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After reading up on quantum mechanics considering entangled qubits I was asking myself this simple question:

If one qubit (A) positioned on earth is entangled with another qubit (B) which is - say - a lightyear away, and we perform a bell-measurenment with (B) and an additional qubit (C), it would cause the entanglement of (A) and (B) to break in an instant, or am I wrong? Wouldn't that mean that a communication (say via morse-code) with speed that's exceeding the speed of light from point a to point b is possible? If so, isn't it also possible to communicate from out of a black hole using this method? (disregarding time-dilation).

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  • $\begingroup$ Yes, entanglement breaks instantly, but why do you think that such measurements could be used to transmit e.g. morse-code? $\endgroup$ – Noiralef Jul 18 at 12:14
  • $\begingroup$ I thought that somehow you could use "entangled" and "not entangled" the same way you use "1" and "0" as states to communicate in binary. For this to work, you would need to be able to see when and if the entanglement breaks, so maybe the question is - is it possible to determine whether your qubit is still entangled? $\endgroup$ – Florian Claaßen Jul 18 at 12:16
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    $\begingroup$ No, it is not. As long as you only have one of them, definitely not. $\endgroup$ – Noiralef Jul 18 at 12:19
  • $\begingroup$ How does this change assuming I have more than one? $\endgroup$ – Florian Claaßen Jul 18 at 12:21
  • $\begingroup$ How do you know whether your one qubit has ceased to be entangled with the other one? How do you detect that? $\endgroup$ – probably_someone Jul 18 at 12:26