Exploring Entanglement Swapping: Post-Selection vs. BSM?

[kurt101]

The way to investigate this question is for you to read the literature and see if you can find papers supporting what you say. If you can, by all means post references to them here. Writing your own personal code is not doing that; as I have said, it is personal theory and is off limits here.

Here is a paper that refutes @DrChinese claims https://link.springer.com/article/10.1007/s10701-021-00511-3#Sec21

From the Conclusions section in this paper:
"Under conventional assumptions—i.e., excluding retrocausality—the sensitivity of such experiments to this Collider Loophole depends on the temporal relation between the entanglement-swapping measurement C and the measurements A and B. CL a threat if the C is in the future of A and B, but not if it is in the past."

 

[vanhees71]

What you personally would call it is immaterial. In the QM literature, the "nonlocal" effects which some have called "spooky action at a distance" and which others have called "acausal" (not just @DrChinese, the term appears in multiple papers in the literature, some of which have been referenced in other threads--for example see some of the posts by @RUTA), are the basis for the violation of the Bell inequalities by QM.

Remember that PF does not allow discussion of personal theories. Your claims about your code amount to a personal theory. That means they are off limits for discussion here. Do not post further about them or you will receive a warning.

Now we are back at this fruitless discussion about what "non-locality" means. As we realized some time ago, it's not a uniquely defined term. In the HEP community we call our relativistic QFTs "local", including that all interactions are "local". This is implemented as a clearly defined mathematical property of the field operators that represent local observables, i.e., the microcausality condition, and this condition excludes faster-than-light causal influences, i.e., in this sense standard relativistic local QFT has no "spooky actions at a distance". In this sense of the term entanglement doesn't describe any non-local interactions but just correlations between far-distant parts of quantum systems prepared in the corresponding entangled states. It's rather what Einstein called "inseparability" than "non-locality".

 

[PeterDonis]

you can simply do the measurments on 1&4 and based on the outcomes select a subset with desired corelations.

That's not a valid way of selecting a subensemble. It would be like me claiming I had a biased coin by flipping it a thousand times and then selecting the "subensemble" of all the flips that came up heads.

The statistics of 1&4 does not change because the density matrix does not change.

Yes, vague ordinary language isn't really suitable here. The density matrix of 1 alone or of 4 alone does not change if the 2&3 BSM is done, but the correlations between 1 and 4 do change--there are subensembles selected by the 2&3 BSM results that violate the Bell inequalities, that aren't there if the 2&3 BSM is not done. Both of those things could be referred to by the word "statistics", so it's good to be clear about exactly what doesn't change and what does.

 

[PeterDonis]

This is an interpretation dependent statement.

No, it isn't. Mathematically, if you want to correctly predict the correlations between 1&4, you have to use the appropriate state based on the result of the 2&3 measurement. That's true no matter what interpretation you adopt.

Different interpretations will tell different stories about why you have to do that.

 

[PeterDonis]

My definition of sub-ensemble is just a subset of a full ensemble,

That's the wrong definition. See the first part of my post #73.

it's also spelled out clearly in this paper, which I brought up last time this was discussed:

Bell Inequality Violation and Relativity of Pre- and Postselection

This paper does not mean what you think it means. It is making the same error that I described in the first part of post #73.

The BSM operation might even select the same sub-ensemble that was selected by just looking at the results.

This makes no sense. You can't pick out "the same results" in two physically different experiments.

 

[PeterDonis]

Here is a paper that refutes @DrChinese claims https://link.springer.com/article/10.1007/s10701-021-00511-3#Sec21

From the Conclusions section in this paper:
"Under conventional assumptions—i.e., excluding retrocausality

This isn't a refutation of anything. It's a proposed claim about a new "loophole" in such experiments. Such claims are ubiquitous in the literature, but they always boil down to one thing: claiming that if an experiment is done that closes the claimed loophole, the predictions of quantum mechanics will be violated. In other words, such claims are made by people who don't want to accept that the QM predictions are correct.

So far, every time a more sophisticated experiment has been done that closes one of these loopholes, the predictions of QM have continued to be confirmed. So the track record of these loophole claims is one of complete failure so far. If you want to maintain the position that this particular loophole won't share this fate, that's up to you; but unless and until an actual experiment is done that (a) closes the loophole, and (b) shows a violation of QM's predictions, you cannot claim that anything is refuted.

 

[PeterDonis]

Now we are back at this fruitless discussion about what "non-locality" means.

"Nonlocality" is just a word. The people who use that word in ways you don't like make it perfectly clear what it refers to in the actual math. If you prefer to call that thing in the math "inseparability", that's fine as far as it goes, but unless and until you have convinced everyone else in the QM community that uses the word "nonlocality" for that thing, to use the word "inseparabilty" instead, it's pointless to complain about it.

 

[DrChinese]

Here is a paper that refutes @DrChinese claims https://link.springer.com/article/10.1007/s10701-021-00511-3#Sec21

From the Conclusions section in this paper:
"Under conventional assumptions—i.e., excluding retrocausality—the sensitivity of such experiments to this Collider Loophole depends on the temporal relation between the entanglement-swapping measurement C and the measurements A and B. CL a threat if the C is in the future of A and B, but not if it is in the past."

LOL. They start by assuming that which they seek to prove. You can't exclude retrocausality by assumption because that is part of the question they seek to answer. One of the authors, Huw Price, has been arguing against retrocausality for over 25 years. Now please keep in mind that I am agnostic on the issue of retrocausality itself. All I assert is that quantum nonlocality violates strict Einsteinian locality, by creating a context that spans spacetime. I think a better term than "retrocausal" is "acausal". (They sometimes use the word "noncausal".)

As to the meat of their argument, it makes absolutely no sense at all. There is no such thing as the "collider loophole" (CL) in quantum physics. And again, this is precisely the point of experiments such as Delft. The CL examples they provide do not involve questions of action at a distance (AAD/AAAD) or locality. Those are classical examples. You cannot get violation of a Bell inequality from a classical preparation. It's that simple.

Quantum theory predicts that our 1 & 4 photons are monogamously bound (entangled) to other photons at creation. Either the swap operation "causes" 1 & 4 to be monogamously entangled (ceasing prior monogamous entanglement), or quantum theory is flat out wrong. You tell me.

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

After moderator review, the thread will remain closed. Thanks to all who participated.