Talk:Superluminal communication

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[edit] Using qubits for superluminal communication

I'm probably missing something here, but it seems to me that you really could use qubits for superluminal communication. The article says:

"If Alice wishes to transmit a "0", she measures (...), collapsing Bob's state (...). To transmit "1", Alice does nothing to her qubit."

But I was thinking, instead of just measuring or not measuring, couldn't Alice measure one qubit to transmit a 0, and measure a different qubit to transmit a 1? It could go something like this: Alice and Bob create a state which is an entanglement of three qubits. Then, after it's created, Alice keeps the first and third qubit. Bob takes the second qubit and goes to some far-off distance. So now:

  • Alice has |\psi\rangle_{A} and |\psi\rangle_{C}
  • Bob has |\psi\rangle_{B}

Then, to transmit a zero, Alice performs a measurement on only |\psi\rangle_{A}, and leaves the other two qubits alone. To transmit a one, Alice performs a measurement on only |\psi\rangle_{C}. Then, after Alice performs her measurement, Bob measures |\psi\rangle_{B}. The three-qubit state could be initally set up so that:

  • |\psi\rangle_{A} has a high probability (greater than .5) of being measured as |0\rangle
  • If |\psi\rangle_{A} is measured as measured as |0\rangle, then, subsequently, |\psi\rangle_{B} has a high probability of being measured as |0\rangle
  • |\psi\rangle_{C} has a high probability of being measured as |1\rangle
  • If |\psi\rangle_{C} is measured as measured as |1\rangle, then, subsequently, |\psi\rangle_{B} has a high probability of being measured as |1\rangle

There would be a probability of error in the transmission, but this error would be less than .5. One could compensate for that error by using many sets of three qubits to transmit each bit. Or maybe one could compensate with classical error-correcting codes.

Would all of that work? I'm guessing that I've missed something, since I'm not an expert on quantum mechanics. Most of what I know is from taking a one-semester class on quantum computing. --Navigatr85 03:10, 22 August 2007 (UTC)

[edit] The example

If Bob creates copies and measures them, wouldn't they all have the same spins on each axis? Conversely, how is Bob measuring one of his copies different from Alice measuring her copy?