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B Communication with no latency using quantum entanglement

  1. Mar 26, 2016 #1
    So I was wandering why it is not possible to use quantum particles to communicate across vast distances. As I understand it we cannot send information using this as it would break the laws of physics but what I was wandering is why can't we us entangled particles in a binary why by changing the spin of a particl to a specific direction depending on what the binary value is. Surely this would result in an identical spin on the entangled particle which we could perceive as the binary value on the other entangled particle. This would technically not be sending information but instead interpreting the change of spin as a binary value. Is there some fundemental law of physics that I don't know about or something else. Many thanks in advance.
     
  2. jcsd
  3. Mar 26, 2016 #2

    phinds

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    There are a great many threads on this topic. I suggest a forum search starting with the threads listed at the bottom of this thread.
     
  4. Mar 26, 2016 #3

    Simon Bridge

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    ... technically, any approach that would allow a message to be sent from one place to another is "sending information".
     
  5. Mar 26, 2016 #4
    Read John Bell.
     
  6. Mar 27, 2016 #5

    jfizzix

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    The short answer is, if you have a single particle and you can do whatever you want with/to it, there is simply no way of telling whether of not that particle is half of an entangled pair without outside information (say about another particle) being communicated to you. That outside information would be communicated at or below the speed of light.
     
  7. Mar 27, 2016 #6

    DrChinese

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    The answers above are all correct. Just to add a bit...

    If a pair of particles are entangled on some basis - say spin (which is binary) - then it is in a superposition and has no definite specific value. You cannot "force" it to take on a specific value (as make it Up or make it Down). If you force it to take a value, instead it will take on a random value. True, that value will (in effect) be communicated to the other of the pair. But that will still be a random value. Ergo, no useful communication has occurred.
     
  8. Mar 27, 2016 #7

    phinds

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    A question about this. I was under the impression that if you were to force spin as described, you would be doing something that would be equivalent to measuring the existing spin and THAT would communicate to the other half of the entangled pair and then they would not longer be entangled and your new value of spin would have no effect on the other half of what was a pair. so, does entanglement persist after measurement or is the forcing of spin not equivalent in effect to measurement?
     
  9. Apr 1, 2016 #8
    I don't think you should feel good about the word "communicated" even if you put it in quotes. There are correlations (e.g. give the same value), mysterious true, but that's as much as one can say.
     
  10. Apr 1, 2016 #9

    phinds

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    @DrChinese, I'm really interested in the question I posed in post #7. Do you know? Thanks.
     
  11. Apr 1, 2016 #10
    Dear Hewiitt,

    Suppose I take a pair of gloves and put each of them in a box. I keep one with me and give the other to you, who then take a rocket to the Moon and there opens it. Instantaneously you know what glove remained with me - there is no "communication" betwin us, as the situation was deffined
    the moment I closed the boxes.

    Einstein didn't understand it. "The ghostly action at distance" is nonsense!
     
  12. Apr 1, 2016 #11

    Demystifier

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    This is not a good analog for quantum entanglement.
     
  13. Apr 1, 2016 #12
    Dear Demystifier,

    Of course not, but I think it clears a fact Einstein didn't understand: that the Universe is non local. Why not accept it as it is, an empirical evidence?
     
  14. Apr 1, 2016 #13
    Your glove parable of post #10 mystifies no one, and sheds no light. Also I know of no evidence that says the Universe (sic) is non local. QM is compatible with locality, of course in that case realism must be abandoned.
     
  15. Apr 1, 2016 #14

    Nugatory

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    Entangled particles do not behave like gloves which have their handedness determined when they go into the box. You may want to try google for "Bell's Theorem" or try http://www.drchinese.com/Bells_Theorem.htm (maintained by our own @DrChinese) for more information, but the key point is that the behavior of entangled particles cannot be explained by any theory in which the properties of the particles are set when the pair is created. Experiments have confirmed many times over that the particles do obey the laws of quantum mechanics and not the commonsense behavior of gloves.
     
  16. Apr 1, 2016 #15

    Nugatory

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    Einstein died a decade before Bell discovered his inequality, so there was no empirical evidence either way during his lifetime.
     
  17. Apr 1, 2016 #16
    What I wouldn't give to know how Einstein would have reacted to Bell's result.
     
  18. Apr 1, 2016 #17

    Nugatory

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    Me too... and DrChinese has expressed the same sentiment as well...

    I'm certain that he would have accepted the result, but the really tantalizing question is what lines of thought would have followed from there.
     
  19. Apr 1, 2016 #18

    jtbell

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    ...or of Bertlmann's socks.
     
  20. Apr 1, 2016 #19
    I'm not convinced. Local realism was a strongly held belief for him. He may have rejected the predictions of QM, and we would then have had to wait for him to see Aspect's results, whereupon he might have said, "Lord why have you forsaken me".
     
  21. Apr 1, 2016 #20

    Nugatory

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    You're right - I was thinking of the combination of Bell's theorem and the experimental confirmation as well.
     
  22. Apr 1, 2016 #21

    Strilanc

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    I'm not sure what you're asking, exactly.

    There's no need to think of one side as "forcing" a spin onto the other, as opposed to one side predicting the measurement of the other. When two qubits are entangled, operations on one will still commute with operations on the other. Any explanation that implicitly asserts an ordering to the measurements (such as one side "forcing" the other), even when the events are space-like separated, is including unnecessary details.

    You can play around with entanglement in Quirk. The linked circuit entangles two qubits into the singleton state, shows their marginal states are maximally mixed, then shows that conditional states are pure and opposite no matter which axis you condition along. Then it shows that making an entangled copy (a reversible form of measuring) weakens this nice correlation by snapping the conditional states to the Z axis. (In real experiments you don't get direct access to the conditional states; they are inferred.)
     
  23. Apr 2, 2016 #22

    phinds

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    Then re-read the post to which I am responding
     
  24. Apr 2, 2016 #23
    It seems you are asking two questions:
    (1) Is forcing (a spin) the same as measurement?
    (2) Does the pair stay entangled after a measurement?

    The answer to (2) (I think you already know) is no.
    As for (1) the term forcing is a bit ambiguous. I am sure the way DrChinese is using the term is as a measurement - force it to take on a "random value".
    One could force it/ make it take on a specific value, say up, as follows: measure it, if the result turns out to be up your done. If the result is down then apply a unitary change to make it up. Of course the latter operation has no effect on the other particle due to (2).
     
  25. Apr 3, 2016 #24

    phinds

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    Yes, that is my understanding

    OK, that's what I was/am no clear about but your response helps.
     
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