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Questions about the delayed choice quantum erasure experiment

  1. Feb 16, 2012 #1
    Hello, the delayed choice quantum erasure experiment seems really intriguing and although many sources seem to state that the results are what has been predicted by quantum mechanics, I'm having a very hard time understanding the results conceptually. What is exactly happening that causes there to appear to be a breach in causality?

    Also, I was wondering if I understood it correctly that the total pattern of the signal photons is always the same, no matter what kind of setup is created for the idler photon?

    And finally, was this experiment a one of, or has this experiment been repeated including different variations? If so, what are those variations?
     
  2. jcsd
  3. Feb 16, 2012 #2
    Well, Wheelers Delayed Choice Experiment will explain it most efficiently in simple terms.

    I will simplify it even further.

    Imagine a photon traversing the universe in some wordline. However according to quantum mechanics, this photon does not travel one wordline alone, in fact, it takes every possible route, even the most improbable routes all dictated by a wave function. Imagine then also, that when a photon has travelled from the past and reaches Earth in our present day which is further observed by some scientist in the lab, those many histories suddenly collapse. A single path for the photon would present itself (upon the square of the wave function which is called the collapse of the wave function). So in theory, what Wheelers thought experiment concludes is that by using the laws of quantum mechanics you can change in some real dynamical way the history of a system. This violates causality they say, because for us to effect the past is an unusual conduct, atleast when you usually consider that the past always preceedes the effect. But it's not really a violation if we assume that quantum information is somehow shared in time. One approach to solve the violation is the well-respected Transactional Interpretation, which was proposed by Doctor Cramer. In his theory, causality is preserved by allowing both the future and past to effect each other using two half-retarded and advanced wave oscillating in time. They cancel out in what is called the transaction. This would mean that information is always changing based on observations made on the present. The future effects us in a statistical way.
     
  4. Feb 16, 2012 #3
  5. Feb 16, 2012 #4
    Thank you, it does indeed make some conceptual sense in the transactional interpretation. But other interpretations have apparently not been ruled out because of the experiment, how do they arrive at the same predictions? You say they somehow share information in time, is this something that is a bit better defined somewhere?

    What I meant is if the delayed choice quantum eraser has been performed with some slight alterations. For example:
    - Splitting off 90% of the photons towards the detectors which will know which path information. How does the data change because of that?
    - Placing the detector receiving the signal photons further than the part that handles the idler photons to see if any of the data changes
    - Removing one of the two detectors that detect the photons where which path information has been erased
     
  6. Feb 16, 2012 #5
    Ok, I got it. No, I don't know if any such alterations have been done.
     
  7. Feb 17, 2012 #6

    Cthugha

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    Generally speaking the transactional interpretation is not very accepted. Theoretical investigations of a proposed quantum DCQE experiment come to the conclusion that "Discussing the delayed-choice experiment, Wheeler concludes: ‘‘In this sense, we have a strange inversion of the normal order of time.We, now, by moving the mirror in or out have an unavoidable effect on what we have a right to say about the already past history of that photon’’ [5]. We disagree with this interpretation. There is no inversion of the normal order of time".
    in R. Ionicioiu et al., Phys. Rev. Lett. 107, 230406 (2011) which is also available on the ArXiv. That proposal has also been realized in S.S. Roy et al., Phys. Rev. A 85, 022109 (2012).

    The interference pattern is always only present in coincidence counting, so there can never be any violation of causality. You just either have a subset of all photons present which allows to retrieve an interference pattern or you do not.

    Having some which-way information will reduce the visibility of the interference pattern. There is a complementarity relation between the visibility of the interference pattern and the amount of which-way information you can get known as the Englert-Greenberger duality relation.

    Both do not introduce any changes.
     
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