Quantum entanglement is a phenomenon in which particles can remain mysteriously connected across space. Another striking phenomenon of quantum mechanics is nonlocal entanglement, even when the particles are separated by vast distances. A measurement on one particle immediately affects the other, a fact that defies classical intuition.
Though countless experiments with photons and atomic internal states have verified this phenomenon, the dynamics of massive particles have remained a largely untested frontier, until now.
ANU quantum physicists have shown that the motion of ultracold helium atoms can be entangled. It is the first experimental demonstration of Bell correlations in the kinematics of massive particles. The findings confirm predictions made more than a century ago: Matter can be in two places at once and can interfere with itself even at those locations.
Bell’s inequality is a litmus test for quantum mechanics. It distinguishes between classical local realism, in which particles are independent and act only under the influence of nearby events, and nonlocal correlations that arise from quantum theory.
It is only when experiments show a violation of Bell’s inequality that we know quantum entanglement is at work, thereby rejecting the idea of hidden classical variables. Many such violations have been reported, mostly in systems based on photon polarization (massless photons) and atomic spin states.
A novel class of quantum particles behaves in unexpected ways
The specific joint probability distributions that Bell tests measure between two particles are called Bell correlations. These correlations serve as a fingerprint of quantum entanglement. The presence of what can be called ghost-lit paths gives a strong hint that the particles are connected nonlocally, which means in a way that classical physics cannot explain.
This new study is novel because it shows Bell correlations in the motion of massive particles, not just in their internal states.
The research team generated pairs of helium atoms with entangled momentum via controlled collisions at ultracold temperatures. To quantify their quantum correlations, they adapted a device called a Rarity-Tapster interferometer, which is normally used for photon experiments, for use with atoms.
Using the Bell-test formalism with extreme rigor, and with it implementing their novel atom–atom correlations that were strong enough to violate Bell’s inequality.
Yogesh Sridhar, lead author and PhD researcher, said, “Experimentally, it’s extremely hard to demonstrate this. Several people have tried in the past to show these effects, and they have always come short.”
A quantum experiment to observe wormhole dynamics
The breakthrough paves the way to answering one of the greatest long-known but unanswered questions of physics: how the strange laws that govern matter on small scales, quantum mechanics, fit together with the universal laws of gravity and general relativity that hold sway over the cosmos.
This accomplishment goes beyond the technical; it signifies a paradigm shift in our understanding of concepts. The scientists have uncovered a new way to put quantum mechanics to the test in situations where mass and movement matter: they found that atomic motion can exhibit Bell correlations.
The consequences are profound: from the design of gravitationally sensitive quantum sensors to the exploration of whether entanglement can survive in a classical-dominated world.
This study has broadened our understanding of the quantum world and its relationship to everyday reality by moving beyond the rules typically used for quantum behavior, which demonstrate that atoms can ‘dance’ in accordance with them.
Journal Reference:
Athreya, Y.S., Kannan, S., Yan, X.T. et al. Bell correlations between momentum-entangled pairs of 4He* atoms. Nat Commun 17, 2357 (2026). DOI: 10.1038/s41467-026-69070-3