B Why is it assumed communication through entanglement would be FTL?

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vanhees71

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Well. . . !!




That seems to narrow things down, a bit. . . . :DD








OK, even though I read everything here, yes everything, I probably shouldn't

post here, so. . .



Carry on . . 👌 . :oldbiggrin:

.
It's a somewhat too narrow view, since to the contrary I claim that I follow the interpretation the great majority of physicists follow, which is the minimal statistical interpretation in the one or the other form. Sometimes it's even called the "orthodox interpretation". How can something be "orthodox" if it's only followed by one completely irrelevant person as I am. Believe it or not, I'm not a guru with many believing followers ;-))). SCNR.
 

DrChinese

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For instance when people say "the measurement here makes instantaneously a change there" they could mean all kinds of things, to me it is so vague that it is almost vacuous. What does it actually mean?
Weinberg phrases the answer as: "Of course, according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem..."

And what he means is: In a normal Bell test, Alice performs a measurement on her previously entangled particle. That projects the particle - previously in some superposition - into a specific pure state. To be specific, let's say Alice's particle is found to be polarized at 10 degrees. It certainly was NOT polarized previously at 10 degrees (out of the infinite number of possibilities), that occurred ONLY as a result of her measurement.

Now Bob's distant particle can be said to be in a specific state, one that it could not have been in prior to Alice's measurement. That can be experimentally verified, and to again be specific: let's have Bob's particle be re-routed so the experimental verification occurs in Alice's frame of reference at a later time. That means the measurement on Bob occurs AFTER Alice's measurement in every possible reference frame. So we miraculously find Bob in a state that can have only been steered by Alice's previous measurement.

Now, please explain to me how QFT or any Bell compliant theory can say that Bob was *not* affected by something Alice did remotely (independently). There is no question that Bob's particle was influenced precisely due to Alice's exact choice of measurement basis. It didn't take on its polarization coincidentally, because it had no polarization at 10 degrees when it was entangled (as Bell tells us).

What is there not to agree with? It doesn't matter how you construct a theory when the above is experimental fact. And no, this is not really a semantic issue: quantum nonlocality is the name of this effect, as can be seen in this book of compiled articles (confusingly titled Quantum Nonlocality :smile: ) released this year (2019):

Entire book:
Abstract:

"It is important to understand what the meaning of nonlocality is in quantum theory.
Quantum theory does not have the strongest and simplest concept of nonlocality, which is the
possibility of making an instantaneous observable local change at a distance. However, all single-world
interpretations do have actions at a distance. The quantum nonlocality also has an operational meaning
for us, local observers, who can live only in a single world. Given entangled particles placed at a

distance, a measurement on one of the particles instantaneously changes the quantum state of the other, from a density matrix to a pure state."

Either these many top writers have not heard of QFT, or perhaps what I am passing on is good ol' fashioned scientific consensus.
 
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Either these many top writers have not heard of QFT, or perhaps what I am passing on is good ol' fashioned scientific consensus.
What you are passing on might indeed be the current consensus, but I do not think that is an adequate response to the obvious point that, since non-relativistic QM is just an approximation to QFT, quantum foundation discussions that are solely based on non-relativistic QM--which is basically all of them--are incomplete. Those many top writers have surely heard of QFT, yes, but that doesn't mean their failure to include QFT in their foundations work can simply be ignored.
 

martinbn

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@DrChinese You didn't answer my specific question about that specific scenario. Can B tell whether A measured or not? If not, in what sense A's measurement caused a change (what change)?

What you describe is something that I have seen many time before, here and other places. But it is just an attempt to write in plain language. It reminds me a lot of "light path bends near stars because mass curves the fabric of space-time". Of course it is not the same, but it seems to be in the same spirit.

I still think it is a matter of language and not science. Because I am only confused when people say action at a distance or A causes B.
 

DarMM

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Given entangled particles placed at a
distance, a measurement on one of the particles instantaneously changes the quantum state of the other, from a density matrix to a pure state.
If you consider the wave function to describe the actual state of the physical object then yes there is an influence. However if the wavefunction is just a tool for calculating probabilities of observations all that has happened is that Alice has conditioned her state on her observation and has increased predictability for Bob's outcomes (mixed -> pure).
 

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DrChinese

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What you are passing on might indeed be the current consensus, but I do not think that is an adequate response to the obvious point that, since non-relativistic QM is just an approximation to QFT, quantum foundation discussions that are solely based on non-relativistic QM--which is basically all of them--are incomplete. Those many top writers have surely heard of QFT, yes, but that doesn't mean their failure to include QFT in their foundations work can simply be ignored.
My point is the other way: experimental fact, plus virtually any assumption about quantum theory (QM or QFT or whatever that includes the HUP) shows us that Alice's choice of measurement basis casts Bob's particle into a pure state determined solely by Alice (from an infinite number of such). Whether you follow QM or QFT is quite irrelevant. Specifically, there is no reference frame where the results would be different for my example. I.e. relativity plays NO part in the predicted outcome. That's why these authors don't make a distinction. If QFT (or any theory) does not feature that which is readily seen in experiment, then it is considered falsified. That is science all day and all night. So I calmly assert that QFT contains action at a distance a la my example, the same as is discussed in the book on Quantum Nonlocality.

Quantum Nonlocality
= (Spooky) Action At A Distance
= "...a measurement in one subsystem does change the state vector for a distant isolated subsystem... "

...Which we learned from EPR+ Bell+Aspect, and every related experiment since.
 

DrChinese

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@DrChinese You didn't answer my specific question about that specific scenario. Can B tell whether A measured or not? If not, in what sense A's measurement caused a change (what change)?

What you describe is something that I have seen many time before, here and other places. But it is just an attempt to write in plain language. It reminds me a lot of "light path bends near stars because mass curves the fabric of space-time". Of course it is not the same, but it seems to be in the same spirit.

I still think it is a matter of language and not science. Because I am only confused when people say action at a distance or A causes B.
I think I did answer directly. You ask what is meant by your statement (I used Weinberg's viewpoint and wording as I thought that make my answer more palatable). I gave a specific example. In that example, Alice makes a measurement decision on one particle A from infinite possible measurements. She then finds the other particle B in a known pure state afterwards (Bob's particle is sharply defined in one observable and undefined in non-commuting observables). Now, perhaps you don't believe this demonstrates remote steering by Alice and Alice alone (and as stated clearly by Weinberg). In that case, I cannot be of help. However, it is scientific consensus as I showed with my book reference. On the other hand, there is no requirement for anyone to accept consensus (other than it is required for PhysicsForums posts).

This just as in the old EPR paradox. As EPR says, if QM were complete, then there must be spooky action at a distance. Now we know there is no more complete specification of a quantum system than the HUP allows, and I am unaware of any theoretical extension to the HUP that changes that conclusion.
 
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experimental fact, plus virtually any assumption about quantum theory (QM or QFT or whatever that includes the HUP) shows us that Alice's choice of measurement basis casts Bob's particle into a pure state determined solely by Alice (from an infinite number of such). Whether you follow QM or QFT is quite irrelevant
No, QM vs. QFT is not at all irrelevant here, because in QFT there is no such thing as a "state" involving a spatially extended system. More precisely, any such "state" is frame-dependent. In QFT the ontology is operators attached to particular events in spacetime; if the events are spacelike separated, the operators must commute.

So the description you are giving here is not a simple straightforward description of what happens in the experiment. It is a theory-dependent description.

Also, the HUP is irrelevant here because we are not dealing with non-commuting observables. We have one Alice observable and one Bob observable, and all of the Alice observables commute with all of the Bob observables. (That is a straightforward consequence of the QFT principle I stated above.)

calmly assert that QFT contains action at a distance
If you define "action at a distance" to mean "correlations that violate the Bell inequalities", then yes, this is true. And that is also Bell's definition of "nonlocality".

However, if you go beyond that to make any specific claims about underlying mechanisms, then no, I do not agree that QFT necessarily contains any such mechanisms.
 

DrChinese

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No, QM vs. QFT is not at all irrelevant here, because in QFT there is no such thing as a "state" involving a spatially extended system. ...
Clearly, Weinberg does not agree with you since he states precisely the opposite (though it appears to make no difference to anyone). And neither do many other top names (I provided a book full of those that are immediately ignored, the book being title Quantum Nonlocality).

I have absolutely no doubt that you and Vanhees71 understand far more about QFT that I ever would in 10 lifetimes. And yet, I have yet to see an single reference or quote that contradicts a single statement I have made. With all due respect, I am following PF guidelines faithfully - but I do not think you two are.

Quantum nonlocality is established by perhaps a thousand experiments. In fact, a backlash is starting to occur on these within the community because (drumroll...) these experiments merely confirm well-established theory. What I don't know any more than anyone else is... what is the mechanism whereby quantum nonlocality operates? If QFT answered that (which it obviously does not), then we wouldn't need interpretations of QM, would we?

I mean no disrespect to you or anyone else. But I disagree that QFT as a theory should be held up as something it is not. It does not cause Bohmians (such as @Demystifier) to reject Bohmian Mechanics, it does not cause MWIers to reject MWI, and it does not cause those who accept Time Symmetric/Retrocausal/Acausal interpretations to reject those. Are those people simply ignorant? Or perhaps someone is overselling QFT.

Either way, I have asked for specific quotes supporting a position counter to mine, and every time you turn the argument around and demand more from me. That is completely unfair. So I ask you: Please provide a straight statement from a recognized authority that support your position and/or reject mine, as stated below.

Your position (I assume it to match that of Vanhees71, although please correct me as appropriate): Quantum nonlocality - spooky action at a distance - is no longer considered a feature of quantum mechanics because orthodox QFT is locally causal.

My position: Perhaps the weirdest feature of quantum mechanics is entanglement, the need to describe even systems that extend over macroscopic distances in ways that are inconsistent with classical ideas. A measurement in one subsystem does change the state vector for a distant isolated subsystem. That demonstrates quantum nonlocality.
 
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Weinberg does not agree with you since he states precisely the opposite
Where does he state that about QFT? Bear in mind that I am not asking what he says about quantum foundations; I've read plenty of what he's written about quantum foundations, and none of it says a thing about QFT; all of his writings on the subject that I have read, like all the other quantum foundations literature that I have read, uses non-relativistic QM as its framework. So the fact that it all talks about states of spatially separated systems does not at all answer the question I am asking, because of course non-relativistic QM assigns states to spatially separated systems. Nobody is disputing that. But that does not mean that QFT does so too. To establish that you need to show me a reference about QFT.

More generally, your argument appears to be that, since all of these well-known scientists are using non-relativistic QM instead of QFT to discuss quantum foundations, QFT must make no difference to quantum foundations. I think that is a weak argument. At the very least, if it really is true that everybody working in the field believes that, it would be nice to see a reference to a textbook or paper where they explain why; I have never seen one, and while I have not read the entire literature in the field, I have spent some time looking since it seems so obvious to me that there should be such an argument if everyone in QM foundations is simply going to ignore QFT. Every time someone posts a link to a new QM foundations paper here at PF, I look at it just to see if QFT is mentioned. So far it never has been.

The fact that I have not found such an explanation, however, does not convince me that it must be the case that QFT makes no difference to quantum foundations. I do not share your confidence that all those well-known scientists could not simply have missed this simple point; if they have, it would not be the first time that a point which in retrospect seems obvious was missed by a lot of very smart people in a scientific field.

Quantum nonlocality is established by perhaps a thousand experiments.
Again, you need to define what you mean by "quantum nonlocality". If it means "correlations that violate the Bell inequalities", then of course you are correct, and nobody has disputed that. Nobody is disputing the actual experimental results. The only disputes are about what kind of story you want to tell in ordinary language about the experimental results, and whether you need to pay attention to QFT in order to tell such a story.

If you mean something else by "quantum nonlocality", then you're going to have to explain what, because at that point "quantum nonlocality" no longer means the thing that is "established by perhaps a thousand experiments", but some other theory-dependent claim.

What I don't know any more than anyone else is... what is the mechanism whereby quantum nonlocality operates?
Neither do I. Neither does anyone else; as you say, if someone did, we would not have all these arguments about interpretations of QM.

However, you are arguing that nobody needs to pay any attention to QFT in order to investigate this question, which is a stronger claim that seems to me to be obviously false--or at any rate seems to me to require some justification that I have never seen provided.

I disagree that QFT as a theory should be held up as something it is not. It does not cause Bohmians (such as @Demystifier) to reject Bohmian Mechanics, it does not cause MWIers to reject MWI, and it does not cause those who accept Time Symmetric/Retrocausal/Acausal interpretations to reject those. Are those people simply ignorant? Or perhaps someone is overselling QFT.
All this is irrelevant, since QFT is not a rival interpretation of QM. QFT is a theory that is more fundamental tha non-relativistic QM, and includes non-relativistic QM as an approximation when relativistic effects can be ignored. As a theory, QFT is just as interpretation agnostic as "shut up and calculate" non-relativistic QM is. So of course nobody needs to give up their pet interpretation because of QFT.

But, once again, that is not at all the same as saying that nobody needs to pay attention to QFT when investigating quantum foundations. The latter is a stronger claim, which again seems to me to be obviously false or at least in need of justification.

Your position (I assume it to match that of Vanhees71, although please correct me as appropriate): Quantum nonlocality - spooky action at a distance - is no longer considered a feature of quantum mechanics because orthodox QFT is locally causal.
That is not my position at all. I won't speak for @vanhees71 , he can explain his position himself.

My position, as should be obvious from the above, is that "quantum nonlocality" in the sense of correlations between spacelike separated measurements that violate the Bell inequalities, is an obvious experimental fact. QFT predicts this experimental fact, so QFT is perfectly consistent with quantum nonlocality in this sense.

QFT is also "locally causal" in the sense that spacelike separated measurements commute. The fact that such measurements commute in no way prevents their results from showing correlations that violate the Bell inequalities; QFT predicts that too.

Those are the only meanings of "quantum nonlocality" and "locally causal" that I am aware of that are well-defined enough for me to have a position on. The term "spooky action at a distance" is too vague for me to care about it; either it means the same thing as quantum nonlocality--correlations that violate the Bell inequalities--in which case we already have a perfectly good term for it and we don't need another one; or it means some kind of hypothetical mechanism that could produce the correlations, but nobody who uses the term ever specifies what that mechanism is (other than to say that we know what it isn't--it isn't a local hidden variable mechanism), so there's no point in discussing it.

My position: Perhaps the weirdest feature of quantum mechanics is entanglement, the need to describe even systems that extend over macroscopic distances in ways that are inconsistent with classical ideas.
I would rephrase this as: the need to explain how measurements on entangled systems at macroscopic spacelike separations can show correlations that violate the Bell inequalities. That makes it precise exactly what experimental facts you are referring to.

A measurement in one subsystem does change the state vector for a distant isolated subsystem.
This is not an experimental fact but a theory-dependent statement. The experimental fact is correlations that violate the Bell inequalities.
 

atyy

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The Bell theorem covers the case of relativistic QFT.
For simplicity, we can restrict ourselves to free relativistic QFT, which already predicts the violation of Bell inequalities.
The Bell theorem means that there are no local variable theories reproducing the predictions of QFT that Bell inequalities are violated.
In particular, QFT is not a local variable theory.
The statements by @vanhees71 basically say that QFT is local variable theory. Those statements are wrong.

As @DrChinese has mentioned, QFT has signal locality, and it does not have local variables. @vanhees71 is confusing signal locality with local variables.
 

Demystifier

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In QFT the ontology is operators attached to particular events in spacetime
I don't think "ontology" is the right word here. In particular, in the path-integral formulation of QFT there are no field operators at all, but ontology should not depend on the formulation.
 

Demystifier

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The Bell theorem covers the case of relativistic QFT.
For simplicity, we can restrict ourselves to free relativistic QFT, which already predicts the violation of Bell inequalities.
The Bell theorem means that there are no local variable theories reproducing the predictions of QFT that Bell inequalities are violated.
In particular, QFT is not a local variable theory.
The statements by @vanhees71 basically say that QFT is local variable theory. Those statements are wrong.

As @DrChinese has mentioned, QFT has signal locality, and it does not have local variables. @vanhees71 is confusing signal locality with local variables.
Instead of talking about "variables", it would be more precise to talk about observables and beables. QFT has local observables (hermitian operators commuting at spatial distances), but the Bell theorem excludes locally interacting beables.
 

DarMM

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I think all this confusion is caused by CHSH etc inequality violating correlations being given the name "nonlocal correlations" or "spooky action at a distance" rather than simply "nonclassical correlations" which is what the quantum foundations community seems to be moving toward.

Under the old name @vanhees71's rejection of "spooky action at a distance" makes it sound like he rejects Bell's inequality violations. Where as it is just the statement that QFT is local, which is in no contradiction with the existence of nonclassical correlations in that theory.

Although I think what's sometimes missing in these accounts is that dropping determinism is not enough to get nonclassical correlations, you also have to drop the existence of countertfactuals.
 

atyy

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I think all this confusion is caused by CHSH etc inequality violating correlations being given the name "nonlocal correlations" or "spooky action at a distance" rather than simply "nonclassical correlations" which is what the quantum foundations community seems to be moving toward.

Under the old name @vanhees71's rejection of "spooky action at a distance" makes it sound like he rejects Bell's inequality violations. Where as it is just the statement that QFT is local, which is in no contradiction with the existence of nonclassical correlations in that theory.

Although I think what's sometimes missing in these accounts is that dropping determinism is not enough to get nonclassical correlations, you also have to drop the existence of countertfactuals.
The problem is that @vanhees71 is claiming that QFT is local in a way that is excluded by the Bell inequality violations.

No one is contesting that QFT is local in the sense of not allowing superluminal communication.

One can see that @vanhees71 is not referring to signal locality, because he objects to the nonlocality of collapse on the basis of what he calls the microcausality of QFT. That is wrong, because the microcausality of QFT is signal locality, and the nonlocality of collapse is consistent with signal locality.
 

DarMM

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The problem is that @vanhees71 is claiming that QFT is local in a way that is excluded by the Bell inequality violations.

No one is contesting that QFT is local in the sense of not allowing superluminal communication.

One can see that @vanhees71 is not referring to signal locality, because he objects to the nonlocality of collapse on the basis of what he calls the microcausality of QFT. That is wrong, because the microcausality of QFT is signal locality, and the nonlocality of collapse is consistent with signal locality.
I've had this discussion with @vanhees71 before and what is happening is a conflict between how he uses the word "collapse" and how you and myself use it. He's rejecting collapse as an actual nonlocal process. So he's saying that state reduction causing a global update to the wavefunction doesn't mean there is a physical nonlocal process occurring.

Basically he only uses "collapse" to refer to state reduction as a physical nonlocal process.
 

atyy

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I've had this discussion with @vanhees71 before and what is happening is a conflict between how he uses the word "collapse" and how you and myself use it. He's rejecting collapse as an actual nonlocal process. So he's saying that state reduction causing a global update to the wavefunction doesn't mean there is a physical nonlocal process occurring.

Basically he only uses "collapse" to refer to state reduction as a physical nonlocal process.
Here I mean collapse as a physical nonlocal process.

It is not correct to use the microcausality of QFT to object to the nonlocality of collapse, because
(1) microcausality does not refer to physical locality
(2) under appropriate assumptions, any physical variables reproducing the quantum predictions must be nonlocal - it is not possible to save locality by rejecting collapse.
 

Demystifier

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Although I think what's sometimes missing in these accounts is that dropping determinism is not enough to get nonclassical correlations, you also have to drop the existence of countertfactuals.
What exactly do you mean by counterfactual? Something that could happen but didn't, or something that happened but nobody observed?
 

DarMM

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Here I mean collapse as a physical nonlocal process.

It is not correct to use the microcausality of QFT to object to the nonlocality of collapse, because
(1) microcausality does not refer to physical locality
(2) under appropriate assumptions, any physical variables reproducing the quantum predictions must be nonlocal - it is not possible to save locality by rejecting collapse.
All that @vanhees71 is saying is that he doesn't see the necessity of collapse as a physical process.

I agree that microcausality of QFT is not an argument against signal-local theories that might be nonlocal in other ways, e.g. Bohmian Mechanics. I'm not sure if @vanhees71 would disagree either as the discussion has been confused by the use of different meanings for "collapse".

I don't think physical variables must be nonlocal to replicate QM predictions, that's just one way of explaining CHSH violations, but not the only one.
 

DarMM

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What exactly do you mean by counterfactual? Something that could happen but didn't, or something that happened but nobody observed?
The standard meaning in Quantum Foundations, that variables unmeasured have values.

Chapter 6 of Peres's monograph "Quantum Theory: Concepts and Methods" discusses it and it is used very explicitly in his proof of Bell's theorem. It's not an assumption called out in the original Bell proof, but it is the assumption Copenhagen rejects so it is important to recognize. He says famously "Unperformed experiments have no results"

I do not mean (and I want to empasize this as it is what people seemed to think it means in previous discussions) the trivial fact that unperformed experiments did not happen.
 

Demystifier

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I don't think physical variables must be nonlocal to replicate QM predictions, that's just one way of explaining CHSH violations, but not the only one.
If we assume counterfactuality, what, in your opinion, are the other reasonable options?
 

DarMM

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If we assume counterfactuality, what, in your opinion, are the other reasonable options?
The other options are superdeterministic, retrocausal, many world or acausal theories. Depending on their formulation they might also reject counterfactuals though not necessarily.
 

vanhees71

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Weinberg phrases the answer as: "Of course, according to present ideas a measurement in one subsystem does change the state vector for a distant isolated subsystem..."

And what he means is: In a normal Bell test, Alice performs a measurement on her previously entangled particle. That projects the particle - previously in some superposition - into a specific pure state. To be specific, let's say Alice's particle is found to be polarized at 10 degrees. It certainly was NOT polarized previously at 10 degrees (out of the infinite number of possibilities), that occurred ONLY as a result of her measurement.
Again, where is this quote from? Weinberg has written the very best QFT books I know of, and there he emphasizes heavily the importance of the cluster-decomposition principle, its relation to Lorentz covariance and very carefully the microcausality of local observables. I think, it's important to know the context, where this is quoted from.

In a normal Bell test, when A performs a measurement, she'll adapt her description of the state, but that doesn't imply that anything happens instantaneously at B's place. Thanks to the cluster decomposition principle, which is valid in usual local (i.e., microcausal QFT) it's clear that nothing changes for B.

Of course the correlations of observables concerning far-distant parts of an entangled system are no contradiction and are in full accordance with the real-world measurements, which have demonstrated that it's not a causal connection due to A's or B's measurement but due to the preparation in an entangled state by demonstrating that the temporal order of the local measurements at A's and B's place don't play any role. Most importantly the correlations are even there if the measurement events at A's and B's place are space-like separated. We have discussed this at length recently in the thread about the entanglement-swapping experiment by Zeilinger's group with four photons.
 

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