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I want to ask about a pair of entangled particles, each one heading towards a different group of experimenters.

When one group observes their particle, is it possible for them to tell whether the other group had observed their particle at that point?

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    $\begingroup$ Why the downvotes? This seems a perfectly good question. $\endgroup$ – John Rennie Sep 15 '15 at 14:44
  • $\begingroup$ I agree with John Rennie and I have given you an upvote to compensate. There is nothing wrong with the question. $\endgroup$ – CuriousOne Sep 15 '15 at 14:48
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    $\begingroup$ @DanielSank: Actually "at that point" seems to imply that the OP has faster-than-light in mind. $\endgroup$ – WillO Sep 15 '15 at 15:46
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    $\begingroup$ @CuriousOne "Only if we combine the two measurements after the fact can we tell if the pair was entangled, or not." -- This is not correct. It would only be correct if we would do a series of measurement on a set of identical qubit pairs. $\endgroup$ – Norbert Schuch Sep 15 '15 at 15:52
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    $\begingroup$ @CuriousOne My point is the opposite. Two measurements on the two qubits of a single two-qubit state will now allow you to determine if the state was entangled. You will need to repeat this several times on several coppies of the same state. $\endgroup$ – Norbert Schuch Sep 15 '15 at 20:35
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No. The relative probabilities of the observables will be exactly the same as they would have been if the other entangled particle hadn't been observed, it's just that if you later compare the measurements on the two entangled particles, you will find that the measurements are correlated.

Edit (in response to Norbert's comment): In fact, the measurements will be correlated in a way that would be impossible with classical particles without faster than light signaling. But the fact remains that nothing you observe about one particle will indicate whether the other particle has been observed.

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    $\begingroup$ You can equally have correlated outcomes with unentangled states. $\endgroup$ – Norbert Schuch Sep 15 '15 at 20:37
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When one side of an entangled pair is observed [...]

Whether and in which way a pair had been entangled can of course only be known from both having been observed, and both observations having been (separately) evaluated and the results having been correlated.

Is it possible for that side to detect or infer whether or not the other side was already observed at that point?

Considering only one side of a pair there cannot be made any inferences about "the other"; it cannot even be inferred whether or not some particular "other side" might have been "in play" at all.

But it is certainly possible for one side to detect the other side having broadcast their results, before collecting observations pertaining to the own ("one") side of the pair. If so, the two events to which the observations belonged were time-like related to each other.

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No, but if there are only two possibilities they will know what the other measurement will be.

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