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I I can determine the photon path in delayed erasure

  1. Dec 23, 2016 #1

    I am going into quantum physics the past days specifically the dual slit because it boggles my mind.
    You can find the Wikipedia here https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

    And this is the picture:

    Now it is said that D3 and D4 never have an interference pattern because we know the path those photons took. It is also said that D1 and D2 always have an interference pattern because we can't know the path those photons took because of the randomness in the green silver plates.

    But here it comes (my thinking): The distance the photon travels to D1 is never the same as the distance to D2. This is because you can never put 2 objects at an 100% exact distance from eachother. There is always the slightest of differences. This is also the case for Mb to BSc and Ma to BSc and PS to Ma and PS to Mb and so on... all these paths have distances that are different from eachother. I can measure the exact distance the photon has travelled by time measurement and speed of light. If I then compare that distance to the experiment layout distances I can puzzle this in and find the exact path. Right? So why is it still an interference pattern if we could theoretically determine its path?

    What is wrong in my thinking :(

    Attached Files:

  2. jcsd
  3. Dec 23, 2016 #2


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    There's uncertainty in the arrival time of the photons, because photons are always a bit spread out and any part of the wave packet might trigger the detector.

    If the two paths differ in length so much that you can reliably tell the difference between them, the experiment won't work in the first place.
  4. Dec 23, 2016 #3
    Thank you that could probably make sense. Much better then other explanations I heard.

    The more I know about this experiment the more my brain get's totally wrecked by it.

    So the print displayed on D0 is not only aware of how its twin photon will bounce on randomized silver plates in the future, it will also know if the future distances of both slits to sensors D1 and D2 are similar enough for the path not to be detected on the twin.

    Now I start wondering what will happen if I change the distances after the photon already landed on D0 but the twin had not arrived on D1/D2 yet. But it would be impossible to act that fast.
  5. Dec 23, 2016 #4


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    If you took an entangled photon coming out of a BBo crystal, and let it go through a double slit setup, you would not see an interference build up. You would first need to make the light coherent. Then it would no longer be entangled on several observables.

    The point being that the details of the setup make all the difference, as Strilanc says. There are many complexities in the DCQE as you noted, more than many of the other core quantum examples.
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