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Suppose we have a pair of entangled photons, one photon sent to Alice and the other to Bob. A and B are at rest in the same reference frame. When A receives and measures the polarization of her photon, the photon sent to B is still traveling and he measures its polarization later. Does B's photon acquire its correlated polarization instantaneously when A measures her photon, or when B makes his measurement later on?

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    $\begingroup$ How would you tell the difference? $\endgroup$
    – tparker
    Commented Nov 24, 2019 at 4:19
  • $\begingroup$ @ReneKail as far as we know, the collapse is instantaneous for the entangled photon. $\endgroup$
    – lineage
    Commented Nov 24, 2019 at 4:41
  • $\begingroup$ see this answer physics.stackexchange.com/questions/446671/… $\endgroup$
    – anna v
    Commented Nov 24, 2019 at 7:48
  • $\begingroup$ A great extreme case for this is the Delayed Choice Quantum Eraser, which has actually been executed and we've confirmed the results are in line with the predictions of QM $\endgroup$
    – Cort Ammon
    Commented Feb 6 at 22:27

3 Answers 3

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Suppose we have a pair of entangled photons, one photon sent to Alice and the other to Bob. A and B are at rest in the same reference frame. When A receives and measures the polarization of her photon, the photon sent to B is still traveling and he measures its polarization later. Does B's photon acquire its correlated polarization instantaneously when A measures her photon, or when B makes his measurement later on?

If you measure one photon of a spatially separated entangled pair of photons, nothing happens to the other member of the entangled pair. The relevant equations of motion are Lorentz invariant so the motion of the systems concerned can't depend on the order of spacelike separated events since that order isn't Lorentz invariant. The expectation values of measurements on each photon don't change as a result of measurements on the other photon.

If we take the equations of motion of quantum theory as an accurate description of reality, then the correlations arise after information about the results have interacted with one another. Decoherent channels such as records on a computer or signals in a wire carry quantum information about those results even though the expectation values of their observables don't depend on that information: this is called locally inaccessible information

https://arxiv.org/abs/quant-ph/9906007

It is common to say that quantum mechanical equations of motion are not accurate as a description of reality, or to fudge that issue. As a result, most accounts of entanglement don't give any explanation of how correlations arise and even when they do they rarely provide any substance about the alleged mechanism such as wavefunction collapse or whatever.

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  • $\begingroup$ Yet another disguised attempt to push a particular interpretation of QM, in this case MWI. Actually, no one knows the mechanism of how measurements on widely separated particles stay synchronized, but we know from Bell's Theorem that the process cannot be causally or deterministically local. Normally, Bell tests do not operate on the entangled pair simultaneously. However, since Lorentz invariance is not in any way, form or fashion relevant to the predicted quantum outcome (in complete opposition to what you say), ordering doesn't matter. At. All. $\endgroup$
    – DrChinese
    Commented Feb 9 at 21:46
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    $\begingroup$ @DrChinese If you want to say anything about what's happening in reality, you're picking a particular "interpretation". Simultaneity for spacelike separated events is frame dependent. Lorentz invariance is relevant to quantum theory, see e.g. Chapter 9 of "Quantum Field Theory for the Gifted Amateur" by Lancaster and Blundell and "The Quantum Theory of Fields Volume 1: Foundations" by Weinberg. $\endgroup$
    – alanf
    Commented Feb 10 at 7:54
  • $\begingroup$ Believe me, Weinberg doesn't talk about MWI or any "interpretation" in your reference. If you're so proud of your personal interpretation choice, why not disclose it up front? That way unsuspecting readers aren't led astray by your comments. Simultaneity being frame dependent is sometimes relevant in QM, but not with entanglement. $\endgroup$
    – DrChinese
    Commented Feb 11 at 14:49
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    $\begingroup$ The sentence I wrote that mentioned Weinberg was about Lorentz invariance not the MWI per se. I explicitly say that I am assuming that quantum mechanical equations of motion are an accurate description of reality. Every non-MWI "interpretation" either explicitly sez they are wrong or fudges the issue. I don't mention the MWI for much the same reason that I wouldn't mention interpreting GR in terms of curved spacetime: there is no serious alternative. $\endgroup$
    – alanf
    Commented Feb 11 at 17:37
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As tparker points out in the comment I just upvoted, there is more than one observationally equivalent way to model this. But according to standard interpretations, your measurement collapses the pair into a new state, the new state is unentangled, and therefore you can say that from that moment on the second particle has a state of its own. Of course you can't expect that state to necessarily be an eigenstate of some measurement that nobody's yet made.

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  • $\begingroup$ Yes, this corresponds to the conclusion I came upon after some reflection. $\endgroup$
    – Rene Kail
    Commented Nov 24, 2019 at 9:00
  • $\begingroup$ It's worth clarifying that by "standard interpretations" you specifically mean the Copenhagen interpretation. In the many-worlds interpretation, the wave function collapse is not instantaneous but gets "transmitted" at or below the speed of light. $\endgroup$
    – tparker
    Commented Nov 26, 2019 at 14:34
  • $\begingroup$ @tparker so are we saying ftl communication is the reality? Or just the interpretation implies ftl? …. The interpretation thereby being somewhat implausible? $\endgroup$ Commented Feb 8 at 15:13
  • $\begingroup$ @ReneKail so are we saying ftl communication is the reality? Or just the interpretation implies ftl? …. The interpretation thereby being somewhat implausible? $\endgroup$ Commented Feb 8 at 15:14
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    $\begingroup$ @PhysicsDave : Nobody said anything about communication. $\endgroup$
    – WillO
    Commented Feb 8 at 15:28
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I think remarkably in order to uphold Heisenberg Uncertainty and Quantum Mechanics Together,

It must be that given particle A and Particle B are entangled:

  1. When one measures a property of particle A, at that moment in simultaneity the property measured in Particle A is 100% correlated in particle B instantly, and with no travel time, no delay

  2. The collapsed state of Particle A and hence B as well was not only undeterminable in any shape or form before the measurement was taken, but the angle and property measured from Particle A made a difference in the outcome of the measurement of Particle A and hence the closer to that time Particle B is measured using the same property, the state observed matches exactly what was learned from particle A.

  3. After the measurement of particle A, Particle B is no longer entangled to Particle B which means the longer particle B same property isn’t measured in particle B, the more it won’t correlate to Particle A since its new wave probability is now disentangled

  4. Any other property of Particle B that is measured during or after the collapse of Particle A, will be specific only to Particle B,

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    $\begingroup$ a) The difference in time of measurement of 2 entangled particles has no significance at all, other than if the second interacts with something prior to its measurement. b) There is no evidence that measurement of the first particle affects the second particle as opposed to vice versa; that is by assumption only. c) Distance is not a factor either. d) Particles can be entangled on multiple bases. If so, it is possible to collapse on one basis without causing collapse on another. $\endgroup$
    – DrChinese
    Commented Feb 6 at 22:45

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