Back in October, while contributing to a thread in this forum, I ran into an interesting description of a entanglement/double slit interference experiment. One of the contributors cited a 1999 "Reviews of Modern Physics" article. This remark is made on page S290: Let me see if I can explain this more simply. If we call that "time reversal" photon a "Klyshko photon", then what David Klyshko was "arguing" in some other 1988 article (that I have not located) was that before the big photon reaches the down-converter and splits into two entangled little photons, a little "Klyshko photon" is emitted from the experiment to meet the big photon at the down-converter. To be clear, this is a thoroughly bizarre notion that could not possibly exist in classical mechanics - and it's not clear that the author even considers it "real". In fact, I'm not even certain that David Klyshko considered it real. But it is testable and I would love to know if anyone has ever checked it. If you put a shutter in the apparatus so that the path of the Klyshko photo is blocked only at the moment when the Klyshko photo would be reaching that shutter, would you still be able to retrieve the interference pattern that is collected in that experiment?