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I Photon detectors in a double slit experiment

  1. Aug 12, 2018 #1
    It is said that if one shoots photons in a double slit experiment, and place a detectors around the slits to find which slit the photon went, one will not see the photon interfere. However, to detect a photon, one must absorb it. So how does the photon detectors work by detecting the photons and still allowing it to reach the screen?
  2. jcsd
  3. Aug 12, 2018 #2


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    One thing you could do would be to put a transparent waveplate over one of the slits to rotate the polarization of light going through one slit 90 degrees while keeping the polarization of the other slit unaltered. Then, you can use polarizing beamsplitters at the detector to see which slit the light came from with each detection event. However, this sort of experiment would show no interference pattern because two light sources polarized at right angles to each other don't interfere. In general, any interaction with the photon that's strong enough to tell you which slit the photon passed through will also be strong enough to reduce the spatial coherence of the light to the point that the interference pattern is destroyed.

    As an interesting side note, it is possible to detect the presence of a single photon without destroying it, but the previous argument holds. See for example:
  4. Aug 12, 2018 #3
    I suspect that the double slit experiment is a thought experiment. I seriously doubt if anyone has ever done the experiment in reality. But I could be wrong. Has anyone ever done it? If so then where were the results published?
  5. Aug 13, 2018 #4


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    I suspect you haven't even try to make a google search. I personally suspect it is one of the most repeated experiment because it does not implies very complicated apparatus.

    They are up to using huge buckyball now.
  6. Aug 13, 2018 #5


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    The double-slit experiment with light has been done already by Huygens and was taken as evidence for the wavelike nature of light early on. Today one can do this experiment with single photons and also with all kinds of massive particles and composite objects, as mentioned in the previous posting, even with the rather large C-60 buckyball molecules, all showing the wave-like aspects predicted by quantum theory.
  7. Aug 13, 2018 #6


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    It's been done. https://arxiv.org/pdf/quant-ph/9903047.pdf uses a rather clever trick to get around the problem in the original post, namely that detecting a photon destroys it; there's a more user-friendly description here. And, although I don't have a link handy, the variant that @jfizzix describes have had been done many times.

    That's specifically with photons, because that's what OP asked about. Nowadays, most experimental effort has gone into demonstrating interference with more massive particles, and there's no shortage of experimental results there. Google for "quantum buckyball interference" for some examples and further links.
    Last edited: Aug 13, 2018
  8. Aug 13, 2018 #7


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    The experiment that most closely relates to what the OP asked is https://arxiv.org/abs/quant-ph/0402146, using C70 molecules instead of photons. By heating the molecules, one could get "which slit" info from emitted infra-red photons, and that leads to a decrease of the interference fringes.
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