A team of physicists has for the first time violated Bell's inequality using not internal states but momentum states—that is, states related to motion—of massive particles. The experiment involved pairs of metastable helium atoms entangled in momentum, which were manipulated in a matter-wave interferometer. The result S = 2,52 ± 0,17 exceeded the classical limit 2 by three standard deviations.
The work was carried out by researchers from the Australian National University: J. S. Atreya, S. Kannan, S. S. Hodgman, and colleagues. They generated pairs of helium-4 atoms in a metastable state through s-wave collisions in an ultracold gas. The pairs were then separated and their trajectories manipulated using coherent matter-wave interferometry—an analogue of an optical interferometer, but for atoms.
The key point: momentum entanglement means that measuring the velocity of one atom instantly determines the velocity of the second, regardless of distance. This is not just correlation but genuine nonlocality in momentum space. Imagine two balls flying apart in opposite directions: if one suddenly 'chooses' a direction, the other immediately adjusts, although no signal could have passed between them.
The experiment completed a long-standing goal of quantum atom optics—to transfer Bell tests from photons and spins to the external degrees of freedom of massive particles. Previously, such tests were conducted only with internal states of atoms or with light photons. Now the inequality has been violated specifically for the motion of heavy objects.
This opens the way to studying quantum nonlocality in a gravitational field. Massive particles feel gravity more strongly, so the next step is to test how gravity affects momentum entanglement. According to the preprint published on 2 September 2026 on arXiv, the work lays the foundation for future experiments at the intersection of quantum mechanics and general relativity.
The results obtained with the matter-wave interferometer show that quantum nonlocality is not limited to the microscopic world of light particles. Massive atoms also obey the same strange rules, and now this has been proven directly.
