Hidden Assumptions in Bell's Theorem?

[vanhees71]

If I understood it correctly, QFT and micro-causality explicitly forbid anything space-like to have any predictive(or even retro) effect on the solution of QM local "prediction". Fine.
But does it not mean the Bell's theorem is (and was) intended to show that if NATURE (the territory) (not QM (the map)), is indeed compatible with QM "logic", then NATURE contains NON-LOCAL "processes" ?

Bell's theorem, i.e., the validity of Bell's inequalities is derived from two assumptions: "locality" and "realism". Locality simply means that the setting of the hidden variables in the very beginning determines their setting once and for all, including their probabilistic description in terms of standard probability theory. It's a very weak condition, and the more constraining condition of "locality" in the sense of "microcausality" of relativistic QFT of course, fulfills this condition.

Realism means that all observables always take determined values given the values of the hidden variables, i.e., the probabilistic description is necessary only because of our ignorance of these values due to the ignorance about the hidden variables, and thus the statistics is in terms of standard probability theory for all observables given any probability distribution of the hidden variables (see Weinberg, Lectures on Quantum Mechanics, 2nd edition).

The latter is of course not fulfilled for any type of QT, including relativistic local QFT. Since relativistic local QFT fulfills for sure the locality assumption and violates Bell's inequalities in accordance with the observations made in Bell tests, it must be "realism" which is violated by QT and obviously also by Nature.

So I think it follows that QM is incomplete, at least for those that think that QM must be a local-causal only theory (in SR sense), and that anything else is irrelevant, and can be safely ignored, or worse deemed a "interpretation issue".

Relativistic local QFT is perfectly local and causal. Causality only means that if the state of the system is given in the past it's uniquely determined in the future (given the dynamical laws of the theory under consideration), and that's the case for all QTs, even in the more restricted sense that there's not even memory, i.e., it's sufficient to know the state at a given point in time to know the state at any later time.

Whether or not QT is "complete" is a pretty empty question. At least in one sense it's incomplete, because there's no satisfactory quantum description of the gravitational interaction ("quantum gravity"). As any so far known physical theory it thus has its restricted realm of applicability.

 

[gentzen]

But does it not mean the Bell's theorem is (and was) intended to show that if NATURE (the territory) (not QM (the map)), is indeed compatible with QM "logic", then NATURE contains NON-LOCAL "processes" ?

Bell's theorem and Bell's inequality are typical normal incremental science, clarifying questions raised by EPR and Bohmian mechanics. And the refinements of Bell's inequality and experiments based on it too are typical incremental science.
They are mostly intended to answer specific precise questions raised by previous research, and only tangentially intended to show that "NATURE (the territory)" would be this or that.

The intention was to make clear unambiguous progress (even if small and incremental), not to open yet another "interpretational can of worms".

 

[vanhees71]

Indeed, it translated the vague metaphysical speculation of EPR into a clear scientific prediction about "local realistic theories" to be tested against (!!!) QT. In this way it promoted vague philosophical quibbles of EPR and made interpretational issues a scientifically decidable problem.

 

[Simple question]

The latter is of course not fulfilled for any type of QT, including relativistic local QFT. Since relativistic local QFT fulfills for sure the locality assumption and violates Bell's inequalities in accordance with the observations made in Bell tests, it must be "realism" which is violated by QT and obviously also by Nature.

So you are saying that QFT is incomplete because it is unrealistic to use standard probabilistic theory ? I agree that this is a perfectly coherent take on QFT

Whether or not QT is "complete" is a pretty empty question.

Well it depends on your philosophy. If you think science is not about explaining/computing/predicting what happens in nature, than yes I suppose you can think "it is an empty question".
Others think we should find how non-local correlation happens, and not just say: because this is "unrealistic" (as per Bell's)

At least in one sense it's incomplete, because there's no satisfactory quantum description of the gravitational interaction ("quantum gravity").

I always thought that QFT (and all other QM flavors) was only about explaining Quantum effect (especially at high energy where space-time curvature may prove important), and cannot care less about (nor-be appropriate for) apple falling or even bigger object

 

[Simple question]

They are mostly intended to answer specific precise questions raised by previous research, and only tangentially intended to show that "NATURE (the territory)" would be do this or that.

I agree with you, I am only tangentially interested in what nature is and more interested in what we deduce about our "maps", their domain of applicability, their completeness is one such characteristic.

I would like that map to "contains" what nature does, like non-local correlation, because we know, because of experiment, that nature is un-realistic in Bell's sense, and QFT deny the possibility to explain(and compute) non-local correlation.

Well, we can use those non-local physical correlation anyway. With enough certainty (maybe not 100%, but above 50%) to base security system around it. If QFT proponent are content with 50% confidence, then well, this is fine by me. What's not is for them to call this "complete", or to deem them "interpretation" issue.

The intention was to make clear unambiguous progress (even if small and incremental), not to open yet another "interpretational can of worms".

I really do agree 100% with you. This is not about "interpretation", but doing hard science, and closing some loophole "found" in Bell's inequality experimental verification.

 

[vanhees71]

So you are saying that QFT is incomplete because it is unrealistic to use standard probabilistic theory ? I agree that this is a perfectly coherent take on QFT

No, Nature follows the predictions of Q(F)T not the predictions of local realistic hidden-variable theories. In this sense Q(F)T is a complete description of the observed phenomena within its realm of applicability.

Well it depends on your philosophy. If you think science is not about explaining/computing/predicting what happens in nature, than yes I suppose you can think "it is an empty question".
Others think we should find how non-local correlation happens, and not just say: because this is "unrealistic" (as per Bell's)

Science is about objectively observable quantifiable phenomena and finding mathematical theories to describe these phenomena. It's not answering questions outside this methodology.

I always thought that QFT (and all other QM flavors) was only about explaining Quantum effect (especially at high energy where space-time curvature may prove important), and cannot care less about (nor-be appropriate for) apple falling or even bigger object

Q(F)T is describing everything known today except gravity, from the subatomic-few-particle level as investigated in collisions at particle accelerators ("vacuum QFT") to composite systems (atomic nuclei, atoms, molecules, condensed matter) ("many-body Q(F)T").

 

[Simple question]

No, Nature follows the predictions of Q(F)

No, QFT probalilistcally predic what nature may do (it is the other way arround), and by your own admission:
** can NOT be use to predict space-like correlation, witch is what swapping is all about.

and not the predictions of local realistic hidden-variable theories.

I've never heard of any such theory, and I am pretty sure Bell's proved it is not possible to come up with one. Why are you bringing that up ?

In this sense Q(F)T is a complete description of the observed phenomena within its realm of applicability.

.. which does not contain space-like events if I understand you correctly. Hence it is incomplete, because those non-local phenomena are observed, in the lab.

 

[vanhees71]

Bell's theorem is about local realistic hidden-variable theories, and the strength in fact is that you don't need to specify a definite one.

I don't know, what you mean QFT doesn't contain predictions about space-like separated events. It predicts, e.g., by construction (microcausality constraint) that space-like separated events cannot be causally connected.

 

[DrChinese]

This way of thinking sounds "dangerous" to me: The results of those experiments with entanglement swap variations are in perfect agreement with the predictions of QM. All interpretations of QM have to give the same predictions as QM (for all practical purposes). Otherwise, they are different theories and not merely interpretations. So if you believe that you can exclude certain interpretation based on the results of those experiments, then there seems to be a high risk that you are misunderstanding that specific interpretation, or that you are "overinterpreting" the results of those experiments.

Yikes, I don't wanna do anything dangerous...

The experimental results match QM, so all is fair in demanding interpretations meet these higher bars as well. I absolutely do not agree all interpretations have mechanisms that can be considered equivalent to QM. If an interpretation claims a biphoton (system of 2 entangled photons) must evolve separately because it cannot be created as a single system with spatial extent: that's unacceptable. If an interpretation denies monogamy of entanglement: that's unacceptable. If an interpretation claims a future distant measurement context cannot influence the results of a Bell test, that too is unacceptable. In other words: from Bell we learned nature is not both local and realistic (causal), but now we know there are additional hoops to jump through. I want to see how each interpretation handles these hoops, and if they deny the need to do so... well, scratch that.

@vanhees71 says in post #71: "Relativistic local QFT is perfectly local and causal." That's wrong on so many levels (ignoring that Bell nixed that long ago). AFAIK he is the only person on the planet that would write that statement. But I have promised PeterDonis that I will stop debating the point, as vanhees71 uses "local" differently than I do. 'Nuff said. I consider QFT a superset of QM, but regardless it is impossible that there is Einsteinian causality as part of any viable theory... even QFT.

And with interpretations such as MWI, trying to picture the how the "splitting" occurs when measurements are occurring all over the place (in a swapping network) makes my head spin. So I don't believe all interpretations reproduce the predictions of QM; they just claim to. I acknowledge that all authors in the field don't see it the same as I, but some have realized that there are interpretations that need re-examination. I freely admit that I am not the best student of MWI and Bohmian Mechanics. But none of the "updating our knowledge" interpretations make any sense in the new experimental order.

Clearly, an action on A here changes distant B's state - there is no updating of our knowledge of a pre-existing state. And just as clearly, the change in B's state can be made to occur before or after the action on A. I don't see that any experimentalist working with swapping would see it otherwise.

 

[PeterDonis]

I don't believe all interpretations reproduce the predictions of QM; they just claim to.

All interpretation use the same (or equivalent) math to make predictions. That's why they all make the same predictions.

Different interpretations tell very different stories about why the predictions are what they are. Nobody thinks the stories told by all interpretations are equally plausible. But not everyone has the same rank ordering, so to speak, of how plausible the various stories are. And at the end of the day, they're all just stories, which cannot be tested by experiment since they all agree on what the experimental results are. That's why QM interpretation is still an open field after a century of QM.

The other difference between QM interpretations is what kind of more comprehensive theories they suggest. For example, collapse interpretations suggest some kind of "objective collapse" theory like the GRW stochastic collapse theory. This is a different theory, not just a different interpretation of QM, because it makes different predictions from standard QM in certain situations. Unfortunately, this theory, AFAIK, has been ruled out because its predictions have not panned out. But if there is ever going to be any kind of resolution to the QM interpretation debate, it will have to be along such lines as these, where some interpretation leads to a more comprehensive theory that does pan out.

 

[Simple question]

but some have realized that there are interpretations that need re-examination.

I wonder what @Demystifier would say about how Bohmian "realistic" trajectories could or would span those swapping network.

Clearly, an action on A here changes distant B's state

To avoid confusion I would say: an action at A make it correlated to a distant B's state. That in itself is spooky enough

 

[gentzen]

If an interpretation claims a future distant measurement context cannot influence the results of a Bell test, that too is unacceptable. In other words: from Bell we learned nature is not both local and realistic (causal), but now we know there are additional hoops to jump through.

I guess you are "overinterpreting" here, in the sense that you believe that the meaning of those word would be less ambiguous than they actually are. Your first sentence reads as if you believed that one could prove a causal influence from a "future distant measurement context" to "the results of a Bell test" performed in its timelike past. Or maybe just spacelike separated. But in any case, correlations in QM often come without a "provable" causal direction, and those swapping experiments don't exhibit such a direction either.

The "(causal)" behind "realistic" doesn't reduce ambiguity either. I would even say that it makes it even worse, because the meaning of "realistic" in the context of Bell's theorem is at least somewhat established, while the meaning of "causal" is much less discussed in that context.

And I am not convinced that the entanglement swap experiments provide "additional hoops to jump through" which are not already required by Quantum teleportation and Bell inequality violations alone.

But none of the "updating our knowledge" interpretations make any sense in the new experimental order.

Clearly, an action on A here changes distant B's state - there is no updating of our knowledge of a pre-existing state. And just as clearly, the change in B's state can be made to occur before or after the action on A. I don't see that any experimentalist working with swapping would see it otherwise.

And here my guess is that you are misunderstanding those "knowledge" interpretations. I am thinking here of interpretations defended by Rudolph Peierls, by QBists, or by some advocates of Copenhagen (like Heisenberg). Which doesn't mean that those interpretations are unproblematic, but they are neither easy to understand nor easy to destroy.

 

[DrChinese]

1. I wonder what @Demystifier would say about how Bohmian "realistic" trajectories could or would span those swapping network.2. To avoid confusion I would say: an action at A make it correlated to a distant B's state. That in itself is spooky enough

1. There are groups who understand the issues with Bohmian trajectories much better than I. I try to stay away from attacks on Bohmian type theories simply because they are explicitly nonlocal, so there should be some angle for them to stay in the game.

2. I specifically meant it how I phrased it. The swap "causes" (where time ordering not relevant) the [1 & 2] monogamous entanglement to become [1 & 4] monogamous entanglement. [1 & 4] become a new quantum system (a biphoton). The state of [1] and [4] changed. Period. And what caused it was an quantum action at a distant place. That is spooky action at a distance, regardless of what anyone's feelings are about those words.

3. And I am not convinced that the entanglement swap experiments provide "additional hoops to jump through" which are not already required by Quantum teleportation and Bell inequality violations alone.

3. Swap = teleportation, so no disagreement. I think Bell + teleportation is definitely a higher bar than Bell alone. Again, this thread is about the De Raedt "local realistic" simulation - and you certainly don't need to waste time looking at a simultation that purports to mimic PDC entanglement when you have entanglement between particles that have never interacted. For their model, it's case closed.

4. But in any case, correlations in QM often come without a "provable" causal direction, and those swapping experiments don't exhibit such a direction either.

4. I couldn't agree more. And that's precisely my point as to why interpretations preserving Einsteinian causality are doomed. Causal direction is completely ambiguous in actual experiments. The only way to preserve causality is by assumption. Which is to say there is no support for it in quantum events.

5. And here my guess is that you are misunderstanding those "knowledge" interpretations. I am thinking here of interpretations defended by Rudolph Peierls, by QBists, or by some advocates of Copenhagen (like Heisenberg). Which doesn't mean that those interpretations are unproblematic, but they are neither easy to understand nor easy to destroy.

5. "According to QBism, many, but not all, aspects of the quantum formalism are subjective in nature. For example, in this interpretation, a quantum state is not an element of reality—instead it represents the degrees of belief an agent has about the possible outcomes of measurements. Regarding quantum states as degrees of belief implies that the event of a quantum state changing when a measurement occurs—the "collapse of the wave function"—is simply the agent updating her beliefs in response to a new experience. Second, it suggests that quantum mechanics can be thought of as a local theory, because the Einstein–Podolsky–Rosen (EPR) criterion of reality can be rejected. The EPR criterion states, "If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of reality corresponding to that quantity."

The above, from Wikipedia, shows exactly how QBism is NOT equivalent to QM - despite its claims otherwise. There is no sense that QBism can describe perfect correlations of distant particles that have never been in contact as any form of "locality" or as being subjective in any way. There is no "updating" of knowledge, and I am surprised that anyone would assert there is with a straight face. It is a fact, one all observers agree on: perfect correlations (probability=100%) of distant particles without any possibility of Einsteinian interaction. There are no degrees of belief! There is no "agent"! Sure, it may be non-realistic, but if so, there still needs to be an explanation of how the perfect correlations of distant particles arise.

Keep in mind that the entire purpose of an interpretation like this is to somehow preserve Einsteinian locality (i.e. there is no action at a distance). But when we can teleport an unknown state from point A to point B without regard for c, then that objective becomes kaput.

-DrC

 

[lodbrok]

The BSM is absolutely NOT simply post-selection, although the selection does herald a successful swap event. We know that because without the swap, pairs [1 & 2] and [3 & 4] are maximally entangled and monogamously so. However, with a successful swap, [1 & 4] end up maximally entangled and monogamously so.

Heralding in the context of these experiments means to signal that an event meets a particular criterion. It is a filtration or selection flag. The following analogy is appropriate:

For each iteration (#i), you send randomly coloured pairs of socks [1&2] and [3&4] to two different remote locations. Midway through the trip, the pairs are separated so one member of each pair (1 and 4) goes to station A, and the other (2, 3) goes to station B. At station B, a BSM experiment is performed, which in this analogy is equivalent to asking the question "are both socks the same colour?". If the answer is "Yes", the supervisor writes down the number (#i) in his journal (aka "Supervisor's Herald"). If the answer is "No", he ignores it and continues evaluating pairs of socks [2&3] as they come in for many thousands of iterations.

Back at station A, another supervisor has been evaluating incoming pairs [1&4] of socks independently also and keeping the results in a table where she writes down the numbers (#i) next to her results "Yes" or "No". Based on the distances from the socks factory to stations A and B, these "measurements" may happen at different times with A happening before B or vice versa.

The day after the experiments, the supervisors both travel to a third location, taking just their journals with them. Supervisor B notes that the entries in her table are completely random switches between "Yes" and "No". Supervisor A says, "let us filter your spreadsheet and use just the rows with just the numbers from my Herald!". After doing that, they find that all those rows are always "Yes".

Does this mean anything was transferred from any particular pair of socks to any other pair of socks? No! It simply means you are using the information from the [2&3] interaction to post-select a subset from the [1&4] interaction data that would show a correlation, despite the fact that the full [1&4] data does not show any such correlation. In other words, the BSM measurement on [2&3] "heralds" a successful bell state between [2&3] and by extension their entangled siblings [1&4].
The correlation between the [1&4] subset can only be obtained when the information from the "Supervisor's Herald" is used to select the subset of the results from station A. Therefore any influence travels only as fast as it takes for supervisors, A and B to meet and compare notes.

 

[Demystifier]

I wonder what @Demystifier would say about how Bohmian "realistic" trajectories could or would span those swapping network.

Sorry, I didn't follow the discussion so I don't know what is swapping network. Can you briefly explain it to me, or point to the post where this is clearly explained?

But more generally, the details of microscopic Bohmian trajectories are pretty much irrelevant. What matters are the macroscopic trajectories of pointers of the measuring apparatuses, for details see the paper linked in my signature.

 

[Fra]

But none of the "updating our knowledge" interpretations make any sense in the new experimental order.

Not sure if this is relevant to the thread, but it seems relevant for me in order to understand your reasoning, so it makes me curious what your own interpretation or opinion is?

There is no sense that QBism can describe perfect correlations of distant particles that have never been in contact as any form of "locality" or as being subjective in any way. There is no "updating" of knowledge, and I am surprised that anyone would assert there is with a straight face.

One problem of standard qbism that even I acknowledge, is that it does not treat the interaction of the agents. This failure is essentially similar to the problem of a fixed classical background (ie which does not interact with backgrounds of other observers).

But I think any ambitious interpretation has a vision of howto progress from the basic stance, that defines the interpretation. But those visions are then no longer just interpretations. That basic qbist stance itself, I agree, solves nothing!

/Fredrik

 

[gentzen]

Again, this thread is about the De Raedt "local realistic" simulation - ... For their model, it's case closed.

Yeah, at least there are excellent reasons to doubt that such a thing could work. Together with existing papers that try to explicitly disprove it, I am fully confident to dismiss it, case closed.

And of course, I am aware that I tried here (in this thread) to provide an „answer“ to the question you raised in the (now closed) thread:
https://www.physicsforums.com/threa...-of-a-quantum-operation-called-a-bsm.1047772/

I found that an interesting question back then, I didn‘t knew the answer, and I postponed following that thread until I had enough time to deeply dive into it. But when that time came, the thread was already closed. And so I didn‘t dive into it.

What has changed now is that I had the impression that your „monogamy of entanglement“ argument did convince PeterDonis. But I still couldn‘t see how it would change the things why I was unsure about the correct answer. Additionally, your „unconditional“ rejection of „locality“ raised for me the question, of how one could even argue in general (for or against anything), if decomposition into „local“ pieces is rejected. This gave me the idea to decompose the entanglement swapping experiment into two Quantum teleportations, and one Bell experiment:

3. Swap = teleportation, so no disagreement. I think Bell + teleportation is definitely a higher bar than Bell alone.

In fact, the teleportations used for Swap are „poor man‘s“ teleportations, because they only use 1 out of the 4 possible measurement results (for each teleportation), and because there are two teleportations, only 1 out of 16 events is usable. (That‘s why I called it „poor man…“.) But there was another question for me, namely am I even allowed to assume that „poor man‘s“ teleportation still works, especially if I admit that you might be right, and something more happens than just learning the values of classical bits? But all the Swap papers I have “browsed“ so far happily assumed that this is allowed. So I should be fine.

Still, this „poor man‘s“ teleportation is what forced me to come up with mental images of what happens, and strategies of how to argue/check whether they are right or wrong. Your „monogamy of entanglement“ argument was actually one part in my checking computation, that this „one single qubit in context can be encoded by two classical bits“ image works, or more precisely works at least in the context of Quantum teleportation (whether „poor man“ or not). (So far, I only checked the general case by decomposition into individual Quantum teleportations.)

Your post with that „dangerous way of thinking“ gave me the opportunity (or excuse) to present my image (single qubit „in context“), without also being forced to present my calculations and reasoning, or provide references to published papers detailing similar reaonings.

 

[vanhees71]

What has changed now is that I had the impression that your „monogamy of entanglement“ argument did convince PeterDonis. But I still couldn‘t see how it would change the things why I was unsure about the correct answer. Additionally, your „unconditional“ rejection of „locality“ raised for me the question, of how one could even argue in general (for or against anything), if decomposition into „local“ pieces is rejected.

Just my usual lamento: Please define, what you mean by "local" or "non-local". I still do not understand, how one can at the same time use QED to describe an experiment (here the entanglement swapping experiment) and at the same time deny that there are no causal connections possible between space-like separated events, and that's for me the only clearly defined meaning of "non-locality" there is.

In the textbook presentations and, as far as I understand it, also in the original paper by Bell locality is a much weaker assumtion, i.e., that the values (reality!) of the observables are determined once and for all by giving the values of the hidden variables in the very beginning, and that the measurement on B is not influenced by any manipulations/measurements on A and vice versa (locality!).

The difference to the local-QFT description is twofold: (a) the preparation does not determine the values of all observables but only the quantum state, which implies exclusively the probabilities for the possible outcomes of measurements on the so prepared system and (b) that locality is even stronger, i.e., that there cannot be any causal influence between spacelike separated events. Usually the argument for locality to be realized in experiments is even the assumption that there cannot be causal connections between spacelike separated events!

This gave me the idea to decompose the entanglement swapping experiment into two Quantum teleportations, and one Bell experiment:

In fact, the teleportations used for Swap are „poor man‘s“ teleportations, because they only use 1 out of the 4 possible measurement results (for each teleportation), and because there are two teleportations, only 1 out of 16 events is usable. (That‘s why I called it „poor man…“.) But there was another question for me, namely am I even allowed to assume that „poor man‘s“ teleportation still works, especially if I admit that you might be right, and something more happens than just learning the values of classical bits? But all the Swap papers I have “browsed“ so far happily assumed that this is allowed. So I should be fine.

I'd say entanglement swapping is one specific kind of teleportation. I don't understand, what you have doubts about: What's done in the final step is a coincidence measurement at three places: One measures the polarization of photon 1 at a place A, one does a coincidence measurement at place B on photons 2&3, which selects the events, where these states are found to be in the polarization-singlet state, which is especially easy to select, because it's the one state, where it's just sufficient that both detectors register a photon to be sure that the two photons are in this state, and a polarization measurement of photon 4 at place C. The measurements, i.e., the detection of the photons can be in any temporal order, and they can even be space-like separated, the outcome is always the same, i.e., that photon 1 and 4 are described also by the polarization-singlet Bell state. This, together with the assumption that micorcausality holds, and there's no reason to doubt this, because everything is described by standard QED, that there cannot be causal influences among the three (local!) measurements at the far-distant places A, B, and C. Although @DrChinese denies this again and again, it's a mathematical fact of standard QED!

Still, this „poor man‘s“ teleportation is what forced me to come up with mental images of what happens, and strategies of how to argue/check whether they are right or wrong. Your „monogamy of entanglement“ argument was actually one part in my checking computation, that this „one single qubit in context can be encoded by two classical bits“ image works, or more precisely works at least in the context of Quantum teleportation (whether „poor man“ or not). (So far, I only checked the general case by decomposition into individual Quantum teleportations.)

I've no clue, how you think you can describe a qubit with two classical bits at all. A qubit can be in a continuity of states, for two classical bits you have only 4 states. So how can there be a one-to-one connection between them?

Your post with that „dangerous way of thinking“ gave me the opportunity (or excuse) to present my image (single qubit „in context“), without also being forced to present my calculations and reasoning, or provide references to published papers detailing similar reaonings.

It's always better to talk in terms of formulae and calculations than in unclear everyday-language claims, which even contradict the mathematical facts about the theory (in this case QED) used to analyze the entanglement-swapping experiments.

 

[Simple question]

For each iteration (#i), you send randomly coloured pairs of socks [1&2] and [3&4] ...

Please read about Bell's theorem, which is not about Socks. Here is a good layman source
Spin do not work like colour, it is truly quatum. In your analogy the colour of the Socks will only match if both boxes are open at a the same angle. The trick is that the angle can be anything.

 

[Simple question]

2. I specifically meant it how I phrased it. The swap "causes" (where time ordering not relevant) the [1 & 2] monogamous entanglement to become [1 & 4] monogamous entanglement. [1 & 4] become a new quantum system (a biphoton). The state of [1] and [4] changed. Period.

I didn't meant to nitpick. But this rephrasing is perfect (bold's mine) because the sentence is complete and will not permit "local absolutist" to counter "A cannot change B" (which is also true). A (or B) can only change A and B.

 

[Simple question]

Just my usual lamento: Please define, what you mean by "local" or "non-local". I still do not understand, how one can at the same time use QED to describe an experiment (here the entanglement swapping experiment) and at the same time deny that there are no causal connections possible between space-like separated events, and that's for me the only clearly defined meaning of "non-locality" there is.

That's your usual straw-man, so I've fixed it.

The creation of correlation is space-like (even with only one pair), this has been measured and checked in the lab.

I think you cannot understand the issue of swapping because of your philosophy about "ensemble preparation" and micro-causality. It cannot be used to analyse the problem because the swapping of 2&3 is not in the past of either 1 or 4, so you cannot consider it as a valid "preparation procedure of ensemble".

There is no quibbles about what non-locality means.

 

[Simple question]

Sorry, I didn't follow the discussion so I don't know what is swapping network. Can you briefly explain it to me, or point to the post where this is clearly explained?

From DrChinese post #15, I understood that you can theoretically swap between as many "BSM" and kind a form a network. So event more complex number of "node" like this:

/\/\  /\/\/\
1234  123456

But more generally, the details of microscopic Bohmian trajectories are pretty much irrelevant. What matters are the macroscopic trajectories of pointers of the measuring apparatuses, for details see the paper linked in my signature.

But if all pointers follow the quantum potential evolution (trajectories or fields value), how can we determine the starting configuration of that field ? Is it theoretically possible ?
It intuitively seems to me that it would be more and more difficult to find one, the more you add nodes. Do you know if the Bohmian community have tackle this problem ?

 

[kurt101]

On the other hand, any interpretation in which nonlocal correlations are explained by reference to "updating" of our knowledge while retaining locality should, IMHO, be excluded as being ruled out by swapping experiments. Not all authors yet agree with me on this point, which is part of the reason I enjoy threads like this. Always looking for someone who has a strong counter-argument, but that hasn't happened yet. So far, hand-waving and not a shred of experimental support.

I gave you the best counter-argument I could think of. I submitted a program that obeys all of the relevant rules for the entanglement swapping experiment. The program demonstrates that the case where the BSM test is done on photons 2 & 3 after measuring 1 & 4 can be explained through causality. I can think of no better counter-argument then a program that simulates this experiment using hidden variables (short of a realistic theory that replaces quantum mechanics).

As far as I can determine the truth on this issue is that @DrChinese is correct that entanglement swapping does demonstrate non-locality, but only in the case where the BSM test done on photon's 2 & 3 is done before measuring 1 & 4. In the alternative case where the BSM test is done after it does not, which preserves causality. I kind of thought submitting the program would be enough to prove it, and put an end to this back and forth, but apparently not. I don't know if @DrChinese didn't read it (which I can understand), but if you are looking for a good counter argument I suggest that you do. And on top of that I also submitted a paper with the same position. So maybe you can tell me what else I can do to prove to you that the entanglement swapping experiment doesn't demonstrate a violation of causality and only demonstrates non-locality and is not so different than the EPR experiment in that respect.

 

[Demystifier]

But if all pointers follow the quantum potential evolution (trajectories or fields value), how can we determine the starting configuration of that field ? Is it theoretically possible ?
It intuitively seems to me that it would be more and more difficult to find one, the more you add nodes. Do you know if the Bohmian community have tackle this problem ?

In practice, we can't find the starting configuration. That's why, in practice, the Bohmian interpretation makes the same measurable predictions as standard QM.

 

[Simple question]

The program demonstrates that the case where the BSM test is done on photons 2 & 3 after measuring 1 & 4 can be explained through causality.

On this forum ? Can you provide a link to it ?

I can think of no better counter-argument then a program that simulates this experiment using hidden variables (short of a realistic theory that replaces quantum mechanics).

The argument would stand if the program could do both (before and after). Otherwise you've just assumed what you want to prove.

 

[martinbn]

In practice, we can't find the starting configuration. That's why, in practice, the Bohmian interpretation makes the same measurable predictions as standard QM.

Usually BM explains entangelment corations with some sort of action/influance between the particles. How does it work when, as in some swapping cases, the particles never existed at the same time!?

 

[vanhees71]

That's your usual straw-man, so I've fixed it.

The creation of correlation is space-like (even with only one pair), this has been measured and checked in the lab.

The creation of correlation is local: You create entangled photon pairs through local processes and then wait long enough for the photons to be detected (again each by a local measurement) at far distant places to detect the correlations described by entangled states. If the registration events of these photons are spacelike separated, due to the microcausality constraint fulfilled by QED, it is for sure not possible that a causal influence of one measurement on the other measurement causes the observered correlations, and indeed according to local relativsitic QFT the correlations are there from the very beginning, i.e., when preparing the entangled photon pair. That's not a straw-man but a mathematical feature of the theory successfully used to predict the outcome of these Bell tests.

I think you cannot understand the issue of swapping because of your philosophy about "ensemble preparation" and micro-causality. It cannot be used to analyse the problem because the swapping of 2&3 is not in the past of either 1 or 4, so you cannot consider it as a valid "preparation procedure of ensemble".

There is no quibbles about what non-locality means.

The swapping is achieved for a sub-ensemble of photon quadruplets measured in coincidence. That doesn't mean that the photons 1 and 4 are measured before or after or at space-like separation to the local measurement on photons 2 and 3, but that's precisely why I say that in such a setup it's impossible that the entanglement of 1&4 in this subensemble is caused by the local measurement on 2&3.

For me locality in connection with local relativistic QFT means that the microcausality constraint is fulfilled (by construction) and that thus there are no causal connections between space-like separated events possible.

If you now say that the phenomena prove non-locality you must mean something different, and I want to know, what you precisely mean by "locality" and thus by "non-locality".

What's often called "non-locality" is in fact "inseparability", i.e., the correlations between observables on long-distant entangled parts of a quantum system, but correlations don't imply causations.

 

[Fra]

Just to defend the subjective information route here..

There is no sense that QBism can describe perfect correlations of distant particles that have never been in contact as any form of "locality" or as being subjective in any way.

In they way I interpret things, the swapping is effectively a post selection of the outomes of the measurement at C (which needs to be communicated to D to work). I don't see the problem with this. Ontop of this the original "mystery" is I think present already in the original experiment without swapping.

This also explains why the causal order between C and D does not matter, except of course that the final conclusion at D can't happen until the measurent at C is done and communicated(by classical means) to D. Because the only "physical interaction" taking place between C and D, is communicating the C results required to post-select at D. Nothing else. This "communication" can in the experiment be "classical".

Sure, it may be non-realistic, but if so, there still needs to be an explanation of how the perfect correlations of distant particles arise.

To truly "explain this", beyond hand waving, one needs a new worked out theory (from the qbist stance). Which would within the accuracy of all known experiements give same predictions as QM, but maybe give more explanatory power and maybe include more interactions, but be constructed in away that provides much more insight on mechanisms in interactions.

I envision conceptually an "explanation" presumably in two parts

1) the correlation itself is explained by a kind of subjective hidden variable, that can not be cloned or copied like a classical variable, and this hidden variable does not imply the measurement results, it supposedly just explains the correlation. (effectively like QM does)

2) one needs in addition to explain why the above HV, does not obey bell inequality and thus doesnt behave as a ignorance HV. Not necessarily by loopholes, but by arguing that the anzats does not hold at all. Either one can come up with a competing theory to QM, and simply show it does not obey it (which is of course a huge task, and it will no longer be an innocent interpretation) or one can as a first step try to conceptually grasp general traits of such a theory and why the bell ansatz fails. This is ths hard part.

I'm not a bohmian but I do see similarities to the above and the solipsist HV. As subjective information of agents or particles are effectively the sort of solipsistic HV I imagine Demystifier entertains at times. If you see it this way, it wold not make sense to treat it as ignorance as it would imply that one observer/agent would try to average over somebody elses sample space, and that makes no sense.

No matter what else I disagree upon, I agree with this message

"Even if such HV's may look philosophically unappealing to many, the mere fact that they are logically possible deserves attention."
-- https://arxiv.org/abs/1112.2034

I am possibly a bigger fan of these ideas than demystifier himself as it comes out of his mouth on only on his bad days, but it comes out of mine every day as I even find it philosophically appealing. So this thing seems like a common denominator of two very different ways of thinking. That two paths independently leads to the same thing is a good sign I think.

/Fredrik

 

[Demystifier]

Usually BM explains entangelment corations with some sort of action/influance between the particles. How does it work when, as in some swapping cases, the particles never existed at the same time!?

When particle creation is involved, then obviously non-relativistic QM, either in standard or Bohmian form, is not enough. So to answer your question, one must deal with Bohmian interpretation of relativistic QFT. There are several different versions of Bohmian QFT, so a precise answer depends on which version one uses. Perhaps the simplest version is the one which postulates an ontological existence of fields, rather than particles. In this version fields have some definite values at all times, everywhere in space, so the non-existence of particles is not a problem.

 

[kurt101]

On this forum ? Can you provide a link to it ?

The argument would stand if the program could do both (before and after). Otherwise you've just assumed what you want to prove.

I will send the program to you. I originally posted it in its own thread and Peter moved it to the thread Is Enanglement Swapping a result of post selection, or .. and then I think he deleted it as I don't see it there anymore. I think Peter deleted it with the reasoning that I was promoting my own theory, but it is not a theory just a toy model to prove DrChinese is misleading everyone in his conclusion. It demonstrates using math and logic what I am unable to communicate to @DrChinese in words.

@DrChinese seems to be the only one promoting the idea that entanglement swapping says more about interpretations than the EPR experiment. I think this forum needs to hold @DrChinese to the same standard everyone else is held to and ask him to provide a paper that support his views.

I see three independent sources that disagree with @DrChinese that use very different methodologies to reach that conclusion:

1. The program I wrote.
2. This paper https://link.springer.com/article/10.1007/s10701-021-00511-3#Sec21 which shares the same conclusion as my program.
3. The microcausality condition of QFT that @vanhees71 often brings up

 

[vanhees71]

@DrChinese seems to be the only one promoting the idea that entanglement swapping says more about interpretations than the EPR experiment. I think this forum needs to hold @DrChinese to the same standard everyone else is held to and ask him to provide a paper that support his views.

Entanglement swapping is nothing else than teleportation in an experimentally more demanding setup, i.e., it's the teleportation of a Bell state by projecting a photon pair (23) to two far-distant photons 1 and 4, where neither (23) nor (14) have been prepared in an entangled state before but (12) and (34) were. So there's nothing different from any other correlations due to entanglement and there's nothing different from any other Bell test, and that's why it's just the same metaphysical quibbles as brought up by EPR which are (dis)satisfied by these experiments.

I see three independent sources that disagree with @DrChinese that use very different methodologies to reach that conclusion:

1. The program I wrote.
2. This paper https://link.springer.com/article/10.1007/s10701-021-00511-3#Sec21 which shares the same conclusion as my program.
3. The microcausality condition of QFT that @vanhees71 often brings up

The difference between the arguments in 2. and 3. is that the former claims to have found a "new loophole", why the latter just uses a mathematical argument that standard relativistic QFT is a local model describing all quantum/entanglement phenomena. My conclusion simply is that thus we have to give up "realism", i.e., the assumption that all observables always take determined values, and the probabilities associated to the outcome of measurements is just to the incompleteness of our description, i.e., that the values of observables are not known but only probabilities for the measurement outcomes is due to our ignorance of the values of the hidden variables. Bell's inequalities need both locality (i.e., that the measurement at part A doesn't causally influence the measurement on the far distant part B of any system) and realism (i.e., that all observables always take determined values, no matter in which state the system is prepared in).

 

[kurt101]

My conclusion simply is that thus we have to give up "realism", i.e., the assumption that all observables always take determined values, and the probabilities associated to the outcome of measurements is just to the incompleteness of our description, i.e., that the values of observables are not known but only probabilities for the measurement outcomes is due to our ignorance of the values of the hidden variables.

Bell says we have a choice, give up locality or realism. You prefer to give up realism, but then you seem to say you are giving realism up because "due to our ignorance of the values of the hidden variables". Did I understand you correctly? You are not denying the possibility of hidden variables, just that there is no practical way for us to measure them. I equate hidden variables with realism.

I prefer giving up locality and prefer the interpretation that there is an underlying realistic model with hidden variables that is equivalent to QFT even if it is not practical for us to ever directly measure the hidden variables. And I don't see such realistic model being incompatible with QFT and the microcausality condition and your definition of locality even though the realistic model must have aspects of non-locality.

 

[DrChinese]

Heralding in the context of these experiments means to signal that an event meets a particular criterion. It is a filtration or selection flag. The following analogy is appropriate:

For each iteration (#i), you send randomly coloured pairs of socks [1&2] and [3&4] to two different remote locations. Midway through the trip, the pairs are separated so one member of each pair (1 and 4) goes to station A, and the other (2, 3) goes to station B. At station B, a BSM experiment is performed, which in this analogy is equivalent to asking the question "are both socks the same colour?". If the answer is "Yes", the supervisor writes down the number (#i) in his journal (aka "Supervisor's Herald"). If the answer is "No", he ignores it and continues evaluating pairs of socks [2&3] as they come in for many thousands of iterations.

Back at station A, another supervisor has been evaluating incoming pairs [1&4] of socks independently also and keeping the results in a table where she writes down the numbers (#i) next to her results "Yes" or "No". Based on the distances from the socks factory to stations A and B, these "measurements" may happen at different times with A happening before B or vice versa.

The day after the experiments, the supervisors both travel to a third location, taking just their journals with them. Supervisor B notes that the entries in her table are completely random switches between "Yes" and "No". Supervisor A says, "let us filter your spreadsheet and use just the rows with just the numbers from my Herald!". After doing that, they find that all those rows are always "Yes".

Does this mean anything was transferred from any particular pair of socks to any other pair of socks? No! It simply means you are using the information from the [2&3] interaction to post-select a subset from the [1&4] interaction data that would show a correlation, despite the fact that the full [1&4] data does not show any such correlation. ...

All: Please read this entire post, as I have attempted to construct a clear explanation of why entanglement swapping experiments are in fact a proof that local causality in untenable. All of my explanation follows standard QM and actual experiment.

@lodbrok: I can't believe you consider this analogous to entanglement swapping. Nothing about your example is suitable.

First: Bell inequalities are violated in your [1 & 4] sample because the quantum world is contextual. Certainly you know about Bell's story about Bertlmann's socks, else why mention socks? Socks don't cut it, we already know this. You cannot hand devise a data set that matches quantum predictions without knowing what is to be measured (I get to decide, not you, otherwise cheating is possible). Gotta handle perfect correlations AND other angles. That cannot be done - i.e. your example failed this test.Second: In actual swapping experiments: the [2 & 3] selection process does not allow for enough information to be collected to determine that the [1 & 4] pairs will be like entangled as you imagine. The actual information the guy in the middle gets:

a) The pair arrives at the same time for examination, meeting the coincidence window requirement.
b) The pair both pass identical filters at a specified wavelength.
c) The pair has known and opposite polarization, having passed through a filter. Some swapping experiments such as one from the Gisin team use a polarizing beam splitter after the d) step, but others such as the Hanson team use polarizers place before the d) step. Note that this step is performed in order to cast/select the psi- Bell state, which requires that the [2] and [3] photons are either HV or VH. Note that the specific orientation of the polarizers is not relevant, just that they are 90 degrees apart. For our example, we will assume the polarizers are are placed as in the Hanson team's, with an H polarizer on one and a V polarizer on the other. Although they don't identify which is which, we will make the assumption that the [2] photon gets a H polarizer, and the [3] photon gets the V polarizer.
d) The pair consists of either both transmitting through a beam splitter, or both reflecting at the same beam splitter.
e) No action here, just a placeholder for later.

Obviously, the first 5 steps a) to e) have a classical analog and will in fact produce a sample. In the Hanson team paper, there were 245 successful swaps. So I grant you: these 5 steps would be fine in your socks example on the Bell State Measurement side - so far.

f) The [2] & [3] pairs are detected in their source indistinguishable state, and are heralded by 2 fold coincidence on the 2 detectors (let's call them L and R). You don't know if the L photon detector measures the [2] photon or the [3] photon (and vice versa). You don't know which photon is [2] and which is [3], because you don't know whether they were both reflected or both transmitted. There is no classical analog to this, and it is a requirement for a successful swap. You can't mix up classical socks to perform this experiment. So your analogy fails again.Third: There are several interesting issues here. Step d) involves having the [2] and [3] photons to overlap in a small physical region of a 50:50 beam splitter. Perhaps they interact in some fashion? No, that is NOT possible: one is H> and the other is V>. By definition, they are fully orthogonal and therefore cannot interact or interfere or otherwise be changed in any manner. However, this step d) does select a subsample from the inputs. Cases in which the [2] photon is reflected and the [3] photon is transmitted (and vice versa) are excluded - because only one of the two detectors (either L XOR R) will click. To get the entangled Psi- case, we need both detectors to click. Yet we do get a subset/sample that "selects" 245 successful swaps that indicate the [1 & 4] pairs will have perfect (anti)correlations and violate a Bell inequality such as CHSH. These will consist of 2 groups that reach the L and R detectors, totaling 245* in the cited experiment:

i) L detector clicks on receipt of the [2] photon (H polarized), R detector clicks on receipt of the [3] photon (V polarized). Let's pretend there are 127* in this group, although we don't actually know.
ii) L detector clicks on receipt of the [3] photon (V polarized), R detector clicks on receipt of the [2] photon (H polarized). Let's pretend there are 118* in this group, although again we don't actually know.

These scenarios should occur with random and near equal frequency, and cause/select/herald/cast a successful swap for [1 & 4] pair. According to the "post-selection" school of thought, there is no further action at the BSM (where the [2 & 3] sample is identified) that could CAUSE the [1 & 4] groups to stop being correlated. How could they, the argument goes, since we have selected our correlated sample of [1 & 4] pairs? They are too distant to CAUSE a change at this point!

Well guess what... and this is the cool part! Suppose we could magically take our sample consisting of cases i) and ii) - all of which herald successful swaps - and identify just group i) experimentally? If you did that, you would no longer meet the source indistinguishability requirement - and the heralded [1 & 4] pairs would no longer be entangled. How, you ask can this be accomplished?

Go back to our step e) above - the one where nothing happens between the Beam Splitter and the L and R detectors. Instead of doing nothing, let's add an H polarizer in front of the L detector, and an V polarizer in front of the R detector. Voila, we can now distinguish case i) : as only the [2] photons can pass the H polarizer in front of the L detector (which will still click), only the [3] photons can pass the V polarizer in front of the R detector (which will also click). This time, QM predicts the 127 [1 & 4] pairs will not demonstrate any particular correlations.**

Our decision to do nothing - or something - for step e) above CAUSES - without any ambiguity whatsoever - the statistics of the DISTANT [1 & 4] pairs to change (from Entangled State statistics to Product State statistics). That is because the entanglement swapping operation is a physical process/event/action that is essential to the outcome. It cannot be considered as a mere "selection" of a subset, as we select the exact same swap events but get different results.
Please note this important caveat: I have intentionally capitalized the word "CAUSE" in order to distinguish it as being a CAUSE in the quantum mechanical sense. Note that this CAUSE (a successful Bell State Measurement/BSM, or not) can occur *before*, *after*, or *during* the EFFECT - which is the observed statistics of the [1 & 4] pairs (i.e. the Bell test is the effect). And importantly, all this happens regardless of DISTANCE (outside of light cones) from the BSM (cause) to the Bell test (effect). Classical causality would require a cause to occur *before* the effect, and within a distance bound by c. So the kind of causality I assert occurs in the quantum world does not meet any kind of classical definition, which matches precisely the predictions of QM.

There is no local causality, and any theory or interpretation that claims otherwise is invalidated by entanglement swapping experiments.

-DrC

PS If you spot an error in the above, please let me know. These experiments are notorious difficult to follow.*Obviously, a rerun of this experiment would generate different numbers than the 245 (= 127 + 118). But they would be similar. In the actual experiment: "We run 245 trials of the Bell test during a total measurement time of 220 hours. Figure 4a summarizes the observed data, from which we find S = 2.42 in violation of the CHSH-Bell inequality S ≤ 2."

** Please note that I have not seen this particular variation performed in a published experiment, but it is a direct prediction of standard QM. Obviously, it is always a requirement of a successful swap that the [2] and [3] photons be indistinguishable: "If the [2 & 3] photons are indistinguishable in all degrees of freedom, the observation of one early and one late photon in different output ports projects the spins at A [1] and B [4] into the maximally entangled state |ψ> − = (|↑↓> − |↓↑>) / √ 2 ..."

 

[LittleSchwinger]

Did I understand you correctly?

vanhees71 is simply giving a definition of realism. Namely that all probabilities are due to an ignorance of an underlying state where all physical quantities take well-defined values and saying that we must give it up.

There are realist formalisms that possibly replicate parts of non-relativistic quantum theory, but there's no such formalism for relativistic quantum theory.

 

[lodbrok]

But is it really ? QM does define entanglement, which was obviously theoretically applicable on paper to state of particle "prepared" in space-like region. Writing it on paper is easy but realizing it physically is more difficult. But now it is done.

"Prepared" doesn't mean what you think it means. In this context, it simply means:
- take the information from the interaction between streams 2 & 3, and use it to filter streams 1 & 4.

Here is what the papers say:

https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.80.3891
Experimental Entanglement Swapping: Entangling Photons That Never Interacted

To verify that this entangled state is obtained, we have to analyze the polarization correlations between photons 1 and 4 conditioned on coincidences between the detectors of the Bell-state analyzer.​

...​

If entanglement swapping happens, then the twofold coincidences between $D^+_1$ and $D_4$, and between $D^-_1$ and $D_4$, conditioned on the $\left| \Psi^- \right>_{23}$ detection, should show two sine curves as a function of Q which are 90± out of phase​

...​

In that case, one could consider Alice performing the Bell-state measurement on photons 2 and 3, telling Bob, who is in possession of photon 4, the result of the Bell-state measurement.​

Experimental delayed-choice entanglement swapping
https://www.nature.com/articles/nphys2294

​

In our experiment, the primary events are the polarization measurements of photons 1 and 4 by Alice and Bob. They keep their data sets for future evaluation. Each of these data sets by itself and their correlations are completely random and show no structure whatsoever. The other two photons (photons 2 and 3) are delayed until after Alice’s and Bob’s measurements, and sent to Victor for measurement. His measurement then decides the context and determines the interpretation of Alice’s and Bob’s data.​

...​

According to Victor’s choice of measurement (that is, entangled or separable state) and his results (that is, |φ+〉₂₃, |φ−〉₂₃ or |H H〉₂₃, |V V 〉₂₃), Alice and Bob can sort their already recorded data into 4 subsets. They can now verify that when Victor projected his photons onto an entangled state (|φ+〉₂₃ or |φ^− 〉₂₃), each of their joint subsets behaves as if it consisted of entangled pairs of distant photons. When Victor projected his photons onto a separable state (|H H〉23 or |VV〉₂₃), Alice’s and Bob’s joint subsets behave as if they consisted of separable pairs of photons. In neither case Alice’s and Bob’s photons have communicated or interacted in the past.​

​