Interference of Bose-Einstein condensates: quantum non-local effects

TL;DR

This paper demonstrates that Bose-Einstein condensates in interferometers can exhibit quantum non-local effects and violations of classical inequalities, even with large numbers of particles, challenging traditional phase explanations.

Contribution

It introduces new quantum non-local effects and violations of Bell and Hardy inequalities using Bose-Einstein condensates without relying on spontaneous symmetry breaking.

Findings

Violations of BCHSH inequalities with two condensates.

New N-body Hardy impossibilities demonstrated.

GHZ-type contradictions with three condensates.

Abstract

Quantum systems in Fock states do not have a phase. When two or more Bose-Einstein condensates are sent into interferometers, they nevertheless acquire a relative phase under the effect of quantum measurements. The usual explanation relies on spontaneous symmetry breaking, where phases are ascribed to all condensates and treated as unknown classical quantities. However, this image is not always sufficient: when all particles are measured, quantum mechanics predicts probabilities that are sometimes in contradiction with it, as illustrated by quantum violations of local realism. In this letter, we show that interferometers can be used to demonstrate a large variety of violations with an arbitrarily large number of particles. With two independent condensates, we find violations of the BCHSH inequalities, as well as new N-body Hardy impossibilities. With three condensates, we obtain new GHZ (Greenberger, Horne and Zeilinger) type contradictions.