  The violation of a Bell inequality is a striking demonstration of how quantum
mechanics contradicts local realism. Although the original argument was
presented with a pair of spin 1/2 particles, so far Bell inequalities have been
shown to be violated using entangled pairs of photons, with recent measurements
closing all possible loopholes in such a scheme. Equivalent demonstrations
using massive particles have proven to be much more challenging, generally
relying on post-selection of data or measuring an entanglement witness that
relies on quantum mechanics. Here, we use a collision between two Bose-Einstein
condensates to generate the momentum-spin entangled pairs of ultracold helium
atoms. We show that a maximally entangled Bell triplet state results and report
a direct observation of a strong Bell correlation witness. Based on the high
degree of entanglement and the controllability of ultracold atomic systems,
extensions to this scheme would allow a demonstration of nonlocality with
massive entangled pairs, following the original idea of Bell. Other
applications include the demonstration of the Einstein-Podolsky-Rosen paradox,
quantum metrology and tests of phenomena from exotic theories sensitive to such
systems including gravitational decoherence and quantum gravity.
