I would like to open this new topic to continue discussions on single photons which turned up in several other topics, but were a bit out of place there. So here the discussion continues. The first two quotes were basically about coincidence counting, photon statistics and the HBT-experiment. The last post was about polychromaticity of single photons.
I am not quite sure I know what you mean by "coming out of one slit". To get back to the HBT experiment: The coincidence count rates basically give you a method to check the nature of your light field. For example one could have a steady flow of exactly one photon every few ms. That would be a non-classical Fock state. You could have a randomly fluctuating number in the same time interval. That identifies coherent light. Or you might have strongly fluctuating photon numbers where you mostly have no light at all and lots of light at few times. hat corresponds to thermal light. I could back that up with some math, but I do not know whether that makes visualization easier.
Yes, you are right. Upon detection you will of course only have one energy "delivered" to the detector. Collapse to some eigenstate via measurement breaks superpositions. This is true for single photons just like in ordinary qm. It is just like asking for the momentum or emission direction of a single photon emitted by a symmetric emitter. You will detect it at some certain position, but until then it is typically in a superposition state of all possible directions. I am not quite sure how similar this is to photoelectronic emission. This is not really a topic I am familiar with, but I remember that it is more complicated than it looks at first sight, so I do not really dare to give a definitive answer.