscopy, confirming theoretical predictions and exploring impurity behavior in a bosonic medium.

Contribution

First experimental demonstration of impurity quasiparticles in a BEC, validating theories including three-body correlations across interaction regimes.

Findings

Observation of well-defined quasiparticle peaks at weak coupling

Spectrum broadening with increasing interaction strength

No significant three-body decay effects detected

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 $^{39}$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.