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Superliminal Information Transfer

  1. Apr 1, 2008 #1

    In a superliminal transfer experiment, a pair of entangled photons goes two different ways. You could schedule a time for both parties to test the photon. Photon A goes to an encoder that either detects the photon (binary 0) or lets it alone (binary 1).

    Photon B arrives at a detector. If there is an interference pattern, the signal must be a 1. If there is no interference the signal is 0.

    I found a setup for this experiment dated 2007, but it was not finished. Has this experiment been carried out? What are the results, and please explain why it doesnt work? I assume it doesn't only because there is no publicity over it :)
  2. jcsd
  3. Apr 1, 2008 #2
    The reason the experiment doesn't work is that it is very difficult to produce an interference pattern using a photon that's entangled with another. The only way it can be done is to detect the other in such a way as to ensure that the position information is destroyed. However, any experiment must match such photons with the entangled twins in order to actually make an interference pattern using the twins - in other words, the subset of photons corresponding to those whose position information has been destroyed must be picked out in order to make the interference pattern. This correlation requires slower-than-light circuitry.
  4. Apr 1, 2008 #3
    Do you have a link for this 2007 set up? I'm going to sound ignorant here, but the only experiments I've heard of that use entanglement, are quantum eraser experiments which I think might be what peter is suggesting. But I'm posting to see if anyone has any other type of experiment where such a set up could be tried?

    Of course quantum eraser experiments support QM.
  5. Apr 1, 2008 #4
    ergh i must be missing something. i will think about it some more.

    maybe explain why I cant take 1 trillion A photons to mars and reproduce a two slit experiment?
    Last edited: Apr 1, 2008
  6. Apr 1, 2008 #5
  7. Apr 1, 2008 #6


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    Luke, you might want to read this thread about the delayed choice quantum eraser experiment, which discussed why the experiment cannot be used to send information FTL. If you're interested in Cramer's proposal about the Dopfer experiment, you could also read the discussion between me and peter0302 on this thread, starting with post #40.
  8. Apr 1, 2008 #7


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    It might surprise you to learn that an entangled photon does NOT produce an interference pattern when it goes through a double slit. Otherwise, as you point out, superluminal communication would be possible. This is a specific detectible difference between an entangled photon and one that is not. (The kind of interference you get in a quantum eraser is a bit different than the double slit kind because of the setup.)

    You can see this in an enlightening article by Anton Zeilinger, p. 290, Figure 2.

    Experiment and the foundations of quantum physics

    "FIG. 2. A source emits pairs of particles with total zero momentum.
    Particle 1 is either emitted into beams a or a' and
    particle 2 into beams b or b' with perfect correlations between
    a and b and a' and b', respectively. The beams of particle 1
    then pass a double-slit assembly. Because of the perfect correlation
    between the two particles, particle 2 can serve to find
    out which slit particle 1 passed and therefore no interference
    pattern arises."
  9. Apr 1, 2008 #8
    The reason what you describe will not work is no pattern of any kind can be shown using SINGLE PHOTON!
    Entanglement is not even an issue.

    Cramer is still working on his experiment - I doubt he will ever find any new results as nothing will change without actually using correlations.
  10. Apr 1, 2008 #9
    Yes indeed, and Jesse thanks again for that discussion and helping me finally figure out what was probably my biggest conundrum in understanding those experiments. I've thought about it more and I agree with you that if you used photons that you _knew_ were orthogonal to the plane of the double slits, i.e. collimated, the HUP will prevent you from knowing their position, and you will always get an interference pattern with those photons regardless whether you think you obtained "which-path" information from the twins.
  11. Apr 2, 2008 #10


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    Not sure if this is the setup you were contemplating or not: If you have double slit setups at Alice and Bob, you won't get an interference pattern at either with entangled photons. If I know which slit it went through at Alice, I can't get an interference pattern at Bob.
  12. Apr 2, 2008 #11
    In a normal situation, that's right. But if you do something to effectively destroy the possibility of knowing which-slit for Alice's photons, you will see an interference pattern at Bob. That's the Dopfer experiment, which is a similar concept as DCQE.

    But again, critically, you must be able to pick out which photons you destroyed position information for at Alice's location in order to see the interference pattern at Bob's location, and that requires slower-than-light coincidence circuitry. Also, even more interestingly, you can't just "ignore" position information for Alice's photons by smashing them all into a brick wall or something. You actually have to make it impossible, in principle, to determine position, and the way Dopfer does that is by focusing the photons onto a single point using a lens, and detecting them all at that single point.

    Unless someone could invent a lens that focuses ALL photons onto a single point irrespective of the angle of incidence - which AFAIK is impossible - Cramer's idea will never work.
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