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Universal Quantum Dynamics of Bose Polarons

Jiří Etrych, Gevorg Martirosyan, Alec Cao*, Christopher J. Ho, Zoran Hadzibabic, and Christoph Eigen†

  • *Present address: JILA, NIST, and Department of Physics, University of Colorado, Boulder, Colorado 80309, USA.
  • †Contact author: ce330@cam.ac.uk
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Phys. Rev. X 15, 021070 – Published 28 May, 2025

DOI: https://doi.org/10.1103/PhysRevX.15.021070

Abstract

Predicting the emergent properties of impurities immersed in a quantum bath is a fundamental challenge that can defy quasiparticle treatments. Here, we measure the spectral properties and real-time dynamics of mobile impurities injected into a weakly interacting homogeneous Bose-Einstein condensate, using two broad Feshbach resonances to tune both the impurity-bath and intrabath interactions. For attractive impurity-bath interactions, the impurity spectrum features a single branch, which away from the resonance corresponds to a well-defined attractive polaron; near the resonance, we observe dramatic broadening of this branch, suggesting a breakdown of the quasiparticle picture. For repulsive impurity-bath interactions, the spectrum features two branches: the attractive branch that is dominated by excitations with energy close to that of the Feshbach dimer, but has a many-body character, and the repulsive polaron branch. Our measurements show that the behavior of impurities in weakly interacting baths is remarkably universal, controlled only by the bath density and a single dimensionless interaction parameter.

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Popular Summary

A particle moving through a quantum medium can often be described as a quasiparticle—composed of the excitations of the medium and the bare impurity. These quasiparticles, known as polarons, were introduced over 90 years ago in the context of electrons moving through a crystal. Ultracold atomic gases offer an excellent platform for studies of polaron physics, where impurity atoms can readily be immersed in a highly controllable majority-atom medium. For fermionic gases, the behavior of polarons is well understood nowadays, and the quasiparticle picture remains valid even under strong interaction. However, less is known about how this picture applies in Bose-Einstein condensates (BECs), fragile quantum systems that can be strongly altered by the presence of an impurity. In this study, we address this gap by exploring the dynamics of impurities in BECs and examining how strong interactions affect quasiparticle behavior.

Using radio-frequency spectroscopy and interferometry, we investigate how the dynamics of K39 atoms immersed in a BEC depend on the strength of their interactions with the surrounding medium. Our findings reveal that, despite the breakdown of the usual quasiparticle picture under strong interactions, the impurity dynamics still adhere to simple scaling laws across all interaction strengths. In cases of repulsive interactions, we even identify two distinct spectral branches: one associated with a repulsive polaron and another related to a broad, attractive dimer state exhibiting complex many-body effects.

Our results suggest that universal scaling laws govern impurity dynamics in BECs, even when traditional models fail. In the future, we plan to explore the effects of higher impurity concentrations and temperatures.

Article Text

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Phys. Rev. X 15, 021070– Published 28 May, 2025
  • Received 11 April 2024
  • Accepted 7 April 2025
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