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B Why doesn't this FTL communication scheme work?

  1. Dec 4, 2017 #1
    If Alice and Bob had a large number of particles entangled together (with state of either 0 or 1), could Alice send information by breaking the entanglement of particles with state of 1 (by flipping the state for example) so that Bob measures 50% of particles to be in the state opposite of 0, that is 1, plus half of the remaining 50% to be in state of 1 (due to 50/50 chance for measurement of either state in the disentangled pairs) resulting in 75% of the states to be 1 - indicating that Alice has broke the entanglement, which would be the information (one bit)?
  2. jcsd
  3. Dec 4, 2017 #2


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    Staff: Mentor

    It doesn't work because Alice can't selectively flip the state of the particles with spin 1 because she doesn't know which they are. Anything that Alice does to her particles is just as likely to turn a one into a zero at Bob's end as to turn a zero into one, so Bob end up with a 50-50 mix no matter what Alice does.

    If there were a way of preparing entangled pairs so that up front Alice knew which particles she would measure to be in one state or the other (or even if on average Alice's partcles were more likely to be in one state than the other) it would be possible to send a signal as you describe. But such a preparation is impossible, and indeed this is baked into the mathematical definition of the entangled state.
  4. Dec 4, 2017 #3


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    Just to add to Nugatory's correct answer:

    Alice can select a measurement basis for the particle she gets. That measurement basis *is* shared with Bob's particle. In other words, that is the only manner in which she affects Bob. There is no other transformation at Alice's end that Alice can control that would be shared with Bob's particle.

    And as fate has it, there is no measurement possible on Bob's side alone that allows him to detect what measurement basis Alice has chosen. Every measurement he makes will yield a random outcome. (Only when Alice and Bob's measurements are brought together do any correlations appear.)
  5. Dec 4, 2017 #4
    I've read about raman transition which is exactly what i described, a method to flip the state of a particle with laser AFTER it has been measured, so that the entaglement with bobs particles is broken and their state in upcoming measurment will be independent
  6. Dec 4, 2017 #5


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    It's easy enough to flip the state of Alice's particle after it has been measured, but the act of measuring her particle breaks the entanglement so the subsequent flip doesn't affect Bob's measurement. Alice knows that if Bob had measures his particle he'll get the opposite of whatever she measured; Bob knows the same thing about Alice's particle, but has no way of knowing whether Alice has subsequently flipped it.

    If that's not consistent with what you read, post up a link to it so we can find where the misunderstanding is.
  7. Dec 4, 2017 #6

    The part starts with "the answer is to"
    Bob would know alice has" flipped" because he would get result of 75% of (1) state instead of 50% each.
  8. Dec 4, 2017 #7


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    No plan that Alice has for operations to perform on her half of an entangled pair can change Bob's expectation for measuring his half away from 50/50.

    The reason your specific plan won't work is that there is no operation that "breaks the entanglement of particles with state of 1 [but not with state 0]". In order to determine that the particle has a state of 1 or not, you must have already broken the entanglement. (Alternatively you may have moved the entanglement into different qubits instead of breaking it. The specific problem depends on unstated details of your plan.)

    For example, in the Forbes article you quoted in your latest post they say:

    This is an example of an operation that moves the entanglement. The qubit that was being stored in the particle's 0-vs-1 state has been moved into the presence-vs-absence of a photon in the laser beam (or something like that). The entanglement wasn't destroyed, it was moved. Then Alice's equipment records a dip (or no dip) in the laser beam, completing the measurement process that happened to be started by moving the quantum information into something easier to work with.

    (Actually, I don't know anything about trapped ions. Other sentences in the article imply that the 0 state is flipping some state of the photon, in which case the information is being copied instead of moved. Regardless of those details, this is not a selective destroy-entanglement-if-1 process.)
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