Being in Two Places at Once Is Not Just a Quantum Quirk. It Happens to Real Atoms, Too

Quantum mechanics makes a very strange demand on reality. Two particles, once entangled, share a fate that persists regardless of the distance between them: measure one, and the other responds instantly, as if space meant nothing at all. Einstein found this so objectionable he spent the last decades of his career trying to explain it away. He failed, and in the sixty years since his death, increasingly rigorous experiments have confirmed it at every turn. What those experiments had never managed, until now, was to demonstrate the effect in atoms moving through space, particles with real mass that fall under gravity, the kind of matter that makes up everything you can touch. A team at the Australian National University has just done exactly that.

The result, published in Nature Communications, is the first observation of so-called Bell correlations in the motional states of massive particles. Every previous Bell test of this kind relied on photons, particles of light that have no mass and do not experience gravity. Helium atoms are different in ways that matter enormously for physics.

The distinction sounds technical. It is not. When physicists test quantum mechanics using light, they are asking whether the universe’s strangest predictions hold for the most abstract, massless constituents of nature. Asking the same question of atoms, objects that follow the same gravitational rules as the moon or a falling apple, opens an entirely different class of experiments. The gap between quantum mechanics and general relativity, the two great theories that together describe almost everything and yet remain fundamentally incompatible, might finally have somewhere to breathe. Momentum-entangled atoms give physicists a lever to pry at that gap directly; a tool, as one recent theoretical proposal put it, for testing theories about what gravity does to quantum states.

The experiment itself begins in a magnetic trap, where roughly 100,000 helium atoms are cooled to within a whisker of absolute zero, forming what physicists call a Bose-Einstein condensate. At that temperature the distinction between individual atoms blurs, and the cloud begins to behave as a single quantum entity.

From there, laser pulses do the work. A carefully timed sequence splits the condensate into several groups moving at different speeds, then sets them on a collision course. When two groups of ultracold atoms collide at this scale, they do not scatter chaotically. Instead, pairs of atoms emerge flying in exactly opposite directions, their momenta linked by conservation laws in the same way that entangled photon pairs are linked in optical experiments. The process is the matter-wave equivalent of spontaneous parametric down-conversion, the workhorse of quantum optics laboratories everywhere.

The pairs fan out into spherical halos in momentum space, each atom at one point of the halo, its partner diametrically opposite. To test whether these pairs are genuinely entangled in the quantum sense, rather than merely correlated in some more mundane way, the team needed to route them through an interferometer and measure their joint behaviour at the output. This required adapting a photonic technique, the Rarity-Tapster interferometer, for use with matter waves, threading each atom pair through separate arms defined by laser pulses rather than mirrors and beam splitters made of glass.

The numbers that emerged were unambiguous. The Bell correlation function the team measured had an amplitude of 0.86, comfortably above the threshold of roughly 0.71 that marks the boundary between what quantum mechanics predicts and what any classical description of hidden variables could plausibly explain. The maximum violation of the relevant steering inequality reached 1.752, about 3.9 standard deviations clear of the classical limit. “A particle can be in two places at once,” said Sean Hodgman, who led the research group, describing a result that confirmed quantum theory’s most counterintuitive prediction in a physical regime where it had never been tested. The team ran more than 35,000 experimental shots to accumulate the statistics needed to make the claim with confidence.

There is a caveat, and it is an honest one. The experiment has not yet closed the so-called locality loophole, the requirement that the two atoms be far enough apart when their measurements are made that no signal, even one travelling at light speed, could connect them. Closing it would require separating the atoms by at least 30 centimetres before measuring; the current detector is 8 centimetres across. This does not undermine the result, which is a genuine observation of Bell correlations, but it means the most rigorous possible Bell test, one that would rule out even the most exotic classical explanations, remains a target rather than an achievement. Future experiments, the authors note, could use independently controllable phase settings to push toward a fully loophole-free violation.

Yogesh Sridhar, the PhD researcher who led the experimental work, noted that many groups had attempted to reach this point over the years without success. The ANU team got there through a series of incremental improvements: more efficient atom detectors, better frequency stabilisation of the laser beams, and tighter filtering of the momentum windows used for analysis.

Where the result points is perhaps more interesting than the result itself. Momentum-entangled atoms of different masses could, in principle, test whether the weak equivalence principle, the bedrock assumption that all objects fall at the same rate regardless of their composition, holds at the quantum level. It almost certainly does, but nobody has checked using entangled quantum test masses, and the answer is not as obvious as it sounds once quantum superposition is involved. The ANU group has already proposed experiments using pairs of helium isotopes, 3He and 4He, for exactly this purpose.

Beyond equivalence principle tests, there is the deeper question of what gravity does to a quantum system at all. Roger Penrose argued decades ago that gravity might play a role in collapsing quantum superpositions, that there is a mass threshold above which quantum states simply cannot maintain coherence. Experiments using momentum-entangled atoms could probe precisely this kind of decoherence, watching for signatures that quantum mechanics alone cannot explain. The helium apparatus in Canberra may not be large enough or sensitive enough to settle that question. But the question now has a new kind of experimental handle on it.

DOI / Source: https://doi.org/10.1038/s41467-026-69070-3