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Basic Double Slit & DCQE questions

  1. Sep 23, 2011 #1
    I'm not a physicist, but have been thinking about these test lately and have hit some roadblocks that should be answerable.

    1. Why can't we measure a single photon to determine if it fell into the interference pattern or acted as a particle?

    I understand this is because I'm thinking of two physical slits and hitting parts of the wall, but in that model any photon that hits outside of the defined two slit area, should be measurable as part of an interference pattern.

    However, in the DCQE, it is explained that you can only retroactively analyze groups of many photons based on the result of whether it was observed or not. It is written that only after looking at which photons were erased could you then look through the data from main detector to determine the interference pattern. So if you look at the observed group you'll see particle behavior, and non-observed an interference pattern, but I don't understand why this difference can't be measured on a single photon.

    2. Effects of Time Dilation in DCQE

    It would seem to me, that if you had paths A&B that later were recombined to "erase" path information, then if you added say 100ft to A, then even after being combined with B, it would hit the detector at a noticeably later time, and thus you could determine which slit it went through.

    Then, knowing time/space is relative, even on the small scale of a lab room, there must be slight undetectable time changes in both paths, so that means there must be some tolerance for timing while still causing an interference pattern to emerge. Has any testing been done to determine how far apart the photons can be while still causing an interference pattern?

    3. Reverse Causality Test

    This may tie into my first question, but it seemed to me a really basic addition to the DCQE experiment would be to turn off (block, whatever) the recombining splitter (or the detectors after that) based on whether a photon acted as particle or wave, but in the opposite way. So turn "eraser" on for particle photon, off for interference photon. This would seem to be a simple test that would explain a lot or at least answer thought experiments about whether the universe is deterministic.

    Obviously it isn't this simple or it would have been done, but why couldn't you just hold the paired photon from a down converter either in a super frozen gas (slows down light, right?) or make it travel through lengths of fiber optic cable which would give enough time for the main detector to send a signal directly to the "eraser" to turn off or not.
  2. jcsd
  3. Sep 23, 2011 #2


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    Welcome to PhysicsForums, daegs!

    I can give you some quick info that may help:

    1. This experiment does not produce quite the results you would expect. It is not exactly like a double slit setup, so your analogy won't work. There is no spot which is only for interference. The pattern only appears when there IS coincidence counting. When there is none, all you see is a blob pattern.

    2. Time dilation is really not an issue for photons. Their speed is always c. The raw distance of separation is not a factor.

    3. There are no causal paradoxes to be seen here. You are correct that you can run a photon through more fiber to delay its arrival.
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