Or can one set of entangled Qubits be collapsed first, with the second set collapsed later and achieve the same values?


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  • $\begingroup$ what “same values” are you talking about? $\endgroup$ – ZeroTheHero Feb 9 at 18:09

No, the measurements on the entangled state do not have to be simultaneous for you to observe correlations in the outcomes.

Consider the maximally entangled state $\frac{1}{\sqrt{2}}(\vert 00\rangle + \vert 11\rangle)$. Say Alice, who holds the first qubit, measures her part of this state in the computational basis. She obtains either $\vert 0\rangle$ or $\vert 1\rangle$ with equal probability. Bob, who holds the second qubit, can measure it whenever he wants and will obtain the same value as Alice.

Connection to special relativity - Simultaneity is not absolute for all observers. Hence, it could even be that there exist three different reference frames and they don't agree on the order in which the measurements happened. For example, it could be that

  1. According to frame 1, Alice measures before Bob
  2. According to frame 2, Bob measures before Alice
  3. According to frame 3, both measure simultaneously.

No matter which reference frame you pick, both Alice and Bob will get the same correlated outcome of the measurement (i.e. both see $\vert 0\rangle$ or both see $\vert 1\rangle$)

  • $\begingroup$ @CCCSuffolk Please accept the answer if you are happy with it. $\endgroup$ – nr2618 Feb 10 at 18:35

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