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Quantum eraser and the experimenter's retina?

  1. Jun 1, 2013 #1
    Quantum eraser and the experimenter's retina!?

    I just ran across the following quote from Brian Greene's "The Fabric Of The Universe":

    Does this erasure of some of the which-path information– even though we have done nothing directly to the signal photons– mean that the interference effects are recovered? Indeed it does– but only for those signal photons whose idler photons [had their which-path information erased]…If we hook up equipment so that the screen displays a red dot for the position of each photon whose idler photons [had their which-path information erased] and a green dot for all others, someone who was color-blind would see no interference pattern, but everyone else would see that the red dots we arranded with bright and dark bands– an interference pattern.​

    This doesn't make sense to me. If a person who can see color would see the interference pattern in the red dots, then, for a colorblind person to not see the interference pattern (albeit, more faintly), the green dots would have to be in a kind of inverse interference pattern to the red dot one, so that the total pattern was random. But my understanding is that the green dots in this experiment are not supposed to display an interference pattern.

    The alternative would seem to be that Greene is saying that the observer's retina is part of the experimental equipment, so that its being able to see color or not would actually affect the outcome. That seems very unlikely to me, and if someone thinks it is true, it seems worthy of its own experiment. But I wasn't able to find such an experiment.
  2. jcsd
  3. Jun 1, 2013 #2


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    I don't know the details of the setup he describes there, but I would expect that this is the key point. The green dots are complimentary to the red dots. Alternatively, there could be two event types for red dots, but then the color description is weird.
  4. Jun 2, 2013 #3
    I'd write this off as a bad analogy.

    I think his point is just that the subset of the photons whose which-path information was potentially indicated but then erased continue to interfere as if no which-path information had been taken in the first place.

    When I read this passage many years ago, I assumed the color-blind thing was just a weak attempt to illustrate the result... sometimes physics writes stretch a little far when trying and render their points in common terms. I suspect you are right though, in fact everyone would see a weak interference pattern, the result of partial interference overlaid with a random distribution. There is no reason -- as you agreed -- that the which-path photons would "reverse-interfere".
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