I thought that it would be possible to get a single photon to a double slit which would be sufficiently coherent to create an interference pattern. You would use entangled photon pairs, which I realize is not coherent light to start with. And use the Alice stream to herald the Bob stream. But cannot you take the Bob stream and run it through some magic box with a pinhole exit (causing it to no longer be entangled - and of course I am kidding about a magic box) and then end up with a coherent source? You would not be obtaining any information about Alice at that point. So I guess I am saying I don't suitably understand how a single photon is incoherent if you can diffract it through a point source and send to a double slit. I can visualize that you cannot have destructive interference from the source, and I guess to some extent that means no uncertainty in the source location. So the basic idea is to build up an interference pattern and proving it is done one photon at a time. I swear I have seen references to just that.