Abstract
Nonlocal entanglement between pair-correlated particles is a highly counter-intuitive aspect of quantum mechanics, where measurement on one particle can instantly affect the other, regardless of distance. While the rigorous Bell’s inequality framework has enabled the demonstration of such entanglement in photons and atomic internal states, no experiment has yet involved motional states of massive particles. Here we report the experimental observation of Bell correlations in motional states of momentum-entangled ultracold helium atoms. Momentum-entangled pairs are first generated via s-wave collisions. Using a Rarity-Tapster interferometer and a Bell-test framework, we observe atom-atom correlations required for violation of a Bell inequality. This result shows the potential of ultracold atoms for fundamental tests of quantum mechanics and opens new avenues to studying gravitational effects in quantum states.
Data availability
The data that support the findings of this study are available from Zenodo at63.
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Research data available from Zenodo at https://doi.org/10.5281/zenodo.17939482.
Acknowledgements
We would like to thank K.F. Thomas for technical assistance with the early stages of the experiment, and S.A. Haine for helpful discussions. This work was supported through the Australian Research Council (ARC) Discovery Projects, Grant No. DP190103021, DP240101346, DP240101441 and DP240101033. S.S.H. was supported by the Australian Research Council Future Fellowship Grant No. FT220100670. S.K. was supported by an Australian Government Research Training Program scholarship.
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Y.S.A. and S.K. performed the experiment and collected the data under the supervision of A.G.T. and S.S.H. All authors contributed to the design of the experiment, the conceptual formulation of the physics and the interpretation of the data. Y.S.A., R.J.L.-S., K.V.K. and S.S.H. wrote the manuscript with input from all authors.
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Athreya, Y.S., Kannan, S., Yan, X.T. et al. Bell correlations between momentum-entangled pairs of 4He* atoms. Nat Commun (2026). https://doi.org/10.1038/s41467-026-69070-3
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DOI: https://doi.org/10.1038/s41467-026-69070-3
