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FTL and Coincidence counter

  1. Jun 12, 2011 #1
    Does that mean if there were zero (or close to zero) "noise" .....faster than light (flt) transfer of information would be possible?

    i.e. if the experiment was conducted in some dark & quite region of space-time.....where only the entangled pairs were allowed strike the detectors (Ds and Dp)


    my first impression/conclusion is that:

    1. the reason (i.e. flt is not possible due to noise) given above is somehow incorrect/incomplete

    or

    2. noise is some fundamental phenomena (tied to entanglement) that we don't fully understand yet

    Assumption: the author assumes/believes FLT is not possible at all
     
  2. jcsd
  3. Jun 13, 2011 #2
    Electromagnetic radiations are present throughout the Universe I doubt you'll find any place which is completely noise free.
     
  4. Jun 13, 2011 #3
    ok...fine.

    1. It does not need to be completely/100% noise free

    2. however then our current understanding (and/or the above quote that i pasted) is saying:

    well the information did arrive/travel FTL it's just that we cannot decode it

    (i.e. we have to wait to compare both the photons, signal and idler)


    then we are saying....the EM radiation noise, not something fundamental, is preventing us from getting the information, otherwise the information did arrive FTL but needs to be decoded/filtered....

    the reasoning does not sound robust/convincing

    it's like saying the ball (with the information) did arrive FTL but its buried in the sand and to locate it in the sand will take time....and we need to compare with the other ball to determine its location
     
    Last edited: Jun 13, 2011
  5. Jun 14, 2011 #4
    Cosidering the feasiblity of FTL communication (and not that of Ftl itself:-)),
    then my friend what's the use if you can get information fast and not be able to use it.
    from your post#1
    "This unavoidably prevents superluminal communication since, even if a random or purposeful decision appears to be affecting events that have already transpired"
     
  6. Jun 14, 2011 #5

    Drakkith

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    Staff: Mentor

    I think there is more than just noise that is preventing FTL. For one thing, I didn't think there was a way to even transmit information through entanglement. Is this incorrect? I thought everything was random.

    Also, the coincidence counter is needed to determine which photons are entangled with which other photons. Without it all you get on one detector is just a stream of particles with random states or polarizations or whatnot. What meaning is there in that?
     
  7. Jun 14, 2011 #6
    I don't think that 'noise' is the issue here; the issue is that the only 'information' you can send is 'randomly determined information' (probably an oxymoron under the standard definitions of terms); meaning that it's useless for most practical types of communication.

    Imagine you owned a pair of dice that have a remarkable property: they are 'quantum linked' so that if you throw them both at the same time , the total score of two dice always equals 7. Imagine you give your friend one of the dice, he takes it a million miles away, and you both throw them. Now: if your friend sees a 2 on his die, he will 'instantly' learn that your die must read 5. But that's not really helpful for communication-- since you couldn't choose the signal that was sent.

    It's not a perfect analogy but perhaps it gets the flavor across... also, I'm new here, so someone correct me if I'm wrong :) .
     
  8. Jun 15, 2011 #7

    there is a way to get around the "randomly determined information", such as a DCQE experiment, where an eraser can be placed/removed at will to make it "purposeful determined information"

    http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser
     
  9. Jun 15, 2011 #8
    I agree that the DCQE seems to suggest a workaround for "purposeful determined information": one thinks, maybe the sender can "choose to erase or not", and the receiver will be able to observe the sender's choice? (i.e. 'erasing the which-path information' causes an interference pattern to be observed) Is that what you meant?

    However I believe it can't work that way since the receiver doesn't actually have any good way to tell if the interference pattern is present or not! The receiver does not see an interference pattern ever on their "own" detector. Rather, they only detect interference as a statistical correlation between their own results and the sender's results. And therefore to realize that pattern exists, you need to ask the sender what they saw on their detector. And the only way to ask them is with 'normal' slower-than-light communication!



    I think that's what the article is saying, here:
    "The total pattern of signal photons at the primary detector never shows interference, so it is not possible to deduce what will happen to the idler photons by observing the signal photons alone"
    and
    "the interference pattern can only be seen retroactively once the idler photons have already been detected and the experimenter has obtained information about them"
     
  10. Jun 16, 2011 #9
    rgmcc - good answer. to validate it, and understand this better, again let's look at various scenarios/tweaks to the DCQE experiment.

    lets go with the walborn experiment, the link is below for the setup see page 7

    http://arxiv.org/PS_cache/quant-ph/pdf/0106/0106078v1.pdf

    the answers are at the end of the paper however they I am looking for answers with the below assumption

    assume a highly hypothetical/impractical case where there is zero electromagnetic radiations, i.e. no stray photons/electrons, radio waves etc i.e. only entangled pairs striking the detectors

    scenario 1 (no QWPs, no polarizer):

    Without QWP1, QWP2 for signal photon and without polarizer for idler photon

    a) what pattern we would see before co-incidence count/pairing?
    b) what pattern we would see after co-incidence count/pairing?

    scenario 2 (no QWPs, but polarizer):

    Without QWP1, QWP2 for signal photon and with polarizer for idler photon

    a) what pattern we would see before co-incidence count?
    b) what pattern we would see after co-incidence count?

    scenario 3 (QWPs, but No polarizer):

    Without QWP1, QWP2 for signal photon and NO polarizer for idler photon

    a) what pattern we would see before co-incidence count/pairing?
    b) what pattern we would see after co-incidence count/pairing?

    scenario 4 (QWPs, and polarizer):

    Without QWP1, QWP2 for signal photon and polarizer for idler photon

    a) what pattern we would see before co-incidence count/pairing?
    b) what pattern we would see after co-incidence count/pairing?
     
    Last edited: Jun 16, 2011
  11. Jun 16, 2011 #10
    http://arxiv.org/PS_cache/quant-ph/pdf/0106/0106078v1.pdf

    assume a highly hypothetical/unpractical case where there is zero electromagnetic radiations, i.e. no stray photons/electrons, radio waves etc i.e. only entangled pairs striking the detectors

    would not the receiver see an interference pattern on their "own" detector?
     
    Last edited: Jun 16, 2011
  12. Jun 16, 2011 #11
    Assumption 1: I assumed that the QWPs would allow only select polarization thus no random polarizations

    Assumption 2: I assumed that the polarizer (eraser) would allow only select polarization thus no random polarizations

    Are both assumptions wrong or only 2nd assumption wrong?

    http://arxiv.org/PS_cache/quant-ph/pdf/0106/0106078v1.pdf
     
    Last edited: Jun 16, 2011
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