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Entangled particle question

  1. Jun 18, 2004 #1
    Could entangled particles be used to send information instantly? For example, a mars rover could be given a particle and earth could be given a particle. If we did something to our particle, it would also happen to the entangled particle. We could write a computer program to interpret these changes. A code could be developed based on changes in the particles. Right? Or am I making too many assumptions?
     
  2. jcsd
  3. Jun 18, 2004 #2

    jcsd

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    No they specifically cannot be used to send information.
     
  4. Jun 18, 2004 #3
  5. Jun 18, 2004 #4

    jcsd

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    That's describing quantum teleporation that uses entanglement. In order to accomplish quantum teleportation you still need a classicla communications channel (which can only send information at c or under).
     
  6. Jun 18, 2004 #5
    I guess I don't quite understand. It seems like if you have two entangled particles and you put one one place and one another you could do something to one that would affect the other. This could very easily be used to send information between great distances.
     
  7. Jun 18, 2004 #6

    jcsd

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    The problem is though a measuremnt on one partcile instantly decide the state of the other particle it doesn't change the values we should expect to measure. Therefore someone measuring the particle whose state was decie instanteously by a measuremnt on that particle has no way of telling if the value was already determined or not.
     
  8. Jun 18, 2004 #7
    I may be wrong, but it sounds to me like you are thinking that entangled particles remain entangled for many measurements. This is not the case.

    The entanglement is lost the instant that one of the particles interacts with something else. The particles don't remain entangled forever. The very idea of "putting" an entangled particle someplace were you can play with it is a completely impossible idea. The only thing you can do to an entangled particle is measure it where you find it. Anything else that you might do to it will cause it to lose any previous entanglement that it might have had with another particle.

    You only get one measurement, and even that measurement must be made before the entangled particle interacts with anything else. If you want to use entangled particles to communicate you would need to use a whole stream of them. But since you can't do anything special to them (like polarize them or something) without destoying their entanglement, then you can't actually use them for communication. Unless you want to communicate a bunch of random noise. They are good for that!

    Unfortunately communicating random noise isn't a very useful conversation.
     
  9. Jun 18, 2004 #8
    Does it also happens in the so-called Non-demolition measurements (measurements that don't change the state of a system)?
     
  10. Jun 19, 2004 #9
    Quantum entanglement is an aspect of quantum information science that is seriously being considered as a way of transporting data.

    SOURCE: http://www.nsf.gov/pubs/2000/nsf00101/nsf00101.htm#b2
     
  11. Jun 19, 2004 #10

    chroot

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    Information cannot be transmitted by quantum-entangelement alone, Imparcticle. You'll also need a classical channel.

    - Warren
     
  12. Jun 19, 2004 #11
    I'm not aware of any so-called Non-demolition measurements that can be made on quons without changing their state? Seems to me that would be in direct violation of the uncertainty principle. You'll have to provide a link to more information about non-demolition measurements on quons so I can read up on this. There must be a catch somewhere.
     
  13. Jun 19, 2004 #12

    LURCH

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    Gore, as has been said more than once, the popular wisdom stats that information cannot be transmitted via quantum entanglement. Althoug I myself remain unconvinced of this statement, it is broadly accepted by the scientific community in general.
     
  14. Jun 19, 2004 #13
    You're right, I was confused. A quantum non-demolition measurement does not perturb the measured observable but the canonically conjugate observable is perturbed. This document assures this:
    http://plato.phy.ohiou.edu/~ulloa/611-612/612papers/Shaleen Shukla--Q Non-demolition.pdf

    Anyway, and I don't want to seem obstinate, I have heard some things about something called entanglement degradation. Does it happens by decoherence of the particles entangled with the medium?If it's this way, this is a measurement that don't destroys the entanglement, only degrades it. But is possible that entanglement degradation can also happen without any interaction of the entangled particles with anything. I'm not sure.
     
  15. Jun 19, 2004 #14
    A "non-demolition" measurement refers to a measurement in which the state of the system is left in the eigenstate corresponding to the eigenvalue observed. This is very easy to achieve with things like polarization and spin, but much harder for a general observable. For example, a measurement of photon number will usually absorb the photons you are trying to measure. A number of clever techniques have been developed in quantum optics in order to measure a general observable in a non-demolition way.
     
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