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B Single photon and double slit experiment

  1. May 29, 2018 #1
    hi
    are the path lengths to the slits strictly equal?
    If the difference of the path lengths is progressively increased, how evolve the fringes visibility?
     
  2. jcsd
  3. May 29, 2018 #2

    sophiecentaur

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    To work out the pattern taking unequal path lengths, you just have to add in the extra bit on the longer path. But it doesn't make a lot of path difference if there is a small tilt on the plate with the slits in it - it will shift the phase of the interference fringes left or right a bit. You should google Two Slit interference pattern. There are many links and you can easily find one that presents things at your level.
    Single photons or a coherent beam of light will have the same pattern so you can deal with the sophistication of very low levels of light later.
     
    Last edited: May 29, 2018
  4. May 30, 2018 #3
    hi thx
    i have found plenty of experiments: correlation with the phase between the path, using satellite, unequal width, unequal path after the slit, two entangled photons, different arm lengths in michelson interferometer, quantum theories, summaries of "what to know", sophisticated experiments and so on, but not what i'm looking for
    I'd like to see how the interference fringes evolve in a double slit experiment with single photons when the length paths difference increases from 0
    Ok, you say that the fringes will move but, is it possible to be more precise? I think that such a study has been done but where to find it? Search engines are a bit confused with my demands...
     
  5. May 30, 2018 #4

    ZapperZ

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    Why don't you shine a simple laser pointer at a double slit, but have it incident on the slits at an angle?

    Zz.
     
  6. May 30, 2018 #5
    i'd prefer experiments with single photons...
     
  7. May 30, 2018 #6

    ZapperZ

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    Why? After all, if you let the single-photon images accumulate over time, it will look like what you would get with a CW source! What extra bit of information will you get with an oblique single-photon source?

    Zz.
     
  8. May 30, 2018 #7
    Because, if there are two sources from the slits, there will be always interference. If there is only one photon, we may expect a different behavior: because of the phase between the arm lengths, the source from the slits may be delayed in time and there should be no interference if the difference is too important
     
    Last edited: May 30, 2018
  9. May 30, 2018 #8

    ZapperZ

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    If what you said is true, then you should see this effect CLEARER when there are a gazillion photons that passed through (one doesn't see interference effect with just ONE photon). So if there's no interference, one should be able to distinguish that with lots of photons.

    And this is why I asked why you insisted on having single-photon source. Why not use a laser and do this test yourself? Your explanation did not indicate ANY need for single-photon source.

    Zz.
     
  10. May 31, 2018 #9
    There is a wave (at least somewhat that looks like to) so we have to take into account a "thickness" in the interference phenomenon: interference is not a phenomenon between 2 point-particles (probability of interference ~ 0 because they have to hit each other). Therefore, no need mount of photons to do the interference. Depending on this thickness, the interference would allow a delay between 2photons if one of them needs the wave of the other to achieve interference. When the arm lengths are equal, only one photon is needed to interfere with itself. If this difference is too important, no interference is expected and a second photon is probably needed to interfere with the delayed wave of the previous photon. Between these events (null or important arm lengths difference), it would be interessant to know how the interference fringes of single photons evolve with the arm lengths difference.

    [Mentor's note - This post has been edited to remove a link to an a source that is not acceptable under the forum rules]
     
    Last edited by a moderator: May 31, 2018
  11. May 31, 2018 #10

    Cthugha

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    This is completely incorrect. For intereference to be seen, the two fields interfering must have a stable relative phase. If the path difference from the slits to the point of detection is longer than the the coherence length of the light field, there will be no interference, regardless of whether you use a laser, single photon source, sunlight or a light bulb. You can easily see that if you shine two different lasers on the two slits of a double slit. This will not show any interferences in the time average.

    Most of this is wrong. Photons are not point particles and the emission process is not instantaneous. Due to the emission process being smared out in time, even a single photon has non-zero detection probability in more than just a single point. This translates to a spatial scale of smearing out that is again exactly the coherence length, just as for a laser or any other light source. By the way, all usual double slits only show single-photon interference. Different (that means: distinguishable) photons never interfere in the double slit. With respect to the double slit, both a laser beam and a single photon beam of the same average spectral and spatial shape are exactly the same light fields. The only difference is that the laser will have a larger amplitude, which is completely irrelevant for the double slit.

    That thickness you mention is the coherence length and covered in any good introductory text on optics. It is just given by the spatial and spectral properties of your light field and not by the photon statistics.
     
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