If one beam from SPDC is sent to a double-slit or similar experiment, it can form interference if the which-way information is erased in the other beam. To detect it, photons have to be counted in coincidence because there will be both interference and "anti-interference" that add up to a single bump with none of the structure. This is a fundamental effect of creating entangled beams. What are the properties of the anti-interference photons? It seems to me that they would have to be 180 degrees out of phase between the two slits in order to reverse the probabilities of where they will land on the screen. When normal light (as in what Thomas Young used, or a non-downconverted laser beam) hits the slits, the wave splits and goes through both slits where each sub-wave is in phase. How can light end up 180 degrees out of phase, or is the mechanism different than this?