A team of physicists has shown that pairs of helium atoms, chilled to near absolute zero and launched into each other, can behave as though each atom travels two separate paths at the same time. The experiment, published in Nature Communications in spring 2026, marks the first time researchers have observed Bell-type correlations in the momentum of entangled atoms, extending a landmark quantum test from the world of light into the heavier, messier world of matter.
Why this experiment matters
For decades, physicists have used photons to probe one of quantum mechanics’ strangest predictions: that two entangled particles can be correlated so tightly that no classical explanation can account for the results. These experiments, known as Bell tests after physicist John Bell, have been refined to the point where every major loophole has been closed for light. But photons are massless. The question of whether atoms, which have mass, inertia, and interact with their surroundings far more readily, can pass the same test has remained largely open.
The new experiment tackles that question head-on. Researchers collided two Bose-Einstein condensates, clouds of metastable helium-4 atoms cooled so close to absolute zero that they merge into a single quantum state. When these clouds smash together, they spit out pairs of atoms flying in opposite directions with tightly linked momenta. Think of it like smashing two water balloons and finding that every droplet on the left is perfectly matched to a partner droplet on the right, not just in speed but in a way that defies any classical bookkeeping.
From spin to motion
This result builds on earlier work by the same research program. In 2019, the group demonstrated Bell-type correlations between the spins of entangled helium atom pairs produced through the same collision technique. Spin is an intrinsic quantum property, somewhat like a tiny internal compass needle. That earlier experiment proved the platform could generate and detect entangled atoms one by one.
The new work shifts the entanglement from spin to momentum, a fundamentally different property that describes how each atom moves through space. To test whether the momentum correlations exceeded classical limits, the team adapted a scheme called the Rarity-Tapster interferometer, originally designed for photons. In the optical version, a photon is split across two paths and then recombined; the resulting interference pattern reveals whether the photon’s behavior can be explained without quantum mechanics. Here, the researchers replaced photons with helium atoms, each about four atomic mass units heavy, and used atom interferometers to split and recombine the atoms’ paths.
The correlations they measured went beyond what any classical model of independent particles could produce, a signature that physicists call Bell-type correlations. In practical terms, the atoms acted as if each one traveled both paths through the interferometer simultaneously, only “choosing” a definite path when detected.
What the experiment does not yet prove
There is an important distinction between observing Bell-type correlations and achieving a full, loophole-free Bell inequality violation. The researchers are careful about this boundary. A comparison between the team’s earlier preprint and the final peer-reviewed paper shows that statistical claims were tightened during review, with updated error bars and clarified language around the strength of the violation. This kind of refinement is standard in experimental physics, but it means the margin by which the correlations exceed classical thresholds is modest, and the robustness of that margin under alternative analysis methods has not been exhaustively tested.
More fundamentally, no experiment with massive particles has yet closed the loopholes that photon-based Bell tests sealed over a decade ago. Two loopholes matter most. The detection loophole requires that a sufficiently high fraction of particle pairs actually be detected; miss too many, and classical models can mimic quantum correlations through selective sampling. The locality loophole requires that the two measurement stations be far enough apart, and the measurements fast enough, that no signal traveling at the speed of light could carry information between them during the test. Atoms move far more slowly than photons, making the locality condition harder to enforce, and single-atom detection efficiency, while impressive for metastable helium, still falls short of the thresholds needed.
The published paper does not claim to have closed either loophole. Doing so will likely require a larger apparatus, faster measurement switching, and improved detectors.
The road ahead for matter-wave Bell tests
Theoretical proposals have outlined how to push these experiments further. One approach would use collisions between two different helium isotopes, helium-3 and helium-4, exploiting their mass difference to create tunable interference patterns and potentially sharpen sensitivity to the quantum correlations. But no group has yet managed to simultaneously cool and collide two isotopes in a single apparatus, and the technical challenges are substantial.
There is also the question of generalizability. The detection method used here relies on a quirk of metastable helium: these atoms carry a large amount of internal energy, so when they strike a detector surface, they release enough energy to be counted individually. Ground-state atoms and heavier species lack this advantage, meaning that extending Bell-type tests to other kinds of matter may require entirely different detection technologies, such as fluorescence imaging or cavity-enhanced readout.
The experiment sits within a broader European research effort, the TwinAtoms project, which has been funded specifically to produce entangled atom pairs for tests of quantum nonlocality. That institutional backing signals sustained interest in the field and suggests that follow-up experiments are already in the pipeline.
Why massive-particle Bell tests open new doors
For physicists, the significance is conceptual as much as technical. Bell tests with photons established that quantum entanglement is real and not just a mathematical convenience. Extending those tests to atoms, which have mass and interact gravitationally, opens the door to probing whether quantum mechanics holds up in regimes where gravity might start to matter. Some theoretical frameworks predict that gravity could cause quantum correlations to break down for sufficiently massive objects. Helium atoms are far too light to test those predictions directly, but they represent a critical stepping stone toward experiments with heavier particles or even small molecules.
For now, the result stands as a clear demonstration that the conceptual toolkit developed over decades of photon experiments can be translated into the domain of matter waves. Entangled helium atoms, born from the collision of quantum clouds and probed with interferometers borrowed from optics, behave in ways that classical physics cannot explain. Closing the remaining loopholes will take years of engineering. But the atoms, at least, appear to be cooperating with quantum theory.
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*This article was researched with the help of AI, with human editors creating the final content.
