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Questions about Quantum erasure with causally disconnected choice

  1. Jun 13, 2014 #1

    Interested mostly-amateur here with some questions about the Quantum erasure with causally disconnected choice experiments conducted in Vienna and the Canary Islands by Ma, Zeilinger et al.

    1. Does the projection event of the system photon's entangled partner occur AFTER the system photon has completed interacting with the interferometer? (I'm assuming it's safe to use terms like before and after since the whole experiment takes place within the same nonaccelerating frame of reference.)

    2. If so, is one possible conclusion that the transactional interpretation of QM is correct and that the experiment manages to break the usual symmetry between events' retarded and advanced waves? In other words, is the environment photon projection event really influencing the system interferometer events that happen BEFORE it?

    3. If one maintains that this reverse causality is NOT occurring, and assuming that Einstein locality is inviolable, what other interpretations can account for the results? The authors state that "no naive realistic picture is compatible with our results because whether a quantum could be seen as showing particle- or wave-like behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether." That's fair enough, but doesn't really narrow the field of possible pictures that ARE compatible with their results!

    4. Describing an earlier quantum eraser experiment by Scully and Drühl, the authors make this provocative statement: "The presence of path information anywhere in the universe is sufficient to prohibit any possibility of interference. It is irrelevant whether a future observer might decide to acquire it." Assuming this also applies to causally-disconnected versions of the experiment, could detectors 3 and 4 (the ones which detect the polarisation basis of the environment photon) be removed from the Vienna experiment without affecting what's observed at the interferometer? If so, would it matter whether they were replaced with empty space or with black objects (which would absorb the photons, presumably destroying their welcher-Weg information but not measuring anything in the process)?

    Many thanks,

  2. jcsd
  3. Jun 13, 2014 #2


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    Welcome to PhysicsForums, Sam!

    You have asked some good questions. And you have jumped into one of the more difficult areas of research. Zeilinger et al's work is fascinating stuff at the forefront of theory and experiment. I will add what I can:

    1. Yes, the projection can be objectively said to occur after.

    2. I don't know if it can be considered a possible conclusion. But it is certainly consistent with retro-causal class interpretations of QM.

    I am something of a proponent of time symmetric interpretations, so let me add this: the rule is that the ENTIRE setup must be considered. Time sequence in and of itself is not a factor. Therefore there is no good sense in which A influences B more than B influences A.

    That point is even more clear in some of Zeilinger's other experiments. In some of the those, causal influences appear to flow in one time direction in one part of the experiment, and the opposite direction in another arm of the same experiment. Try this:


    3. Bohmian, for one.

    4. You wouldn't see anything different at the interferometer.
  4. Jun 14, 2014 #3
    The TI account of quantum erasure...

    The TI account of quantum erasure can be found in my book at the end of Chapter 5. You will see that it makes no difference whether the choice to erase is 'delayed' or not. Thanks for your interest in TI.

    Also, if interested in the issue of whether quantum states can be interpreted as being primarily about our knowledge, see my new blog post.
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