Hippies, Bell tests, and a career studying quantum entanglement

Lukas Neumeier, Jun Matsuura, James S. Douglas

David I. Kaiser

F.J. Duarte

Y. S. Athreya, Suresh Kannan, X. T. Yan +4 more

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 first 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. Momentum-entangled atom pairs are used to demonstrate quantum non-locality, where changing one atom in an entangled pair instantly alters the state of the other atom. This result paves the way to study interactions between quantum states and gravity.

Xianming Meng

Quantum entanglement is an intriguing concept that is vital to quantum computing, quantum communication and teleportation. The numerous experiments called Bell tests act as evidence of quantum entanglement. However, the exact meaning of quantum entanglement is somewhat vague. One interpretation is the instant inner connection between totally separated entangled particles, or the ‘spooky action at a distance’ termed by Einstein. The paper has examined the Bell tests from different perspectives and discovered that although the Bell tests have disproved the local hidden variable hypothesis and proved nonlocal correlation, they do not automatically endorse any inner connection between totally separated entangled particles, such as superluminal communication and superposition over a large distance. The measurement on one entangled particle does not change the quantum state of its counterpart, so any inner connection is not necessary. The nonlocal correlation shown by Bell tests can be explained by coherence states of entangled particles and the common interaction between particles and measuring instruments. The paper concludes that the essence of quantum entanglement is the coherence of quantum states.

Hoshang Heydari