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I Detectable interference between counter-propagating modes

  1. Nov 2, 2017 #1
    Is it possible (in principle) to make a very thin phtotodetector that has the same sensitivity with respect to light coming from either side, so that we can see interference between two light waves arriving from opposite sides?

    That is, as we vary the relative phase shift between the right hand side and left hand side inputs, could we see the detection count increase and decrease? Of course, this would call for a detector thinner than a wavelength so that the phase difference would be nearly constant over the thickness. And we assume very good collimation with the beams perfectly normal to the detector surface.

    I'd like to know if there is a fundamental reason that would make this impossible. And if there is no such reason, what technologies currently exist.
  2. jcsd
  3. Nov 2, 2017 #2
    Dear Moderators,
    Please consider moving this back to Quantum Physics, because (although I didn't stress this in my post) one key consideration that affects the presence or absence of interference here is this : Can a detector be built, wherein events triggered by photons from the left and from the right are mutually indistinguishable events?. This particular aspect, of course, belongs squarely in quantum physics.

    Thank you.
    Last edited: Nov 2, 2017
  4. Nov 2, 2017 #3


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    1. The quantum efficiency DROPS as you make the detector very thin.

    2. Transmission photocathodes typically have lower QE than reflection photocathodes.

    3. #1 and #2 together means that you are not guaranteed that you will get to detect "interference between two light waves arriving from opposite sides".

    4. Besides, how would you even detect this within a photodetector? If they destructively interfered inside the detector, how is this different than no interaction, and no signal, from the photocathode?

    5. What exactly is the aim of all this? What is being measured, and what is being tested here?

  5. Nov 2, 2017 #4
    Yes, well, I guess my original post suffers from a confusion of different things that I was trying to get my head around, while in the process of thinking through a badly defined thought experiment (as I often tend to do, unfortunately).

    Let me focus a bit more sharply on one point. Daendliker says (https://spie.org/etop/1999/193_1.pdf) the following:
    My question is, if the modes have the same phase profile but opposite directions of propagation, do they interfere? Of course, the photon efficiency would be low - but my question is whether the probability of detection (however low it may be) is a function of the local phase difference between the modes? If we think in terms of standing waves, it seems reasonable that events would occur with a higher probability at nodes rather than antinodes.

    To me, this seems like it's about distinguishability. Can a photosensitive molecule, for example know which path the photon came through?
    Last edited: Nov 2, 2017
  6. Nov 3, 2017 #5


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    I still don't understand what you are hoping to accomplish here, and more importantly, why you think that this particular technique (using photodetector and figuring out what is happening INSIDE the detector) is the way to do this.

    I don't quite understand whether you are talking about single-photon head-on interference, or if you actually just need coherent sources interfering from opposite direction of propagation. You DO know that practically all of the interference phenomena that we are familiar with are single-photon interference and not multi-photon interference, don't you? So I do not understand this issue of (i) having photons in opposite direction and (ii) how you think they will somehow and coincidentally bump into each other right in the very thin, transmissive photocathode that will have a very poor QE.

    If all you care about are modes of interference of light waves, why not just use a waveguide?

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