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Understanding delayed choice experiment

  1. Jan 12, 2013 #1
    What I understand from the delayed choice double slit experiment, is that , only after we see in the detector of the whereabout of the particle, we get the patter according to it. That is if we have destroyed the detector and then we observe the screen we get an wave pattern. Barbecue we didn't have the access to the information of the particle whereabout.

    That is only after we decide whether to check the detector or not, we find the result on the screen according to our decision/action i.e before that there was no pattern on the screen. Which is why it is called delayed choice experiment.

    So what will be the pattern on the screen? when the detector record which slit the particle passed, but the detector is not looked at first instead the screen is observed first for the pattern and there is an sincere intention from our side to destroy the detector once the whatsoever pattern is observed on the screen without looking at the detector as we had honestly intended to destroy it and we did. What will be the pattern found?

    Because after observing the screen, anyhow we are destroying the detector/information completely i.e we will be not having any information of the particle which slit it passed. So according to it, we should get an wave pattern?

  2. jcsd
  3. Jan 12, 2013 #2

    Jano L.

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    I am no expert on this topic, but I think it is perfectly possible first to observe interference pattern consisting of many spots and then check the history of recorded detection on both detectors.
  4. Jan 12, 2013 #3


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    No matter what you do, the patterns found directly on the screen are never interference patterns, but always just simple diffraction patterns or something similar. You cannot influence these patterns seen at the screen at all.

    Patterns will only occur in coincidence counting. That means you take all the detection events at the screen, keep only those detection events that happened simultaneously with some other well defined detection event at another detector and throw away all the other ones. This specific subset can give an interference pattern depending on whether a detection on that second detector carries which-way information or not.

    So the delayed basically comes from the fact that the decision whether the detection at that second detector offers which-way information or not is made after the first photon has already been detected. However, as the pattern is only visible in coincidence counts, this does not change the pattern at the screen at all, but only the detections at that second detector.

    What you intend to do does not create any changes. You do not influence the observed pattern.

    The interference pattern will only occur in coincidence counts. Destroying all the detectors and information recorded will basically just rob you of the opportunity to pick the correct coincidnce count subset which would give you the interference pattern. Also in this situation, destroying detectors does not change the pattern on the screen.
  5. Jan 12, 2013 #4


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    You cannot destroy the detector and expect the results to change. The ordering of the actions (ie whether the choice is delayed or not) does not change the results.

    The standard rule is that if, in principle, it was possible to learn which slit the particle went through, there is no interference. It does not matter whether you actually view the answer or not. The particle does not care whether the information was used or not, just whether the setup could have been used for that purpose.

    The erasing part must occur within the system for it to make any difference. You cannot erase once the information exits the system.
  6. Jan 13, 2013 #5
    This is what is interesting in the delayed choice experiment, if I'm right, if the detector is destroyed without looking at the information, and then we observe the screen, we get an interference pattern.
  7. Jan 13, 2013 #6


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    The answer to that question has already been given to you:

    What you do to the detectors is completely irrelevant and there are absolutely no signs of the presence or absence of conscious observers having any influence on the outcome of the delayed choice experiment. Also, note that the interference pattern is NEVER directly visible on the screen in delayed choice experiments as I already pointed out in my last post. You always need postprocessing to get it.
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