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Why wouldn't this experiment allow superluminal communication?

  1. Apr 19, 2013 #1
    I know there's a flaw here as superluminal communicaton is impossible.

    I was considering this scenario:
    As with the initial stage of the delayed time quantum eraser experiment, put two light sources through a beta barium borate chrystal creating two pairs of entanged photons. Send one pair to station on Mars and the other to a ground station.

    Now I gather that the way the photon is observed will force its entagled partner on the other side to behave like a particle or a wave. So what's preventing instantaneous communication. You could easily send morse code by changing the way you observe the photon, so turning on/off an interference pattern at the other side.

    What am I missing?
  2. jcsd
  3. Apr 19, 2013 #2


    Staff: Mentor

    You cannot measure "does the object behave like a particle or a wave?" in the way you can influence it on the other side.
  4. Apr 19, 2013 #3
    do you mean you measure momentum on one side then the particle collapses as a wave, or you measure position and the its a localized particle and this should force the particle on the other side ? But then the problem is you dont know which measurement (position or momentum )you should make on the other side so you cannot transmit information. For example if you measured momentum on earth and position on mars the particle will localize on mars and wont produce an interference pattern so that we couldnt transmit that particle was a wave on earth.
    Last edited: Apr 19, 2013
  5. Apr 19, 2013 #4


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    Science Advisor
    Gold Member

    Entangled photons do not produce an interference pattern in the first place. That is why the idea will not work. See page S290 in the following reference, Figure 2, which makes this more clear. This paper is from Zeilinger, one of the premier researchers in this field.

  6. Apr 19, 2013 #5
    bcrelling good question.

    i had the same idea/question a few years back.

    i think the answer is that there is "noise" in it. to separate the noise.....we need to compare both the photons....

    various kinds of noise:

    1. non-entangled photons
    2. random photons which never came from the "source" used in the experiment
    3. and a few more categories

  7. Apr 19, 2013 #6
    Well basically the idea fails because it is not enough that you know your photon you also need to know the other guys one , you may say that well check yours and you will know what kind the other one has , it would work perfectly if it wouldn't be for the fact that you cannot know the outcome before measurement + the measurement itself can make your photons "information" useless now this all together makes information sending via this way useless , well if random bits of photon states is considered information then you have a FTL machine but if they are not then you don't have such a wonder.
  8. Apr 20, 2013 #7
    I just remembered that even if there was no noise information could still not be got/had/extracted.

    because, as DrChinese has rightly pointed out,:

    entangled photons don't produce interference patterns

    Coherence and entanglement are complimentary -- and I think -- this (like other complimentary pairs) is hinting at something very fundamental/deep about reality/universe
    Last edited: Apr 20, 2013
  9. Apr 21, 2013 #8
    Thanks everyone, I think I get it- it's only when we examine the measurements from both ends after it has been done that a meaningful pattern arises.

    However I don't understand the statement "entangled photons do not cause interference", as the delayed choice quantum eraser does produce interference(if only for entangled photons which have their "which path information" erased).
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