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What are the implications of weak-measurement on entangled particles, and how does that resolve the problem of non-superluminal quantum "communication"? If I understand correctly, entangled particles X + Y are basically in superpositions that are anti-correlated with each other. As soon as one of the pair decoheres (or "wave function collapses"), say by an attempt to measure X by Alice, the other member of the pair Y observed by Bob will have an opposite value and is no longer itself in a proper superposition. The normal explanation why no information is "communicated" in this process is that distinct information is only present when the system is taken as a whole.

But say Alice weakly measures X, and learns that its wave function has not collapsed yet. Weak measurement is theorized to enable restoration of the superposition state of the qubit. Can she then logically infer that Bob has not collapsed? Wouldn't this imply FTL communication, since Alice can "poll" her member of the pair waiting for Bob to force Y to de-cohere? In this case, communication occurs because Alice knows Bob has forced measurement (the fact that the measurement has occured itself can an arrange signal between Alice and Bob.)

I'm not trying to argue anything or break physics, just trying to make sure I properly understand what does and doesn't happen when entangled particles resolve to classical states. Other answers such as this seem to provide an answer only for strong measurement. And this answer, while helpful, doesn't address the problem of measurement that doesn't collapse the superposition.

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  • $\begingroup$ I don't understand what the "problem of non-superluminal quantum communication" should be? There is no superluminal signaling going on in entanglement experiments. The correlations can never be revealed in less time than a light signal will take from A to B. $\endgroup$ – CuriousOne May 10 '16 at 20:34
  • $\begingroup$ "learns that its wave function has not collapsed yet" How can he learn this, if he only measures one side of an entangled state? Also, I would be careful with the term wave function collapse here. $\endgroup$ – Mikael Kuisma May 11 '16 at 0:14
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Weak measurements don't let you learn about the system without disturbing it. They let you make tradeoffs, where you disturb/decohere/collapse less by revealing less, but you still have to pay for whatever the measurements do reveal.

You can't combine many weak measurements into a "free" strong measurement.


But say Alice weakly measures X, and learns that its wave function has not collapsed yet.

There is no such measurement available to Alice. Alice's actions and measurements commute with Bob's actions and measurements in this situation.


just trying to make sure I properly understand

Did you translate the situation in question into math and then explore the behavior of the resulting math thing? If not, then no.

QM is known for being counter-intuitive precisely because the reasoning by over-simplified analogy that you're trying to do doesn't work. You need to do the math to build any kind of useful intuition about QM.

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  • $\begingroup$ "There is no such measurement available to Alice. Alice's actions and measurements commute with Bob's actions and measurements in this situation." - so where am i wrong? Is it that Bob's measurement doesn't affect the collapse of Alice's side of the qubit? Or that Alice has no way to distinguish between a superposition and collapsed state? $\endgroup$ – Jeremy Gilbert May 10 '16 at 20:44
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    $\begingroup$ @JeremyGilbert Alice has no way to distinguish between the entangled case and the collapsed state. $\endgroup$ – Craig Gidney May 10 '16 at 21:28

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