The decay of ortho-positronium into three photons produces a physical realization of a pure state with three-party entanglement. Its quantum correlations are analyzed using recent results on quantum information theory, looking for the final state which has the maximal amount of GHZ-like correlations. This state allows for a statistical dismissal of local realism stronger than the one obtained using any entangled state of two spin one-half particles.
@article{arxiv.quant-ph/0007080,
title = {Three-party entanglement from positronium},
author = {A. Acin and J. I. Latorre and P. Pascual},
journal= {arXiv preprint arXiv:quant-ph/0007080},
year = {2009}
}