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vanesch
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#15
May21-08, 10:53 PM
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Quote Quote by peter0302 View Post
Patrick - does your proof take into account the possibility of interference exhibited in one entangled particle, depending on the type of measurement performed upon the other, i.e., the Dopfer / Zelinger experiment?
Of course, that's exactly what it does. ANY measurable quantity (such as "there's an interference pattern") is - in quantum theory - always, there's no exception - an expectation value of an operator.

Well, it turns out (it's the proof) that the expectation value of an operator which corresponds to a measurement on only one branch of an entangled pair is independent of what is measured or even done to the other branch, simply because this expectation value is the trace of the product of said operator and the "reduced density matrix", which itself is independent of any measurement or unitary transformation of the opposite branch.