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Single particle single slit interference - question about the experiment

  1. Apr 14, 2012 #1
    Came across the below experiment, on Wikipedia, but don't understand how it was performed...

    It was shown experimentally that:

    in a double-slit system where only one slit was open at any time, interference was nonetheless observed provided the path difference was such that the detected photon could have come from either slit.

    The experimental conditions were such that the photon density in the system was much less than unity.

    Question: if only one slit is open --
    how is the path difference created?
    how could the photon have come from either slit?

    Reference is:

    Sillitto, R.M. and Wykes, Catherine (1972). "An interference experiment with light beams modulated in anti-phase by an electro-optic shutter". Physics Letters A 39 (4): 333–334. Bibcode 1972PhLA...39..333S. doi:10.1016/0375-9601(72)91015-8.
  2. jcsd
  3. Apr 14, 2012 #2


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    Guessing from the title ...
    It looks like they used a phase shift in the two paths. The thing to remember is that the photon paths are paths through space and time.

    Try it this way, imagine a regular double slit experiment but with a shutter opening the two slits for a very short time.

    Now let one slit be farther forward and let its shutter therefore open and close earlier so that both slits are never open at the same time.

    Next, instead of moving one slit forward, instead put an equivalent delay I the path to that one slit. This is what It sounds like they did.
  4. Apr 14, 2012 #3
    Good guess jambaugh, thanks.

    And is the delay equal to one wavelength (or some multiple) so that coherence is maintained between both the paths? to get interference
  5. Apr 15, 2012 #4


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    Possibly a quarter wavelength so one path's peak equals the other path's node. That would effect a shift in the interference pattern but you'd still get the double slit interference.

    and again speculating from the title, the "slits" would be optically controlled, say with a laser so that as the peak passes it opens (or closes) each slit in turn.
  6. Apr 16, 2012 #5
    well said...

    ok....open just in time to allow the peaks to pass through?
    does the interference start (to happen/evolve) at the second/further/delayed slit?
    Last edited: Apr 16, 2012
  7. Apr 17, 2012 #6


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    The interference pattern will not show up you've at a range of points where a path from both slits is possible. It is better, I think, to visualize the interference pattern at infinity, i.e. as a function of the angular position from the slits.
  8. Apr 21, 2012 #7
    ?....we just agreed that there would a shifted interference pattern....

    not sure what you are saying.....however you have piqued the curiosity

    do the range of points start post (or prior) the second/further/delayed slit?

    if we were to place a screen after the second/further slit, we would see an interference pattern and as we move it backward the pattern would continue to show up

    however if we moved the screen closer...and before the second slit (i.e. between the first and second slit)......i guess we would not see an interference pattern.....right?

    the actual experiment might not be done that way.....however the above analysis can still be done, i guess...

    does it not seem as if something is physically travelling through all the paths?
    Last edited: Apr 21, 2012
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