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Understanding superposition and entanglement

  1. Sep 29, 2010 #1
    I am a newbie here, just an enthusiast with an above average understanding of math and physics, but exposed to QM after I left college.

    I have looked through the posts and did not see a concise summary to the following. Please forgive me if I overlooked some threads.

    There are some assertions I have which I want to verify are generally accepted to be true.
    A. Measuring entangled particle A only impacts particle B by causing both to decohere where measured and both to go into superposition on a noncommuting measurement.
    B. The no communication theorem states that measuring one entangled particle cannot provide measurable information to the other entangled particle.
    C. There is no way to measure whether a particle is in superposition for a particular measurement.

    Do I understand all of this correctly?

    Many thanks,
  2. jcsd
  3. Sep 29, 2010 #2


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    Welcome to PhysicsForums, Marty!

    Yes, I would say that you have it stated pretty well. I would comment on your A. that a measurement terminates the superposition and sends the particles into a mixed state, at least for non-commuting observables. Occasionally, you can observe entanglement continuing for commuting operators.
  4. Sep 29, 2010 #3
    Thanks for the fast reply!

    Extending the above, if there were a way to determine whether or not an entangled particle was in superposition for a particular observable, then one could violate the no-communication theorem?

    Thanks again,
  5. Sep 29, 2010 #4


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    That's correct, you cannot determine if the superposition is still effective by a local operation. You must correlate results from both Alice and Bob to learn this after a series of operations, which of course defeats the objective of sending an FTL signal.
  6. Sep 29, 2010 #5
    Thanks again for staying with me on this and feel free to point out at any time where I am off base.

    Suppose I had a device which produced two entangles particles A and B with non-commuting observables X and Y. In addition, suppose that X is not in superposition and is set to a predefined state (stay with me on this fantasy).

    Now if we measure observable Y on particle A, then observable X on both particles is in superposition. Once observable X on partible B is measured, it will either have the expected state, which tells us nothing, or it is in the unexpected state, which means that particle A has had observable Y measured.

    Where did I go wrong?

  7. Sep 29, 2010 #6


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    If X and Y are non-commuting, then they cannot have a relationship in which X is entangled and Y is not. Either they are both in an entangled state, or neither are.

    Also: they cannot be in an entangled state with a known X or Y. X/Y must be unknown prior to the measurement. The result will therefore be random. You cannot send much of a signal using a random sequence.
  8. Sep 29, 2010 #7

    Many thanks for the clarification.

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