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In ultra-peripheral relativistic heavy-ion collisions, a photon from theelectromagnetic field of one nucleus can fluctuate to a quark-antiquark pairand scatter from the other nucleus, emerging as a $\rho^0$. The $\rho^0$production occurs in two well-separated (median impact parameters of 20 and 40fermi for the cases considered here) nuclei, so the system forms a 2-sourceinterferometer. At low transverse momenta, the two amplitudes interferedestructively, suppressing $\rho^0$ production. Since the $\rho^0$ decay beforethe production amplitudes from the two sources can overlap, the two-pion systemcan only be described with an entangled non-local wave function, and is thus anexample of the Einstein-Podolsky-Rosen paradox. We observe this suppression in200 GeV per nucleon-pair gold-gold collisions. The interference is $87% \pm 5%{\rm (stat.)}\pm 8%$ (syst.) of the expected level. This translates into alimit on decoherence due to wave function collapse or other factors, of 23% atthe 90% confidence level.

Observation of Two-source Interference in the Photoproduction Reaction $Au Au \to Au Au \rho^0$