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I'm referring to this recent paper, "Experimental Proof of Nonlocal Wavefunction Collapse for a Single Particle Using Homodyne Measurements" by Fuwa et al. published in Nature Communications. Non-paywalled arXiv version here. From the abstract:

...the choice of measurement in one lab really causes a change in the local quantum state in the other lab

My understanding of entanglement (whether it is two particles, or one as in this article) is that Bob's measurement can be affected by the (uncontrollable) result of Alice's measurement, but not her choice of measurement. (the latter would seem to enable a form of non-local communication). So is it just a badly-worded abstract? (the full article uses similar language)

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From my initial reading of the paper it looks like the the quantum state Bob has access to is changed by Alice's choice of measurement but in such a way that he can't tell until Alice communicates her choice of measurement to him.

In particular they present a way for Alice to prove to Bob that she has influenced the part of the state he has access to even if he doesn't trust her (so even if Alice is malicious Bob can be confident that they shared an entangled state initially).

The phase that Bob measures looks to be uniformly distributed if he doesn't condition on anything. However if he conditions his measurements on the numbers Alice gives him (basically he splits the data up into groups depending on what settings Alice says she chose) structure emerges as shown in figure 2.

This is quite a general feature of entanglement. Alice can "steer" the quantum state Bob has access to in what appears to be a violation of special relativity but only in such a way that Bob can't tell unless Alice sends him supplementary information over a classical channel (so they can't break causality).

I agree that there is a huge amount of superstition / misinformation around about quantum effects though (especially entanglement effects).

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  • $\begingroup$ I'm trying to understand this but it makes no sense. What they are arguing is that it's a non-local effect, i.e. that something Alice does affects Bob's measurement. However I don't see why that is non-local, because if Alice has to tell Bob she did something (using non-FTL communications), then clearly, there has been time for the effect of her measurement upon the wavefunction to propagate over to Bob's lab, where it is now local. It's like throwing a rock in a pond, then waiting for the waves of your rock to get to the other side, and saying, "look at my waves." How is that non-local? $\endgroup$
    – CommaToast
    Apr 3, 2015 at 0:37
  • $\begingroup$ @CommaToast Her and Bob's measurements are time like separated which means nothing causal (slower than light) can transfer information between them. Bob initially gains results which look random. Then Alice sends over her information and Bob splits his results into bins depending on the settings Alice chose which reveals the structure. So the structure was always in Bob's data but he didn't know how to see it until Alice sends her causal message. $\endgroup$
    – or1426
    Apr 3, 2015 at 11:17
  • $\begingroup$ Ok, I think I'm following. So, forgetting about any "communication" between the two observers, but just looking at the order in which they make their observations: does Alice take her actual measurements prior to Bob taking his actual measurements? Or is it that Bob takes his measurement prior to Alice taking hers? Are they in different buildings with different electrical power systems? Is there any shielding that would block interference from Alice's apparatus from affecting Bob's? Just trying to understand. $\endgroup$
    – CommaToast
    Apr 4, 2015 at 17:41
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    $\begingroup$ I've read the study, I know what they are saying happens. However, what's not clear to me is how they know it's not light pollution feeding back upstream from Alice to Bob that's causing the shift. I mean, it's not like they're able to do the experiment on a single photon. They need to leave the laser turned on for a significant time and average their readings out over many photons. Sometimes effects are just artifacts of lab environments. And if every similar such experiment ever done was in such an environment, then it's a self-reinforcing prophesy. $\endgroup$
    – CommaToast
    Apr 10, 2015 at 3:48
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    $\begingroup$ Oh, I have no doubts as to QM's accuracy. I just want to know what is causing this effect. Saying Alice's measurement causes it does not explain how it does. $\endgroup$
    – CommaToast
    Apr 14, 2015 at 7:09

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