Two dancers on the same stage mirror each other's moves even when a curtain separates them. In the quantum world, such pairs are called entangled, and physicists have spent half a century probing how far this connection reaches. Until now, all tests with massive particles concerned their internal properties, such as spin, but not their motion itself. Yet it is motion that determines how matter spreads through space and responds to gravity.
Now Australian physicists have made the atoms' momenta themselves dance. They cooled a cloud of helium into a Bose-Einstein condensate, where atoms behave like a single wave, and then a short laser pulse split that wave into three parts. When the parts collided, they produced pairs of atoms flying in opposite directions, like two dancers who receive a shared push from a single shove. Their momenta were entangled: knowing the direction of one allowed predicting the direction of the other.
After splitting, the pairs passed through an interferometer that brought their paths together, like two light beams in a classic experiment. A detector on microchannel plates reconstructed the three-dimensional momenta of the atoms, allowing correlations to be measured. The result was incompatible with any prearranged scenario: the CHSH parameter reached 2.52 ± 0.17, exceeding the classical limit of 2 by about three standard deviations.
This means the connection between the atoms' momenta cannot be explained by a common past or hidden instructions. The dancers truly sense each other across the void, and their motions are correlated more strongly than any local realism permits. For the first time, nonlocality has been demonstrated for the external degrees of freedom of massive particles, that is, for their very motion through space.
The practical benefits are not yet obvious, but the platform opens the way to quantum sensors and tests of quantum gravity. Perhaps one day such experiments will help understand how quantum mechanics coexists with curved spacetime. For now, two helium atoms flying apart continue their silent dance, confirming that the quantum world recognizes no boundaries.
The detector was located 848 mm below the trap, and the atoms fell onto it for about 0.416 seconds, reaching a speed of roughly 65 mm/s at the moment of detection.