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Lets say there are two entangled Qubits A, B, very far away from each other and described by the state

$|\Psi\rangle = |++\rangle + |--\rangle$

  • In a reference system S, A measures + and one hour later B certainly also measures +.

  • But in another system S', moving relative to S, B could measure + first and one hour later A measures +.

Therefore, it doesn't give sense to state that exactly one of the observers is "responsible" for collapsing the state. However, the scenario is often mentioned in such a way.

I often read that the moment A makes a measurement, the (later) measurement for B is already determined because A collapses the state. Isn't that misleading? Even worse, it is often claimed that B's state changes instantaneously when A makes a measurement.

What is the correct way of looking at this?

MichaelW
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1 Answers1

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As Tobias indicates, there are some similar questions that have good answers. However, in some ways they really beg the question (so to speak). Entangled systems have spatiotemporal extent. There are no particular restrictions on them due to special relativity, at least none that can be demonstrated by experiment. I would point out the following:

  1. When the order of measurement is unambiguous (say A first): There is no evidence that A causes collapse from A to B as opposed to vice versa; except by pure assumption. So "relativity of simultaneity" isn't really a factor, because nothing changes (as to the quantum mechanical expectation) when order changes anyway.
  2. A and B need not ever have been in a common light cone: High-fidelity entanglement swapping with fully independent sources
  3. A and B do not even need to exist at the same time: Entanglement Between Photons that have Never Coexisted
  4. A and B can become entangled after they cease to exist: Experimental delayed-choice entanglement swapping

It cannot be said in particular that "collapse" (assuming there is such a thing) occurs at any place or time. And obviously distance is not a factor, i.e. the speed of light c is not a limit*. If there is Einsteinian causality, it is not clear how it applies given the variety of experiments I linked which undermine its concepts. So I would not conclude that "special relativity" supercedes "quantum mechanics"; or the other way either.

*Experiments have shown that if collapse is not instantaneous, it must be at least 10,000c.

DrChinese
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