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Delayed choice quantum eraser

  1. Feb 27, 2007 #1
    Question: What would happen if this was delayed even further? We are talking nanoseconds here, but what if the particle was slowed or sent far away, before being reflected back? What I'm getting at is... What if the delay wasn't nanoseconds, but seconds? What if it was minutes? What if it was enough time to manually change whether the second photon was observed or not?

    In other words, the photon behaves one way if we observe the second photon nanoseconds later and another way if we don't. So we start the experiment and we observe our detector. Depending on whether we see an interference pattern or not, we observe each secondary photon or we don't and that's the point.

    I'm been thinking about this for a while and I can't figure out what would happen. I searched Google and found http://uplink.space.com/showflat.php?Board=sciastro&Number=141733 where someone else asks the same question, but unfortunately no one really seems to understand the experiment, so he gets no real answer.

    This is exactly what I'm asking too. Now, depending on how you choose to interpret the delayed choice quantum eraser experiment, there's no getting around that we're either changing the past or predicting the future. So what happens if we delay it long enough to change whether we'll detect or erase the information about the second photon's after observing the results?

    This is not just a theoretical question, this is something that should be possible to test for someone who has access to this kind of equipment, but unfortunately I don't. However, many people have in the past predicted the outcomes of quantum experiments correctly, so I hope someone here can predict the outcome of this experiment as well.

    Thank you for your time. :)
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Mar 1, 2007 #2


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    Referring to the first set of photons as "signal photons" and the second set as "idler photons", the way it works is that the total pattern of signal photons actually never shows interference--even if you measure all the idlers in such a way that the which-path information is erased, you will see interference if you do a "coincidence count" between signal photons and idlers which went to a certain detector, but if you add all the subsets, they add up to a non-interference pattern. So, it's impossible to find an interference pattern until after the idlers have already been measured, ruling out the possibility of any backwards-in-time hijinx. This was discussed on this thread a while ago. And here's my summary from this thread:
    Last edited by a moderator: Apr 22, 2017
  4. Mar 9, 2007 #3


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  5. Mar 12, 2007 #4


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    For those who have not read the paper above, here is a plain-English summary:
    The delayed-choice experiment looks paradoxical if you think that the photon passes either through both slits or through one slit only. The paradox removes if you assume that something (the wave) passes through both slits, while something different (the particle) passes through one slit only. But then you need to assume that the photon consists of two separate things, which contradicts the standard interpretation of QM. Fortunately, there is an interpretation - the Bohmian interpretation - that provides such a wave-and-particle picture consistently.
  6. Mar 17, 2007 #5
    Thank you for your comments, I greatly appreciate it.
  7. Mar 17, 2007 #6


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    It might be interesting to do a search in this forum on delayed choice quantum eraser. Many threads have discussed this in the past here.

    BTW, although Bohmian mechanics can give an explanation for the phenomenon, every self-consistent interpretation of quantum theory can do so.
  8. Mar 19, 2007 #7


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    I agree. But why then this phenomenon is regarded as a big deal, even by experts?
  9. Mar 19, 2007 #8


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    I already pointed out a few times that many experimental papers on this issue, which are *experimentally* very rich, are very misleading/poorly written on the interpretation side.

    I guess that both Bohmians and MWI-ers don't find it a big deal (although nevertheless quite fun). The "projectionists" are of course a bit puzzled because they don't know anymore when to project :smile:
    (although, if you really understand von Neumann, you wouldn't be puzzled either).
  10. Mar 20, 2007 #9

    I too have seen this alot and have found a one-line version of the question:

    What would you see if you looked at the detector in a DCQE before the choice was made?

    Now as for the implementation, I see Spacezilla's point also that the implementation would seem at least as straightforward as the original experiment.
    I would imagine you would use a Bose-Einstein condensate to considerably slow the signal photons. While you replaced the mirrors with a manual (manually controlled electronic) switch which merged the signal photon path from slit B into the path from slit A.
  11. Mar 20, 2007 #10
    Understanding Bohmian


    So, under the bohmian interpretation of quantum mechanics, every particle is a particle and a wave always.
    And that the wave influences the particle based on where the wave is most intense.

    But does that mean manipulating the which-way information affects the wave aspect? Otherwise it would always show an interference pattern (over time).
  12. Mar 20, 2007 #11
    Which pattern?

    Now take a modification of the experiment where many photons are sent to the interferometer and hit the detector so that a pattern could emerge (and those are the idlers?).

    While the signal(?) photons are still travelling for such a time that the last idler has hit the detector before the first choice is made on a signal photon.

    Then what might you see on the detector if measured prematurely?
  13. Mar 20, 2007 #12
    RP, if you look at only the detector, you just see noise. Gaussian. Doesn't matter whether the "choice" has been made "yet" or "not". Read those other threads.
  14. Mar 20, 2007 #13

    So if you look at the detector prematurely you see noise?
    Because if you always see noise this isn't much of an experiment.
  15. Mar 20, 2007 #14
    Signal vs Idler

    Sorry, you had them correct JesseM,
    The signal is measured by the detector, and the idler is put through the choice.
  16. Mar 20, 2007 #15
    RP, the only way not to see noise is by looking at correlations.. which is obviously impossible until after you receive information about the result of the measurement on the other (idler) set of photons.
  17. Mar 20, 2007 #16

    Ok, now I think I've been confused all this time.

    Here's how I thought the measurement was taken (of the signal photons, D0 in the paper):

    The signal photons either exhibited a particle or a interference tendency (although only to be seen over meny repetitions).
    D0 was on a stepper motor moving left and right to capture the photons in a certain position.
    D0 would be moved to the next position after a good number of photons were sent through from the pump through the whole setup.
    Then the total number of hits from each position was mapped together with each other position to form a sort of 2D graph from which either an interference pattern or a particle pattern would be seen.

    Please correct me if I'm wrong.
  18. Mar 20, 2007 #17


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    Good points! :approve:
  19. Mar 20, 2007 #18
    ..or you could just carefully read the original journal article yourself. Or those other threads you've been directed to here.

    Think of it this way: if it worked the way you were assuming, it would be possible to tell the future or transmit information faster than light (and if there's one thing that physicists certainly would have noticed by now...).
  20. Apr 10, 2007 #19
    Kim et al.

    I thought that the signal photons were passing through a Young's slit type apparatus - in which case shouldn't they usually show an interference pattern overall ??? - in other words why should the interference pattern only appear with the correlations ??
  21. Apr 10, 2007 #20
    not if there's some way (in principle) to figure which slit each went through
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