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Realization of Repulsive Polarons in the Strongly Correlated Regime

R. Henke1, J. Levinsen2, M. M. Parish2, J. Boronat3, G. E. Astrakharchik3, H. Moritz1,4,*, and C. R. Cabrera1,4,5,†

  • *Contact author: hennning.moritz@uni-hamburg.de
  • †Contact author: cesar.cabrera@uni-hamburg.de
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Phys. Rev. X 16, 041040 – Published 13 November, 2026

DOI: https://doi.org/10.1103/m5qj-j8bm

Abstract

Mobile impurities interacting with a quantum medium form quasiparticles known as polarons, a central concept in many-body physics. While the quantum impurity problem has been extensively studied with ultracold atomic gases, repulsive polarons in the strongly correlated regime have remained elusive. Typically, the impurity atoms bind into molecules or rapidly decay into deeper-lying states before they can acquire an appreciable dressing cloud. Here, we report on the realization of polarons in a strongly repulsive two-dimensional quantum gas. Using a superfluid of Li6 dimers, we introduce impurities by promoting a small fraction of the dimers into higher levels of the transverse confining potential. These novel synthetic-spin polarons give access to the strongly repulsive regime where common decay channels are suppressed. We extract key polaron properties—the energy, quasiparticle residue, and effective mass—using trap modulation and Bragg spectroscopy. Our measurements are well captured by a microscopic T-matrix approach and quantum Monte Carlo simulations, revealing deviations from mean-field predictions. In particular, we measure a significant enhancement of the polaron mass, with values exceeding twice the free-dimer mass. Our demonstration of a stable repulsive Bose polaron establishes a platform for studying impurity physics in low-dimensional and strongly correlated systems.

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Popular Summary

An impurity moving through a quantum medium can attract or repel surrounding particles, become dressed by its environment, and form a quasiparticle known as a polaron. However, repulsive polarons typically decay before the strongly interacting regime can be explored. We overcome this fundamental limitation and realize a stable repulsive Bose polaron in a two-dimensional quantum gas by encoding the impurity and its surrounding medium in different vibrational states of the confining potential. This technique suppresses dominant decay processes, thereby allowing us to access the strongly interacting regime and measure fundamental properties including the energy, quasiparticle residue, and effective mass. Through these measurements, we observe that many-body interactions increase the polaron mass to more than twice the bare mass. Our approach provides a platform for investigating how quasiparticles emerge and move through strongly correlated quantum matter in low dimensions.

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Phys. Rev. X 16, 041040– Published 13 November, 2026
  • Received 11 December 2025
  • Accepted 24 August 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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