Bell correlations between momentum-entangled pairs of $^4\text{He}^*$ atoms

TL;DR

This paper reports the first experimental observation of Bell correlations in the motional states of momentum-entangled ultracold helium atoms, demonstrating nonlocal quantum entanglement in massive particles' motion.

Contribution

It presents the first demonstration of Bell correlations in the motional states of massive particles, using ultracold helium atoms and a Bell-test framework.

Findings

Bell inequality violation observed in motional states

Momentum-entangled pairs generated via s-wave collisions

Potential for fundamental quantum mechanics tests with ultracold atoms

Abstract

Nonlocal entanglement between pair-correlated particles is a highly counter-intuitive aspect of quantum mechanics, where measurement on one particle can instantly affect the other, regardless of distance. While the rigorous Bell's inequality framework has enabled the demonstration of such entanglement in photons and atomic internal states, no experiment has yet involved motional states of massive particles. Here we report the experimental observation of Bell correlations in motional states of momentum-entangled ultracold helium atoms. Momentum-entangled pairs are generated via $s$-wave collisions. Using a Rarity-Tapster interferometer and a Bell-test framework, we observe atom-atom correlations required for violation of a Bell inequality. This result shows the potential of ultracold atoms for fundamental tests of quantum mechanics and opens new avenues to studying gravitational effects in quantum states.