An experiment on extremely cold helium atoms has demonstrated Bell correlations in the momentum states of massive particles. The result may open the way for new experiments at the interface between quantum mechanics and gravity.
Quantum entanglement is one of the strangest and most well-known phenomena in quantum mechanics: Two particles can be correlated in such a way that a measurement of one is immediately related to the result measured in the other, even when they are separated from each other. For decades, such entanglement has been demonstrated in light, atomic spins, and other quantum systems. Now a team of researchers from Australia and the United States has reported another step: an experimental demonstration of Bell correlations between pairs of helium atoms entangled in their state of motion—that is, their momentum.
The study was published inNature Communications.. The researchers used helium-4 atoms in an excited state and cooled them to extremely low temperatures, creating a Bose-Einstein condensation — a state in which the collection of atoms behaves as a single quantum system. They then split the atomic cloud into different orbitals, created collisions between the cloud components, and measured the pairs of atoms that scattered in opposite directions. According to the paper, this is an experimental demonstration of Bell correlations in the motions of massive atoms. (report)
Not just “where is the atom,” but also how does it move?
The uniqueness of the experiment is that it does not deal with an internal property of the atom, such as spin, but with its state of motion. The momentum of a particle describes the direction and speed of its motion along with its mass. When it comes to photons, massless particles of light, momentum entanglement is more familiar. In atoms, which are particles with mass and affected by gravity, it is more difficult to show cleanly that the motion itself is in an entangled state.
To do this, the researchers created pairs of helium atoms that scattered in coordinated pairs after s-wave collisions. Each such pair carried shared information about the momentum of both atoms. The researchers then used a version of the Rarity–Tapster interferometer for matter waves and examined the correlations between the atoms in a Bell test. The purpose of such a test is to see whether the correlations can be explained by a local classical picture, or whether they require a nonlocal quantum description. (report)
Bell test for atoms with mass
Bell tests are a key way to test the difference between the predictions of quantum mechanics and theories of local “hidden variables.” Until now, such experiments have been done mainly on photons or internal states of atoms. In the new paper, the researchers report that they have measured correlations between the momentum of atoms that are not explained by a wide class of classical local theories. (report)
This is important not because it “proves” that quantum mechanics is correct for the first time, but because it expands the type of systems in which it can be tested. If entanglement can be created and measured in the motion of massive atoms, it will be possible in the future to design experiments in which such quantum systems are also affected by gravitational fields. This is one reason why researchers point to the possibility of using cold atoms for fundamental tests of quantum mechanics and the effects of gravity on quantum states. (report)
A small step towards big questions
The experiment does not create a new quantum computer and does not lead to a practical gravity sensor tomorrow. It belongs first and foremost to fundamental physics: a precise examination of the way nature behaves when particles with mass are in delicate quantum states. However, this kind of fundamental physics is sometimes the basis for future technologies. Atom interferometers, for example, are already being used to develop precise sensors for measuring acceleration, gravity and tiny changes in physical fields.
Next, similar experiments might try to create even more complex situations—for example, entanglement between different types of atoms or between states of motion that respond differently to gravity. If successful, such experiments could provide a new way to test the boundary between quantum mechanics, which describes the tiny world, and general relativity, which describes gravity and space-time.
Therefore, the importance of the research is not in the drama of “action at a distance,” but rather in the exact opposite: in the ability to take a known quantum phenomenon, transfer it to a system that is more difficult to measure—massive atoms in motion—and test it with precise experimental tools. This is another step on the long road to connecting quantum physics with the world of heavier and larger phenomena.
Scientific source:
YS Athreya, S. Kannan, XT Yan et al., “Bell correlations between momentum-entangled pairs of 4He* atoms”, Nature Communications 17, 2357, published February 4, 2026. DOI: 10.1038/s41467-026-69070-3. (report)
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Short FAQ:
What is quantum entanglement?
Quantum entanglement is a situation in which two particles are described as a single system, such that a measurement of one is related to a measurement of the other in ways that are not explained by local classical physics.
What's new in this experiment?
The novelty is that entanglement was demonstrated in the state of motion, or momentum, of helium atoms with mass, and not just in light or the internal properties of particles.
why is it important?
Atoms with mass are affected by gravity, so such experiments may allow for new tests of the relationship between quantum mechanics and the theory of gravity in the future.
Will this immediately lead to new technology?
Not immediately. This is primarily an experiment in fundamental physics, but it may contribute in the future to the development of precise quantum sensors and experiments in the field of quantum gravity.
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A great article that explains complex physics concepts in a simplified way