Single Photon Detectors

[lesolee]

I have been reading a book on QED by Richard Feynman (The Strange Theory of Light and Matter). In it he describes what appears to be a real dual slit experiment using a low intensity source into a single photon detector. I have found avalanche detectors and photomultipliers and that is all well and good. Where I have a problem is when he puts a detector near each slit to see if the photon went that way, saying that the interference effect goes away when you "look" like this.

 

My problem is that he seems to be describing a real experiment but I can't understand how a detector can detect a photon as it passes by. The single photon detectors I have seen "consume" the photon in order to detect its presence. I am not looking for an explanation of the single photon dual slit experiment, what I would like to understand is what sort of apparatus is being used to perform the real world sensing of the photon path.

[swansont]

Oner way is to first send the light through a nonlinear crystal and get two entangled photons out. You can detect one while the other passes through the slit.

[lesolee]

Thank you. A very understandable answer.

 

My concern is that the text says ""(it is possible to design a detector that can tell whether a photon went through it)". Now we know from classical experiments that it is necessary to "split the incoming wavefront" by using an earlier slit. This ensures good spatial coherence to both later slits. If we "regenerate" the photons by down conversion there seems no reason to suppose the resultant beams would be coherent, and if not coherent then no interference would be seen - as indeed is the case. The text's interpretation of this experiment then seems quite bogus ... unless there is a better "special detector" of course.