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An electron and its antiparticle — the positron, predicted by Paul Dirac, — upon meeting, momentarily form Standard Model: a tiny pair circling each other, like in an atom but without a nucleus. Almost instantly it decays, emitting two photons that turn out to be mysteriously linked. This connection, entropy, resembles a pair of magic coins: no matter how far apart they fly, if one shows 'heads,' the other always shows 'tails.' John Stewart Bell proved that such behavior is not a trick but a fundamental property of the microworld.

To test this, physicists came up with the following: they 'push' one photon against an electron (a phenomenon known as spectroscopy). By seeing how the photon bounces, one can tell which side our coin landed on. And as soon as we know that, the second photon instantly becomes definite, even if it has already traveled to the other end of the galaxy. This experiment will pave the way to creating perfectly secure communication channels and quantum computers.

Original abstract

as written by the authors arXiv:2606.31726

Electron and positron can form a meta-stable bound state called positronium that decays via pair annihilation. We show how polarization-dependent Compton scattering can be used to verify that the two annihilation photons in the spin-zero case (para-positronium) are emitted in a maximally entangled Bell state. Our theoretical approach based on two-photon density matrices connects concepts from relativistic quantum electrodynamics and quantum information theory.