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Quantum communication

  1. Oct 28, 2015 #1
    Quantum Communication. All systems described by quantum mechanics can display so-called entanglement, for example, the spin of one electron cannot be known in advance of a measurement yet will be perfectly correlated with the other, even if it is in a distant location. This has recently been verified (Oct 26, 2015). Read more at: http://phys.org/news/2015-10-physicists-quantum-spookiness-schrodinger-cat.html#jCp

    Suppose there are two observers, one on Earth the other on Mars, and each possesses a “box” of particles entangled with particles in a corresponding “box” of the other party. At an agreed upon time, the observer on Earth influences the particles and the party on Mars observes the states and both then know the initial states of each box. At a certain time later, the party on Earth influences the particles so as to change the states of some according to a pre-agreed “code”. Could not the party on Mars then “instantly” determine the states and the message from Earth based on the code?
     
  2. jcsd
  3. Oct 28, 2015 #2
    This is a frequent question and the answer is well known and no. Entanglement can't be used for communication. A person on Mars has no way of telling if a person on Earth has made a measurement on the entangled pair.
     
  4. Oct 28, 2015 #3
    No, because measuring states on one side doesn't change the outcome probability on the other side. The correlations are there but you don't know what's correlated or not until you confront the results under classical communication.
     
  5. Oct 28, 2015 #4

    Nugatory

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    Staff: Mentor

    The first measurement made on either side ends the entangled state; any subsequent measurements have no effect on the other particle. Thus that sequence of measurements you describe won't work.

    Say Mars-guy measures a particle and gets spin-up. He then knows that Earth-guy's first measurement of his particle will be spin-down, but he has no way of knowing whether Earth-guy has already made that measurement, might make that measurement in the future, or will never make it. This, he has no way of knowing whether he's sending "be spin-down" to Earth-guy's particle or instead he's receiving a "be spin-up" from Earth-guy's particle. All he knows is that he got spin-up, Earth-guy's result is spin-down, and neither had any control over the outcome.
     
  6. Oct 29, 2015 #5
    Thanks to all (and especially, Nuqatory). The heart of my question was "why this code system would not work" which would by necessity require entanglement to continue for any conversations to take place. But since the first measurement on either side ends the entangled states then that answers the heart of my question. Thank you.
     
  7. Oct 29, 2015 #6

    jtbell

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    There's also the problem that Mars-guy can't "force" his particle's spin to turn out one specific way or the other. To make a crude analogy, imagine spelling out a word by pulling the letters out of a box one after the other. Now imagine that you want e.g. an "A", but the box gives you a random letter each time you pull one out.
     
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