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Formation of ultracold K39Cs133 Feshbach molecules

Charly Beulenkamp1,*, Krzysztof P. Zamarski1,*, Robert C. Bird2, C. Ruth Le Sueur2, Jeremy M. Hutson2, Manuele Landini1, and Hanns-Christoph Nägerl1,†

  • 1Universität Innsbruck, Institut für Experimentalphysik und Zentrum für Quantenphysik, Technikerstraße 25, 6020 Innsbruck, Austria
  • 2Joint Quantum Centre (JQC) Durham-Newcastle, Department of Chemistry, Durham University, South Road, Durham, DH1 3LE, United Kingdom

  • *These authors contributed equally to this work.
  • †Contact author: christoph.naegerl@uibk.ac.at
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Phys. Rev. A 112, 062821 – Published 22 December, 2025

DOI: https://doi.org/10.1103/kqyr-mcx9

Abstract

We report the creation of an ultracold gas of bosonic K39Cs133 molecules. We first demonstrate a cooling strategy relying on sympathetic cooling of Cs133 to produce an ultracold mixture. From this mixture, weakly bound molecules are formed using a Feshbach resonance at 361.7 G. The molecular gas contains 7.6(10)×103 molecules with a lifetime of about 130 ms, limited by two-body decay. We perform Feshbach spectroscopy to observe several new interspecies resonances and characterize the bound state used for magnetoassociation. Finally, we fit the combined results to obtain improved K-Cs interaction potentials. This provides a good starting point for the creation of ultracold samples of ground-state K39Cs133 molecules.

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Phys. Rev. A 112, 062821– Published 22 December, 2025
  • Received 7 July 2025
  • Accepted 28 October 2025
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