I've read in many places that if, in a Young's double slit experiment, you can determine by whatever method through which slit your photon goes through (enhancing the "particle" behavior of light), then the interference pattern dissapears. For this reason Zeilinger says that entangled photons do not produce interference patterns (because one could always use one photon of the pair to determine through which slit the other photon goes through). And related with this is the fact stated by Dirac that interference between different photons never occurs. BUT: a) A friend of me works at GSI with neutral pions. Neutral pions decay to two entangled photons going in opposite different directions. And she says that they work routinely with the interference of these photons. b) In 1963, Mandel and Magyar published an experiment in Nature (doi:10.1038/198255a0) where they produced interference fringes using two independent laser beams. The exit window of every laser can be considered as a single slit, so you have a double slit experiment which produces an interference pattern, where you know perfectly through which slit every photon has passed. How these facts reconcile with the above statements?