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Observation of a Halo Trimer in an Ultracold Bose-Fermi Mixture

Alexander Y. Chuang1,*, Huan Q. Bui1,*, Arthur Christianen2,3,4,*, Yiming Zhang1, Yiqi Ni1, Denise Ahmed-Braun5, Carsten Robens1, and Martin Zwierlein1

  • *These authors contributed equally to this work.
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Phys. Rev. X 15, 021098 – Published 20 June, 2025

DOI: https://doi.org/10.1103/flty-9d72

Abstract

The quantum mechanics of three interacting particles gives rise to fundamental and universal phenomena, such as the staircase of Efimov trimers predicted in the context of nuclear physics and observed in ultracold gases. Here, we observe a novel type of halo trimer using radio-frequency spectroscopy in an ultracold mixture of Na23 and K40 atoms. The trimers consist of two light bosons and one heavy fermion, and they have the structure of a Feshbach dimer weakly bound to one additional boson. We find that the trimer peak closely follows the dimer resonance over the entire range of explored interaction strengths and across an order-of-magnitude variation of the dimer energy, as reproduced by our theoretical analysis. The presence of this halo trimer is of direct relevance for many-body physics in ultracold mixtures and the association of ultracold molecules.

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Popular Summary

When atoms form molecules, quantum mechanics determines how they bind. One well-known consequence is zero-point energy, the residual energy of the lowest bound state. Less familiar is the prediction of a highest bound state, where atoms remain loosely bound at distances far exceeding the range of their interaction. These are known as halo states. In our work, we report the observation of a novel type of three-particle halo state made of one potassium atom and two sodium atoms. Remarkably, the system’s size is comparable to that of a small bacterium.

Halo states are delicate and appear only in very cold, dilute gases. Halo dimers, consisting of two particles, are routinely created and play an important role in studies of pairing in quantum gases. Halo trimers, however, are rare and have so far only been directly observed in the case where the particles are identical and attract each other. Our experiment shows a new kind of halo trimer where the two sodium atoms repel one another and the difference in mass between sodium and potassium usually prevents trimer formation. Yet, when the interaction strength between sodium and potassium is just right, a trimer forms, consisting of a weakly bound Na-K dimer with the second sodium atom orbiting much farther away.

This finding broadens our understanding of few-body quantum states and opens new paths for studying universal few-body physics. Since interactions between small groups of particles can influence the behavior of larger systems, the existence of this trimer state could help explain and control the phases of complex quantum mixtures.

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Phys. Rev. X 15, 021098– Published 20 June, 2025
  • Received 7 November 2024
  • Accepted 1 May 2025
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