I Realism in the entanglement swap experiment

[DrChinese]

Careful. Statements like this...

...are interpretation dependent. They are not "objective results". We can observe statistics, but we can't directly observe causation.

The measured statistics change, according to the experimenter’s choice. I’d call that objective. Correlated/anti correlated vs no correlation at all.

Of course, there is no FTL signaling. You must bring the Bell State data (ie which events are identified as psi- or whatever) plus the experimenter’s choices (whether delay switch flipped or not) together with the outcome of the 1,4 pair measurements. Only then does the nonlocal action become visible. And that requires classical communication. So nonlocal action, no nonlocal signal.

 

[PeterDonis]

The measured statistics change, according to the experimenter’s choice. I’d call that objective.

That's not the claim I quoted as being interpretation dependent.

 

[Morbert]

You get this same information (identifying psi- or psi+) whether the experimenter flips the photon 2 delay switch (which fails the swap) or not.

I am suspicious of this claim. It sounds like you are carrying out an intermediate measurement which would affect the [2,3] pair, such that if you register a psi- or psi+ you can no longer be sure you would have registered that same result even if you hadn't added the delay. Can you provide the explicit time-evolution of the two scenarios (delay vs no delay)? Or some expression a la equation 3 in this paper that distinguishes the the two cases?

 

[DrChinese]

I am suspicious of this claim. It sounds like you are carrying out an intermediate measurement which would affect the [2,3] pair, such that if you register a psi- or psi+ you can no longer be sure you would have registered that same result even if you hadn't added the delay. Can you provide the explicit time-evolution of the two scenarios (delay vs no delay)? Or some expression a la equation 3 in this paper that distinguishes the the two cases?

Fair question.

Sure, I am familiar with your reference. Equation 3 presents all four Bell states. We will focus on a single of those four, the phi- Bell state, which they seek. The BSM uses a projecting PBS instead of a beam splitter for the first test, but the principle is the same as other setups. The second test consists of a polarizing beam splitter at each of the output ports of the projecting PBS. There are then photon detectors at each of those output ports, four in total.

The desired phi- state requires the 2,3 photons to exit different output ports of the projecting PBS. They must end up as hv or vh in the final polarization test. In this case, the 1,4 photons become cast into a perfectly anti-correlated state. Analogously, when the phi+ State occurs, the 1,4 photons become cast into a perfectly correlated state.

Interestingly, this experiment actually demonstrates exactly what I am saying. Not sure why I missed this on previous reading, but here it is in black and white. See figure 3c and related text (the paragraph beginning with “One can also choose…”). They used Temporal delay to create the distinguishability. The results are exactly as I predicted. Which is, of course, in keeping with usual quantum mechanics.

Once again: the experimenter may choose to have a successful swap, or not, while retaining all the information needed to discriminate between Bell states which occur randomly. When the experimenter adds delay to cause the swap to fail due to distinguishability, the remote 1,4 photons objectively change their state (as demonstrated from the density matrices of figure 3) from entangled to unentangled.

 

[Morbert]

Interestingly, this experiment actually demonstrates exactly what I am saying.

Indeed, this is why I thought it would be the best example to better understand your claim.

Fair question.

Sure, I am familiar with your reference. Equation 3 presents all four Bell states. We will focus on a single of those four, the phi- Bell state, which they seek. The BSM uses a projecting PBS instead of a beam splitter for the first test, but the principle is the same as other setups. The second test consists of a polarizing beam splitter at each of the output ports of the projecting PBS. There are then photon detectors at each of those output ports, four in total.

The desired phi- state requires the 2,3 photons to exit different output ports of the projecting PBS. They must end up as hv or vh in the final polarization test. In this case, the 1,4 photons become cast into a perfectly anti-correlated state. Analogously, when the phi+ State occurs, the 1,4 photons become cast into a perfectly correlated state.

Interestingly, this experiment actually demonstrates exactly what I am saying. Not sure why I missed this on previous reading, but here it is in black and white. See figure 3c and related text (the paragraph beginning with “One can also choose…”). They used Temporal delay to create the distinguishability. The results are exactly as I predicted. Which is, of course, in keeping with usual quantum mechanics.

Once again: the experimenter may choose to have a successful swap, or not, while retaining all the information needed to discriminate between Bell states which occur randomly. When the experimenter adds delay to cause the swap to fail due to distinguishability, the remote 1,4 photons objectively change their state (as demonstrated from the density matrices of figure 3) from entangled to unentangled.

From the paper

In this case [distinguishability], the phase between the two terms of the |φ〉 projected state is undefined, resulting in a mixture of |φ+〉 and |φ−〉 in the projected state, and the first and last photons do not become quantum entangled but classically correlated.

I read this to mean, when distinguishability is established, the apparatus can no longer project onto |φ+〉 or |φ−〉 and instead projects onto some state $a|\phi^+\rangle\langle\phi^+| + b|\phi^-\rangle\langle\phi^-|$. How do you square this with your claim

You get this same information (identifying psi- or psi+) whether the experimenter flips the photon 2 delay switch (which fails the swap) or not.

because it sounds like you lose the ability to identify phi+ or phi- and must settle for a mixture. The caption under figure 3c calls this case a failure to project.

 

[vanhees71]

Correct. We have never had a disagreement on these points.

Fine, so where's finally the disagreement? Let's see...

Let’s focus on a single Bell state, psi-. This state occurs 25% of the time, and is of course random. The identifying characteristics are the 2,3 photons emerging from separate Beam Splitter ports, and polarizations orthogonal.

Yes, that's the most simple case, because here you can deal with a simple beam splitter. The disadvantage is that you can only filter out one of the four interesting projections, i.e., only to the "singlet Bell state". So a complete Bell-state analysis is favorible, because then you can a posteriori (delayed choice!) look at the four interesting sub-ensembles. But anyway, for the discussion of the locality/causality issue it's enough.

Our experimenter at the BSM sees those random cases that identify as psi-. That means 2,3 are triggering 2 detectors that identify them per above. When those specific combinations are registered, the related 1,4 pairs will be perfectly anti-correlated at all angles… but only if the 2,3 photons are indistinguishable! So when the 1,4 pairs that have been identified as psi- are reviewed, there will be no statistical relationship when the experimenter flips the switch one way, and will be a perfect match to the quantum expectation when the switch is flipped the other way.

If the 2,3 photons are projected to $\psi^-$, they are indeed indistinguishable. That's the whole point of this state being a maximally entangled state, i.e., a "Bell state". Of course, for this perpose the experimenter doing the projection must project to this Bell state. This experiment then doesn't tell you anything about what would have been found when doing another experiment, i.e., QT does not tell anything about experiments that haven't been performed or measurements that cannot be performed given the measurement really done. E.g., in an SG experiment you can measure the spin component in one direction but not at the same time in another direction, because the direction is uniquely chosen only with a correspondingly directed and taylored magnetic field. The same holds here for polarization measurements. You have to choose which observations you want to make, i.e., measuring the polarization of single photons in a specific direction, which then of course never leads to the projection to an entangled state or you can decide to project to a Bell state.

The experimenter makes a choice, flipping a switch, causing the distant 1,4 to become anti correlated - or be completely uncorrelated. Depending on whether there is indistinguishability…

I think, we agree up to this point.

This is an objective result, not subject to interpretation. I say it rules out an entire class of interpretations. There is no FTL signaling, but strict Einsteinian causality is not preserved. The order of measurements is not a factor, and light cones are not a limiting factor.

Now, you are contradicting yourself again. If there is no FTL signalling possible, then relativistic causality is preserved, i.e., in other words, there are no causal connections between space-like separated events possible, then relativistic causality is fulfilled. Maybe you have another definition for what you call "Einsteinian causality". Maybe what you in fact mean is not causality but determinism or, in the sense of the original paper of Bell's (obviously he changed his terminology over time, as I learnt recently from some paper, mentioned in one of the threads her on PF, which I can look for again if needed) "realism", i.e., the believe that all observables of a system must always take determined values, independent of the state of the system. This contradicts standard QT and that's why "hidden variables" are invoked, that are unknown and unobservable, so that the probabilities come in only due to "ignorance" (which may not be avoidable from first principle, if for some reason the hidden variables are not even in principle determinable in any way), i.e., as in classical statistical physics.

However, together with the fulfillment of the relativistic causality principle (i.e., no FTL signalling possible), this possibility is ruled out by the very argument leading to Bell's inequalities within such "local realistic HV theories" and the empirical finding that indeed these inequalities are precisely violated in the way predicted by relativistic QFT, which by construction fulfills the locality constraint (i.e., no FTL signalling possible) via the microcausality constraint on local observable-operators.

 

[mattt]

Please see my post #58, where I analyze the psi- case specifically.

When the experimenter fails the swap, he still gets the same information about the Bell state that would have been been obtained had the swap succeeded. There’s no difference in which detectors are triggered. The only change is distinguishability, which fails the swap but still allows the Bell state to be identified (if the swap had succeeded).

You are assuming that, in a single run where the experimenter fails to do the swap, we know what would have happened exactly (wrt output ports, polarization outputs, phi+, phi-) if he hadn't failed to do the swap.

 

[PeterDonis]

Thread closed for moderation.

 

[berkeman]

Thread reopened after a cleanup of misinformation and a thread band for @Simple question

 

[DrChinese]

Indeed, this is why I thought it would be the best example to better understand your claim.From the paper I read this to mean, when distinguishability is established, the apparatus can no longer project onto |φ+〉 or |φ−〉 and instead projects onto some state $a|\phi^+\rangle\langle\phi^+| + b|\phi^-\rangle\langle\phi^-|$. How do you square this with your claim because it sounds like you lose the ability to identify phi+ or phi- and must settle for a mixture. The caption under figure 3c calls this case a failure to project.

Sure, the same detectors are present, and so that data becomes available just as when a swap occurs. It shows which Bell state would’ve been identified, if the swap had actually occurred. Of course, no swap… no true Bell state. From the paper:

One can also choose to introduce distinguishability between the two projected photons. In this case, the phase between the two terms of the |φ⟩ projected state is un- defined, resulting in a mixture of |φ+⟩ and |φ−⟩ in the projected state, and the first and last photons do not become quantum entangled but classically correlated. We observed this when we introduced a sufficient temporal delay between the two projected photons (see Fig. 3c). It is also evidence that the first and last photons did not somehow share any entanglement before the projection of the middle photons.

I would say their result is unambiguously objective and physical. A decision by an experimenter to execute (or fail) a set of swaps causes distant photons 1,4 to be entangled (or not be entangled at all). The Bell state measurement does not reveal pre-existing entanglement. It causes it (again, without violating signal locality).

 

[DrChinese]

If there is no FTL signalling possible, then relativistic causality is preserved, i.e., in other words, there are no causal connections between space-like separated events possible, then relativistic causality is fulfilled. Maybe you have another definition for what you call "Einsteinian causality". Maybe what you in fact mean is not causality but determinism …

However, together with the fulfillment of the relativistic causality principle (i.e., no FTL signalling possible), …

I think you have cleared something up for me. You use the words “locality” and “non-locality” differently than most authors, including the most recent reference from Eisenberg et al: “The observed quantum correlations manifest the non-locality of quantum mechanics in spacetime.” They use non-locality the same way I do, which means in terms of quantum non-locality. Standard garden variety quantum theory is non-local AND respects signal locality.

You, on the other hand, choose to see locality only in terms of signal locality. If a theory respects signal, locality, then in your mind, it is a local theory. That viewpoint conflates two very different ideas. First, that no signal can exceed the speed of light. Second, that no action can occur that exceeds the speed of light, even if there’s no signal transmitted. As I say, quantum theory respects the first and rejects the second.

You have made a completely unwarranted assumption: that in the physical world, where signals cannot exceed c, that means no action can have distant consequences. And yet this experiment, and hundreds of others, state exactly the opposite: they demonstrate nonlocality and say so explicitly.

So, as per usual, I challenge you to provide suitable quotes from references other than yourself that support your position (as I have). I remain confident that you will ignore my challenge, to the same extent that you ignore the usage of these keywords by thousands of authors.

 

[martinbn]

I think you have cleared something up for me. You use the words “locality” and “non-locality” differently than most authors, including the most recent reference from Eisenberg et al: “The observed quantum correlations manifest the non-locality of quantum mechanics in spacetime.” They use non-locality the same way I do, which means in terms of quantum non-locality. Standard garden variety quantum theory is non-local AND respects signal locality.

You, on the other hand, choose to see locality only in terms of signal locality. If a theory respects signal, locality, then in your mind, it is a local theory. That viewpoint conflates two very different ideas. First, that no signal can exceed the speed of light. Second, that no action can occur that exceeds the speed of light, even if there’s no signal transmitted. As I say, quantum theory respects the first and rejects the second.

You have made a completely unwarranted assumption: that in the physical world, where signals cannot exceed c, that means no action can have distant consequences. And yet this experiment, and hundreds of others, state exactly the opposite: they demonstrate nonlocality and say so explicitly.

So, as per usual, I challenge you to provide suitable quotes from references other than yourself that support your position (as I have). I remain confident that you will ignore my challenge, to the same extent that you ignore the usage of these keywords by thousands of authors.

You use nonlocality in two differwnt ways. One is the violation of Bell's inequality, and this is how it is used in all your references. The second that there is a causal relation between space-like events. You haven't cited a single paper that says that. Then you make the claim that the first implies the second and that experiments have proven the second. This is the reason for these debates. You don't claim that it is a posible explanation or that it is an interpretation you choose, but that it is an experimentaly proven fact.

 

[Lord Jestocost]

“Quantum nonlocality vs. Einstein locality” by H. D. Zeh

https://www.thp.uni-koeln.de/gravitation/zeh/nonlocality.html

 

[vanhees71]

I think you have cleared something up for me. You use the words “locality” and “non-locality” differently than most authors, including the most recent reference from Eisenberg et al: “The observed quantum correlations manifest the non-locality of quantum mechanics in spacetime.” They use non-locality the same way I do, which means in terms of quantum non-locality. Standard garden variety quantum theory is non-local AND respects signal locality.

That's precisely the problem, I'd like to discuss in this thread. It's obviously impossible to discuss it without all the philosophical gibberish.

So my aim in this thread is to clarify, what all these superficial writings mean when they call QT non-local. The meaning of local in my field of research, where pracitioners use local (sic!) relativistic QFT in various forms, locality means just a relativistic field description of interactions, where there's no faster-than-light signal propagation possible and thus the causality principle is realized. In classical electrodynamics you simply choose the retarded propgator for the calculation of the electromagnetic field, which is signal propagation with the speed of light and a unique selection of a causal time direction. In relativistic QFT it's the microcausality constraint for local observable-operators, which also implements relativistic causality, including a choice of time direction, with the preparation of the state in the initial and measurements performed at later times, and due to the microcausality constraints, that's a well-defined Poincare invariant concept. Among these very fundamental properties, necessary for consistency of relativistic QFT with the structure of Minkowski spacetime, the microcausality principle also predicts the spin-statistics relation, PCT symmetry, unitarity and Poincare invariance of the S-matrix, the cluster-decomposition principle.

So my question to you is, since you say you understand what all these authors in the foundations-of-QT community but also sometimes even people like Zeilinger at all seem to use the words "locality" or "non-locality" in different ways, what does non-locality mean mathematically and scientifically without all the philsophical gibberish around it. As discussed in many threads, it's pretty clear that Zeilinger rather follows the usual meaning of locality as used in my field of study, which doesn't surprise me, because after all quantum optics is just the application of the very same theory, i.e., QED to the theory of the electromagnetic field and its interaction with matter. Many-body QED of course also obeys the causality principle of relativistic physics as does the vacuum theory. It's by mathematical construction!

My suspicion is that there is not a clear scientific meaning of this alternative use of the notion of locality/non-locality. Often it seems simply to mean the violation of Bell's inequalities all its variations. That's of course misleading, because (at least within the standard interpretation of relativistic local QFT) it's not locality in the scientific sense, which is violated by relativistic local QFT but "realism" or "determinism", i.e., the assumption that observables always have determined values independent of the state of the system. In other words it counterfactual definiteness, which clearly is violated by all QTs, including relstivistic local QFT.

You, on the other hand, choose to see locality only in terms of signal locality. If a theory respects signal, locality, then in your mind, it is a local theory. That viewpoint conflates two very different ideas. First, that no signal can exceed the speed of light. Second, that no action can occur that exceeds the speed of light, even if there’s no signal transmitted. As I say, quantum theory respects the first and rejects the second.

You are contradicting yourself repeatedly in this point since if the "first meaning" is fufilled, the "second meaning" follows. If there's no FTS signal propgation possible there cannot be causal influences between spacelike separated events. Local relativistic QFT respects both!

You have made a completely unwarranted assumption: that in the physical world, where signals cannot exceed c, that means no action can have distant consequences. And yet this experiment, and hundreds of others, state exactly the opposite: they demonstrate nonlocality and say so explicitly.

They demonstrate strong correlations between far-distant parts of an entangle quantum systems. Is it really so difficult to understand the elementary difference between "causation" and "correlation"? All the experiments obviously demonstrate the opposite of what you say: E.g., the entanglement swap does not depend on the temporal order the measurement on photons (2,3) and photons 1 as well as 2 are performed. They may be even performed in no specific temporal order, i.e., such that the corresponding measurement events (i.e., the irreversible storage of the measurement results at far distant places) are space-like separated and thus have neither a definite temporal order nor are they causally connected within local relativistic QFT thanks of the assumption of the microcausality constraint.

So, as per usual, I challenge you to provide suitable quotes from references other than yourself that support your position (as I have). I remain confident that you will ignore my challenge, to the same extent that you ignore the usage of these keywords by thousands of authors.

I have not the time to quote again all the papers we discussed at length for years in this forum. If you want to understand the meaning of the QFT formalism, I recommend (in the order of sophistication)

S. Coleman, Lectures of Sidney Coleman on Quantum Field
Theory, World Scientific Publishing Co. Pte. Ltd., Hackensack
(2018), https://doi.org/10.1142/9371

This book from the very beginning starts constructing relativistic QT emphasizing the role of microcausality as the only known consistent relativistic QT discovered, i.e., the necessity of the QFT formulation including the microcausality constraint.

S. Weinberg, The Quantum Theory of Fields, vol. 1,
Cambridge University Press (1995).

This book builds relativistic QFT from the representation of the Poincare group together with the strong emphasize of the microcausality constraint, mostly arguing with relativistic S-matrix theory.

A. Duncan, The conceptual framework of quantum field
theory, Oxford University Press, Oxford (2012).

This book is complementary to Weinberg, discussing some important foundational topics, not covered by Weinberg (e.g., Haag's theorem, the impossibility of spontaneous symmetry breaking of gauge theories and Elitzur's theorem). In the foundations it's the same as Weinberg, also with the strong emphasis of the microcausality constraint.

 

[Fra]

I Suspect this depends on your interpretational stance, like at what "level" are you making inferences? From the abstrsct math level, or from the perspective of a physical observer/lab?

You have made a completely unwarranted assumption: that in the physical world, where signals cannot exceed c, that means no action can have distant consequences.

From my stance I wonder, how do you distinguish between a locally generate "message" for signalling, from a local action that propagates into a remote disturbance?

Seems quite related, signalling requires a transmitter and receiver, action/consequences requires local and remote observers.

And in both cases remote sites must compare their views to make a conclusion. This again requires a communication, or a response as REaction back to the first observer/transmitter. Which is localized to the transmitter. So isnt the same thing?

But in normal QM the situation is a bit weird as on one hand the QM-observer at least as per copenhagen is the all of the lassic/macroscopic environment. Which is indees delocalized. But at the same time we have the "classical" observers, that are localized within the classical environment that are supposed to form an equivalence class of quantum perspectives that respect SR. So we have QFT for this.

This tension gets problematic only when a quantum system is also a classical observer. To me this strange situation requires some kind of interpretational stance, so one can get and hierarchy of what we take as primary perspectives or constratins, and what we expect to be emergent. I suspect these difference is at the root of thed discussions.

/Fredrik

 

[vanhees71]

Classicality is an emergent phenonmenon. Macroscopic matter is as well described by QT as are single particles, nuclei, atoms, and molecules. There's no sharp boundary between a "classical world" and a "quantum world". It depends on the sophistication of the experiment, whether a classical description of macroscopic systems is sufficient to describe it or not.

 

[Fra]

Classicality is an emergent phenonmenon.

I share the same view. There is no "classical reality" as you say, and it isn't my argument.

BUT; the problem is that the way quantum theory is constructed, it REQUIRES the existence of this emergent background already! It requires it for the stable inferences, collection of statistics etc. Where else would you build your lab, your accelerators, and how cold you TRUST the records of the last years data, if the whole lab was a quantum mess?

So QM partially assumes existing of a limiting case, that is always out of hand, of what is to be proved. This is ugly.

And this is I think most honestly admitted by the founders in the Copenhagen interpretation. This is why I somehow think these original ideas on how QM was born is important to analyse.

Supposed that the physical phenomena that we cant fully describe that supposedly does emerge into the classical background, isn't at equiblirium, by constantly evolves or reaches some other states, then QM will not be reliable.

Ineede this is not an issue for "atomic" or "molecular physics" in human lab, but it is a potential, conceptual and principal problem for theory building if we consider unification and add gravity and cosmology.

But the takeaway for me is not to keep thinking classical reality is real, but that we need a reconstruction of QM, that does NOT presume the existing of the classical limit; in a way that still makes it work, and can exaplain emergent classicality.

/Fredrik

 

[Morbert]

Sure, the same detectors are present, and so that data becomes available just as when a swap occurs.

I really don't think this is the case. By introducing distinguishability, you've introduced a correlation with the environment that prohibits a determination of a Bell state suitable for selecting subensembles that would exhibit strong correlations between 1 and 4. Hence the mixture and the corresponding subensemble that only exhibits weak correlation.

Perhaps a skilfull experimentalist can carry out a measurement that projects onto a pure Bell state after distinguishability has been established, but whatever the outcome of that measurement, we cannot conclude we would have gotten that result even if we had not established distinguishability. If I have two complementary observables A and B, and I measure A and then B, I cannot say the outcome I got for B would have been the same even if I had not measured A.

It is also evidence that the first and last photons did not somehow share any entanglement before the projection of the middle photons.

These type of statements (distant photons becoming entangled upon projection of the middle photons) can't be made rigorous under a minimalist interpretation. Events are analysed in terms of preparations, instrument settings, and correlations present in macroscopic data.

 

[gentzen]

So, as per usual, I challenge you to provide suitable quotes from references other than yourself that support your position (as I have). I remain confident that you will ignore my challenge, to the same extent that you ignore the usage of these keywords by thousands of authors.

I have not the time to quote again all the papers we discussed at length for years in this forum. If you want to understand the meaning of the QFT formalism, I recommend (in the order of sophistication)

S. Coleman, Lectures of Sidney Coleman on Quantum Field Theory,
...
S. Weinberg, The Quantum Theory of Fields, vol. 1,
...
A. Duncan, The conceptual framework of quantum field theory,
...
This book is complementary to Weinberg, discussing some important foundational topics, not covered by Weinberg (e.g., Haag's theorem, the impossibility of spontaneous symmetry breaking of gauge theories and Elitzur's theorem). In the foundations it's the same as Weinberg, also with the strong emphasis of the microcausality constraint.

As expected, this challenge was clearly won by DrChinese. Of course, the question remains whether winning this challenge helps DrChinese's case in any way. Just because I can win against a weak opponent doesn't mean that I am strong. It is no secret that providing appropriate references is not a strength of vanhees71.

You use nonlocality in two differwnt ways. One is the violation of Bell's inequality, and this is how it is used in all your references. The second that there is a causal relation between space-like events.

I don't think this is correct. The quantum teleportation part seems to be what is important to DrChinese, the role of the Bell's inequality part for him seems to be just to verify that teleportation indeed took place.

You haven't cited a single paper that says that. Then you make the claim that the first implies the second and that experiments have proven the second. This is the reason for these debates. You don't claim that it is a posible explanation or that it is an interpretation you choose, but that it is an experimentaly proven fact.

Given that others seem to believe that "Causality" by Judea Pearl or "The Quantum Theory of Fields, vol. 1" by Steven Weinberg would be appropriate references, it feels strange to me to criticise DrChinese for not providing sufficiently appropriate references.
But I want to actually provide and discuss some references. Recently, I responded to the question "Question: can you educate me more about your ideas on dense coding?" by "... David Mermin's book. In chapter "6 Protocols that use just a few Qbits", both "6.4 Quantum dense coding" and "6.5 Teleportation" should be nicely explained, and easy to follow."
Then it occured to me that some of Mermin's papers are actually excellently suited for making the case that "The DrChinese Paradox" is not as trivial as it may appear. He has papers like In praise of measurement (2006) or Copenhagen Computation: How I Learned to Stop Worrying and Love Bohr (2003) that advocate that a literal "discontinuous collapse" interpretation of measurement works perfectly fine in quantum computer science. But this seems to be the interpretation used by DrChinese to arrive at the conclusion that those swap experiments prove that nature is irreducibly nonlocal. And Mermin even explicitly talks about the dense coding and teleportation papers in his Copenhagen Computation paper:

Referee’s Report: Bennett et al., "Teleporting. . ." LZ4539
This is a charming, readable, thought-provoking paper. It presents a novel application of EPR correlations. The character of the quantum state (how much is inherent in the physical system, how much is a representation of our knowledge) is still an extremely elusive notion. This novel method for duplicating a quantum state somewhere else by a combination of quantum correlations and classical information will become an important one of the intellectual tools available to anybody trying to clear up this murkiness.

C. H. Bennett, G. Brassard, C. Crépeau, R. Jozsa, A. Peres, and W. Wootters. Teleporting an unknown quantum state via dual classical and EPR channels. Phys. Rev. Lett., 70:1895–1899, 1993.

Bennett and Wiesner, "Communication via one-and two-particle. . ." LT4749
Your question was: Does this qualify as "strikingly different" enough to publish? I have never read anything like it, and I have read a lot on EPR, though far from everything ever written. So as far as I know it is different.
But strikingly? The argument is very simple, so shouldn’t the point be obvious? After reading the paper I put it aside and spent the next week working hard on something totally unrelated. Every now and then I would introspect to see if some way of looking at the argument had germinated that reduced it to a triviality. None had. Last night I woke up at 3am, fascinated and obsessed with it. Couldn’t get back to sleep. That’s my definition of "striking".
So I say it’s strikingly different and I say publish it.

C. H. Bennett and S. J. Wiesner. Communication via one- and two-particle operators on Einstein-Podolsky-Rosen states. Phys. Rev. Lett., 69(20):2881–2884, 1992.

Did I have the time to read those short papers from 1993 and 1992 before posting them here as references? Is this even a requirement when posting references? Have I read at least Mermin's papers at some point?

In a certain sense, those are probably the wrong questions. The more interesting question seems to be why DrChinese prefers to score easy victories against vanhees71, instead of addressing the specific points brought up by Peter Donis, Morbert and others above? And the same goes for vanhees71: Why does he discuss locality of QFT with DrChinese, instead of addressing specific points brought up by Demystifier, A.Neumaier, and others elsewhere?

 

[vanhees71]

In a certain sense, those are probably the wrong questions. The more interesting question seems to be why DrChinese prefers to score easy victories against vanhees71, instead of addressing the specific points brought up by Peter Donis, Morbert and others above? And the same goes for vanhees71: Why does he discuss locality of QFT with DrChinese, instead of addressing specific points brought up by Demystifier, A.Neumaier, and others elsewhere?

I don't understand, why you claim that the self-contradictory claims by DrChinese were victories against my arguments, which are simply based on standard relativistic QFT. That's quite ridiculus!

 

[gentzen]

I don't understand, why you claim that the self-contradictory claims by DrChinese were victories against my arguments, which are simply based on standard relativistic QFT. That's quite ridiculus!

You have not read correctly what I wrote. The victory is just with respect to providing relevant references, not with respect to the argument itself. I explicitly addressed that point:

Of course, the question remains whether winning this challenge helps DrChinese's case in any way. Just because I can win against a weak opponent doesn't mean that I am strong. It is no secret that providing appropriate references is not a strength of vanhees71.

 

[vanhees71]

You have not read correctly what I wrote. The victory is just with respect to providing relevant references, not with respect to the argument itself. I explicitly addressed that point:

If three thoroughly written textbooks about the subject are not relevant references, I don't know, what you want. Should I look for all the original papers of relativistic QFT? They are quoted in the standard textbooks anyway.

Also once more: The statements by DrChinese are self-contradictory. On the other hand he accepts the mathematical fact that relativistic, local QFT excludes causal connections between spacelike separated events. On the other hand he says there must be some in view of the Bell-test results. This is a contradiction, because the Bell-test results are precisely what's predicted by standard relativistic, loacal QFT, which excludes such causal connections!

 

[DrChinese]

“Quantum nonlocality vs. Einstein locality” by H. D. Zeh

https://www.thp.uni-koeln.de/gravitation/zeh/nonlocality.html

Finally, a start of a reference. Always helps when a quote joins the reference. And it’s not really a paper at all. But it contains some reasonable and relevant discussion and is not too long a read. Quotes are in italics:

Quantum theory is kinematically nonlocal, while the theory of relativity (including relativistic quantum field theory) requires dynamical locality ("Einstein locality"). How can these two elements of the theory (well based on experimental results) be simultaneously meaningful and compatible? How can dynamical locality even be defined in terms of kinematically nonlocal concepts?

So already, Zeh acknowledges QT is nonlocal in what he calls the kinematic sense. And that is different from relativistic sense of locality vs nonlocality, which he refers to as dynamical.

Dynamical locality in conventional terms means that there is no action at a distance: states "here" cannot directly influence states "there". Relativistically this has the consequence that dynamical effects can only arise within the forward light cones of their causes. However, generic quantum states are "neither here nor there", nor are they simply composed of "states here and states there" (with a logical "and" that would in the quantum formalism be represented as a direct product). Quantum systems at different places are usually entangled, and thus do not possess any states of their own. Therefore, quantum dynamics must in general describe the dynamics of global states. It may thus appear to be necessarily nonlocal.

This discrepancy is often muddled by insisting that reality is made up of local events or phenomena only. However, quantum entanglement does not merely represent statistical correlations that would represent incomplete information about a local reality. Individually observable quantities, such as the total angular momentum of composed systems, or the binding energy of the He atom, can not be defined in terms of local quantities. This nonlocality has been directly confirmed by the violation of Bell's inequalities or the existence of Greenberger-Horne-Zeilinger relations. If there were kinematically local concepts completely describing reality, they would indeed require some superluminal "spooky action at a distance" (in Einstein's words). Otherwise, however, such a picture is questionable or meaningless. In particular, nothing has to be teleported in so-called quantum teleportation experiments. In terms of nonlocal quantum states, one has to carefully prepare an appropriate entangled state that contains, among its components, all states to be possibly teleported (or their dynamical predecessors) already at their final destination…

These kinematical properties characterize quantum nonlocality. But what about Einstein locality in this description?

So although he describes the situation in somewhat different terminology than I would, we agree that quantum theory is quantum nonlocal, which has been directly confirmed.

From Zeh’s other page:
Accepting the physical reality of nonlocal entangled quantum states eliminates any need for spooky action at a distance, and in particular for any advanced action that seems to occur in a delayed choice experiment. This delayed choice simply determines which property of the controllably entangled wave function that has unitarily arisen in a virtual "measurement" is finally irreversibly ("really") measured. Paradoxes arise only if one attempts to describe quantum physics exclusively in local terms.

Here Zeh essentially says: take your choice, it’s either nonlocality or spooky action at a distance you must accept. I don’t have any issue with this view at all, I use the terms interchangeably anyway. We either have a nonlocal quantum context, or there is action (interplay) between the distant components. No meaningful (predictive) difference between the 2 views in my book, so label as you prefer. You say there is a single nonlocal context, or a group of local contexts that exhibit action at a distance. Same thing. In both cases, signal locality is respected.

So I generally agree with Zeh’s analysis in the above. He does discuss the QFT microcausality condition, which he sees as a) not contradicting the above; and b) referring to that which respects relativistic signal locality.

 

[DrChinese]

If three thoroughly written textbooks about the subject are not relevant references, I don't know, what you want. Should I look for all the original papers of relativistic QFT? They are quoted in the standard textbooks anyway.

Also once more: The statements by DrChinese are self-contradictory. On the other hand he accepts the mathematical fact that relativistic, local QFT excludes causal connections between spacelike separated events. On the other hand he says there must be some in view of the Bell-test results. This is a contradiction, because the Bell-test results are precisely what's predicted by standard relativistic, loacal QFT, which excludes such causal connections!

As per usual, your concept of quotes relates to entire books. That’s not a quote! There are many things presented in an entire book or paper. Be specific.

I am not self contradictory; you just can’t see any options other than an entrenched position.

Specifically: sure I agree that QFT requires signal locality. And sure I agree that nonlocal action requires a classical signal to perceive. But there is nonlocal action as all of the references say. No paper says anything different.

Quantum contexts/events/actions are generally probabilistic as to outcomes. That’s true whether the contexts are local or nonlocal. If you use the word “causal” to describe an experimenter’s choice which must have a determined specific outcome (such as a specific bell state): then yes, QFT would be locally causal - precisely because experiments do not allow the experimenter the opportunity to make that choice.

But neither I nor most authors require such a strict view when discussing these experiments. Quantum theory predicts an experimenter can choose to execute a swap here and objectively change the state of a pair that is distant. That change is to a state that is randomly occurring, itself outside the control of the experimenter. Further, the experimenter can make his choice before or after 1,4 pair detection. In my book, that’s a violation of strict Einsteinian causality. But of course… not a violation of most tenets of special relativity. That being quantum predictions of relativistic QM.

 

[mattt]

Finally, a start of a reference. Always helps when a quote joins the reference. And it’s not really a paper at all. But it contains some reasonable and relevant discussion and is not too long a read. Quotes are in italics:

Quantum theory is kinematically nonlocal, while the theory of relativity (including relativistic quantum field theory) requires dynamical locality ("Einstein locality"). How can these two elements of the theory (well based on experimental results) be simultaneously meaningful and compatible? How can dynamical locality even be defined in terms of kinematically nonlocal concepts?

So already, Zeh acknowledges QT is nonlocal in what he calls the kinematic sense. And that is different from relativistic sense of locality vs nonlocality, which he refers to as dynamical.

Dynamical locality in conventional terms means that there is no action at a distance: states "here" cannot directly influence states "there". Relativistically this has the consequence that dynamical effects can only arise within the forward light cones of their causes. However, generic quantum states are "neither here nor there", nor are they simply composed of "states here and states there" (with a logical "and" that would in the quantum formalism be represented as a direct product). Quantum systems at different places are usually entangled, and thus do not possess any states of their own. Therefore, quantum dynamics must in general describe the dynamics of global states. It may thus appear to be necessarily nonlocal.

This discrepancy is often muddled by insisting that reality is made up of local events or phenomena only. However, quantum entanglement does not merely represent statistical correlations that would represent incomplete information about a local reality. Individually observable quantities, such as the total angular momentum of composed systems, or the binding energy of the He atom, can not be defined in terms of local quantities. This nonlocality has been directly confirmed by the violation of Bell's inequalities or the existence of Greenberger-Horne-Zeilinger relations. If there were kinematically local concepts completely describing reality, they would indeed require some superluminal "spooky action at a distance" (in Einstein's words). Otherwise, however, such a picture is questionable or meaningless. In particular, nothing has to be teleported in so-called quantum teleportation experiments. In terms of nonlocal quantum states, one has to carefully prepare an appropriate entangled state that contains, among its components, all states to be possibly teleported (or their dynamical predecessors) already at their final destination…

These kinematical properties characterize quantum nonlocality. But what about Einstein locality in this description?

So although he describes the situation in somewhat different terminology than I would, we agree that quantum theory is quantum nonlocal, which has been directly confirmed.

From Zeh’s other page:
Accepting the physical reality of nonlocal entangled quantum states eliminates any need for spooky action at a distance, and in particular for any advanced action that seems to occur in a delayed choice experiment. This delayed choice simply determines which property of the controllably entangled wave function that has unitarily arisen in a virtual "measurement" is finally irreversibly ("really") measured. Paradoxes arise only if one attempts to describe quantum physics exclusively in local terms.

Here Zeh essentially says: take your choice, it’s either nonlocality or spooky action at a distance you must accept. I don’t have any issue with this view at all, I use the terms interchangeably anyway. We either have a nonlocal quantum context, or there is action (interplay) between the distant components. No meaningful (predictive) difference between the 2 views in my book, so label as you prefer. You say there is a single nonlocal context, or a group of local contexts that exhibit action at a distance. Same thing. In both cases, signal locality is respected.

So I generally agree with Zeh’s analysis in the above. He does discuss the QFT microcausality condition, which he sees as a) not contradicting the above; and b) referring to that which respects relativistic signal locality.

The mathematics and experimental results have always been clear.

The problems or disagreements arise only when we (different people) try to describe it all in English or in any other human language.

I quite like the way this article describe in English the whole situation.

He uses the term "kinematically nonlocal" as a synonym of "existence of quantum entangled states" which is a synonym of "existence of states that violate Bell-type inequalities".

Me, vanhees, martinb, Cthunga, Morbert, gentzen and just about everybody obviously accept this, and we have never disputed it.

He also says that accepting the physical reality of these entangled quantum states (which we all obviously accept, because the mathematics say so, and the experiments confirm it) eliminates any need for spooky action at a distance, (and in particular for any advanced action that seems to occur in a delayed choice experiment).

This is exactly what I (and many others here) have been saying for months. You don't need any instant action at a distance to describe these experiments.

If anything, this article seem to describe the whole situation almost exactly the same way as we have been describing it for months here.

It is you (and perhaps some others as well) who insists in the necessity of accepting nonlocal causation (actions here instantly affecting physical things there) as the only way to describe these type of experiments.

 

[DrChinese]

The mathematics and experimental results have always been clear.

The problems or disagreements arise only when we (different people) try to describe it all in English or in any other human language.

I quite like the way this article describe in English the whole situation.

He uses the term "kinematically nonlocal" as a synonym of "existence of quantum entangled states" which is a synonym of "existence of states that violate Bell-type inequalities".

Me, vanhees, martinb, Cthunga, Morbert, gentzen and just about everybody obviously accept this, and we have never disputed it.

He also says that accepting the physical reality of these entangled quantum states (which we all obviously accept, because the mathematics say so, and the experiments confirm it) eliminates any need for spooky action at a distance, (and in particular for any advanced action that seems to occur in a delayed choice experiment).

This is exactly what I (and many others here) have been saying for months. You don't need any instant action at a distance to describe these experiments.

If anything, this article seem to describe the whole situation almost exactly the same way as we have been describing it for months here.

It is you (and perhaps some others as well) who insists in the necessity of accepting nonlocal causation (actions here instantly affecting physical things there) as the only way to describe these type of experiments.

If you say QM is nonlocal (as hundreds of papers say verbatim, and you seem to agree with) but there is no action at a distance: Ok. I just never hear you or others admit that essential point loud and clear. Just say QFT is nonlocal! As I have pointed out repeatedly: the experimenter’s choice to execute a swap here objectively (ie experimentally proven) changes the state of unrelated distant photons to an entangled one, which did not previously exist. (Without any debate on my part, that nonlocal swap cannot be detected without a classical signal.)

Please don’t tell me you think that is demonstration of local causality. No experimental papers on swapping are entitled “proof of quantum locality”. Just ones like this:

Experimental Nonlocality Proof of Quantum Teleportation and Entanglement Swapping​

Characterizing the nonlocal correlations of particles that never interacted​

Quantum nonlocality vs. Einstein locality​

 

[DrChinese]

A) I really don't think this is the case. By introducing distinguishability, you've introduced a correlation with the environment that prohibits a determination of a Bell state suitable for selecting subensembles that would exhibit strong correlations between 1 and 4.

B) Perhaps a skilfull experimentalist can carry out a measurement that projects onto a pure Bell state after distinguishability has been established…

C) These type of statements (distant photons becoming entangled upon projection of the middle photons) can't be made rigorous under a minimalist interpretation.

A) introducing a time delay, as was done in the experiment, does not change which detectors are triggered. there is no change to the interaction with the environment whatsoever. What you end up with is an indication of which Bell State would’ve been selected if one had been created.

If you are unsure of this point, let’s remind ourselves what is being measured at the BSM. We measure whether both photons emerge from different output ports of the projecting beam splitter (T/R or R/T). We also measure whether the photons have the same polarization or are orthogonal. How would inserting a delay on one side to cause distinguishability change any of these T/R or polarization outcomes? (That is, if you don’t think there is a non-local link between a successful swap, and those remote photons.)

B) that is completely impossible. If there’s distinguishability, there is no swap, end of subject.

C) I think Eisenberg et al do a pretty good job of just that.

 

[PeterDonis]

What you end up with is an indication of which Bell State would’ve been selected if one had been created.

I'm not sure this claim is justified. If the incoming 2 & 3 photons are distinguishable, no swap is produced; and that means you cannot make counterfactual claims about what would have happened if a swap had been produced. All you can say is that, for that particular run, no swap was produced. The 2 & 3 photon measurements at the output of the BSM apparatus are measurements of non-entangled photons and give no information about photons 1 & 4.

 

[Morbert]

A) introducing a time delay, as was done in the experiment, does not change which detectors are triggered. there is no change to the interaction with the environment whatsoever. What you end up with is an indication of which Bell State would’ve been selected if one had been created.

It's the significance of the detector trigger that is changed. To illustrate this, consider an abstract bell measurement circuit involving a CNOT (CN) state, a Hadamard (H) gate, and spin-z measurements (M)

We can see that a preparation $|\Phi^+\rangle$ will yield $$|\Phi^+\rangle \xrightarrow[\text{CN}]{} \frac{1}{\sqrt{2}}\left(|0\rangle+|1\rangle\right)|0\rangle\xrightarrow[\text{H}]{}|00\rangle\xrightarrow[\text{M}]{} M_{00}$$I.e. Repeated experimental runs will yield 0,0 100% of the time. Similarly, the preparation $|\Phi^-\rangle$ will yield 1,0 100% of the time. Now lets say we intervene between preparation and measurement such that, whether or not the preparation was $|\Phi^+\rangle$ or $|\Phi^-\rangle$, we now have $$\frac{1}{2}\left(|\Phi^+\rangle\langle\Phi^+|+|\Phi^-\rangle\langle\Phi^-|\right) = \frac{1}{2}\left(|00\rangle\langle00|+|11\rangle\langle11|\right)$$as an "intermediate" preparation. Now we have the evolution $$\frac{1}{2}\left(|00\rangle\langle00|+|11\rangle\langle11|\right)\xrightarrow[\text{CN}]{}\frac{1}{2}\left(|00\rangle\langle00|+|10\rangle\langle10|\right)\xrightarrow[\text{H}]{}\frac{1}{2}\left(|00\rangle\langle00|+|10\rangle\langle10|\right)\xrightarrow[\text{M}]{}\frac{1}{2}\left(M_{00}+M_{10}\right)$$You can see that approx. half our experimental runs will trigger a result 0,0 and the other half 1,0. But we can't naively divide our ensemble into two subensembles and say "these 0,0 runs would have been 0,0 even if we didn't intervene"

I suspect something similar (but not identical) is happening in the actual experimental setup. Distinguishing photons leaves the phase undefined, and so instead of inferring a bell state from detectors, a mixed "correlated" state is inferred.

*I invite other people to double check this math, but I think the conclusion holds. If we do not have phase information, we cannot infer a bell state from a set of detector outcomes.

[edit] - fixed math

[edit 2] - I think the above is still a little bit hand wavey. My interpretation of the paper is, when indistinguishability is maintained, the photons are projected onto a one of two bell states depending on whether the detector results are correlated or anti-correlated. If distinguishability is established, then the photons are projected onto a mixed state whether or not the detector results are correlated or anti-correlated. The above look at an abstract circuit maybe hints at why but doesn't fully explain why. I will have to try to understand the minutia of the particular experiment.

[edit 3] - Rewrote to try and make the relevance more concrete

 

[DrChinese]

It's the significance of the detector trigger that is changed. To illustrate this, consider an abstract bell measurement circuit involving a CNOT (CN) state, a Hadamard (H) gate, and spin-z measurements (M)
View attachment 330458
We can see that an input $|\Phi^+\rangle$ will yield $$|\Phi^+\rangle \xrightarrow[\text{CN}]{} \frac{1}{\sqrt{2}}\left(|0\rangle+|1\rangle\right)|0\rangle\xrightarrow[\text{H}]{}|00\rangle\xrightarrow[\text{M}]{} M_{00}$$I.e. We infer a bell state from the reading 00. But now lets say our input is the state $$\frac{1}{2}\left(|\Phi^+\rangle\langle\Phi^+|+|\Phi^-\rangle\langle\Phi^-|\right) = \frac{1}{2}\left(|00\rangle\langle00|+|11\rangle\langle11|\right)$$Now we have the evolution
$$\frac{1}{2}\left(|00\rangle\langle00|+|11\rangle\langle11|\right)\xrightarrow[\text{CN}]{}\frac{1}{2}\left(|00\rangle\langle00|+|10\rangle\langle10|\right)\xrightarrow[\text{H}]{}\frac{1}{2}\left(|00\rangle\langle00|+|10\rangle\langle10|\right)\xrightarrow[\text{M}]{}\frac{1}{2}\left(M_{00}+M_{10}\right)$$You can see that a mixed state has a 50/50 chance of triggering a detector result 00.

I agree with all of the above.

And so we are not automatically justified in inferring a Bell state from the same detector outcomes.* I suspect something similar (but not identical) is happening in the actual experimental setup. Distinguishing photons leaves the phase undefined, and so instead of inferring a bell state from detectors, a mixed "correlated" state is inferred.

If we do not have phase information, we cannot infer a bell state from a set of detector outcomes

You have it backwards though! If there is no Bell state, there is no swap. Phase information is perfectly available but not meaningful when a swap does not occur. That because the 1,2 pair does not develop a relationship with the 3,4 pair.

But for those of you out there that don’t believe anything physical happens at the 2,3 measurement that creates the 1,4 entanglement: what difference would that make? As I indicated, the detectors indicate Reflect/Transmit at the projecting splitter and the polarization. What you call phase is simply part and parcel of those same properties.

Seen another way: tell me what happens to 1,4 as a result of a successful swap where the 2,3 photons come close to each other - as opposed to when a delay is introduced and no swap occurs. The coincidence time window has most 2,3 photons detected within 3 ns and usually 1 ns apart. 1 ns is about 1 foot. So they do not precisely overlap. Further, orthogonal photons cannot interact (and that happens in 1/2 the cases). So explain what is different when a delay is inserted to change any readings when our experimenter flips his swap/no swap switch.

Or accept the conclusion of the paper: In conclusion, we have demonstrated quantum entanglement between two photons that do not share coexistence. … This is a manifestation of the non-locality of quantum mechanics not only in space, but also in time.