Realistic interpretation of QM

[DrChinese]

1. I have never heard of a counter example where bell violating entanglement was not prepared through local preparation.2. If you have photon pair A and B that are entangled through SPDC and photon pair C and D that are entangled through SPDC and you have B and C interact locally, then I would say A and D are not entangled prior to B and C interacting locally and are not entangled after B and C interacted locally. If you want to say A and D are entangled right when B and C interact, that is fine with me, but using a realistic description, I would say that the local interaction at B and C drops the previous entanglement B had with A and C had with D and creates a new entanglement between B and C.

1. For example:

https://arxiv.org/abs/0911.1314
Theory: Characterizing the nonlocal correlations of particles that never interacted

https://arxiv.org/abs/0809.3991
Experiment: High-fidelity entanglement swapping with fully independent sources

https://arxiv.org/abs/1606.07503
Experiment: Entanglement Swapping with Independent Sources over an Optical Fibre Network

[Edited to include experimental sources in addition to theoretical treatment; see post #37 by PeterDonis.]2. A and D become entangled. Does that entanglement occur before or after B & C interact? No one can say, as the timing of the B/C interaction does not change the outcome. Earlier or later, makes no observable difference.

However, that is not relevant to your premise 3. In that, you say A & D must be entangled by a local interaction. They have no interaction at all. The are never even present in a common light cone, as they are created by independent SPDC sources that are "distant" to each other. So how can there be a local realistic description of A & D becoming entangled so that there spins are 100% correlated when measured at any chosen angle?

Your premise requires their hidden variables to be set at their creation - and they will NOT be entangled unless something else occurs after they are on their merry way to the detectors. In a local realistic world, they are totally immune from such later influence (if they are sufficiently far apart).

 

[PeterDonis]

For example

This is not an experimental paper but a theoretical one. I believe you have referenced experimental papers on entanglement swapping in other threads; such a paper would be a good reference here.

 

[kurt101]

2. A and D become entangled. Does that entanglement occur before or after B & C interact? No one can say, as the timing of the B/C interaction does not change the outcome. Earlier or later, makes no observable difference.

Are you saying that if you measure both A and D before B and C interact that you will still see the Bell violation between A and D?

 

[Maarten Havinga]

Are you saying that if you measure both A and D before B and C interact that you will still see the Bell violation between A and D?

Although this surprises you kurt101, it does not pose a threat at all to your realistic interpretation if viewed from General Relativity. Let me explain why:

In general relativity, a localisation is a point in spacetime. That is, space has a vectorvalue and time has a value. This means that for a phenomenon (entanglement) to be NON-local, that: it can affect a distant particle instantaneously, not only distant in space, but also distant in time!

Of course this only happens in the right circumstances, such as an experimental setup, where it is guaranteed that the two particles become entangled by quantum physics. That is, therefore, ALSO true in cases where entanglement is ensured but in entirely differing timeframes for two particles.

Quantum physics is, though difficult, indeed interpretable in a realist view.

 

[PeterDonis]

In general relativity, a localisation is a point in spacetime. That is, space has a vectorvalue and time has a value. This means that for a phenomenon (entanglement) to be NON-local, that: it can affect a distant particle instantaneously, not only distant in space, but also distant in time!

Where are you getting this from? Can you give a reference? Please bear in mind the PF rules regarding personal speculation.

 

[Maarten Havinga]

If you don't like the idea, think differently. I just thought it was logical in the setting of this discussion.

I'd say for a reference, my previous post might be the original "academic" source. It's a generalization of the concept of nonlocality.

 

[PeterDonis]

I'd say for a reference, my previous post might be the original "academic" source.

That's not how things work here. PF is not for developing personal research. It's for discussing and helping people to understand mainstream science. That's why you need an actual reference--a textbook or peer-reviewed paper. "My previous post" obviously is neither of those.

 

[Maarten Havinga]

OK!

 

[lodbrok]

1. For example:

https://arxiv.org/abs/0911.1314
Theory: Characterizing the nonlocal correlations of particles that never interacted

https://arxiv.org/abs/0809.3991
Experiment: High-fidelity entanglement swapping with fully independent sources

https://arxiv.org/abs/1606.07503
Experiment: Entanglement Swapping with Independent Sources over an Optical Fibre Network

[Edited to include experimental sources in addition to theoretical treatment; see post #37 by PeterDonis.]2. A and D become entangled. Does that entanglement occur before or after B & C interact? No one can say, as the timing of the B/C interaction does not change the outcome. Earlier or later, makes no observable difference.

However, that is not relevant to your premise 3. In that, you say A & D must be entangled by a local interaction. They have no interaction at all. The are never even present in a common light cone, as they are created by independent SPDC sources that are "distant" to each other. So how can there be a local realistic description of A & D becoming entangled so that there spins are 100% correlated when measured at any chosen angle?

Your premise requires their hidden variables to be set at their creation - and they will NOT be entangled unless something else occurs after they are on their merry way to the detectors. In a local realistic world, they are totally immune from such later influence (if they are sufficiently far apart).

We have to be very careful in interpreting those types of results in this way. Firstly, these experiments (eg, https://www.nature.com/articles/srep09333) do not measure just 4 photons (A, B, C, D). They measure 4 streams where streams A & B (C & D) consist of entangled pairs. Secondly, the interaction between B & C is not simply an interaction between one photon from the B stream and one photon from the C stream. Rather it is a coincidence experiment between the two streams. A coincidence counting experiment is akin to a filtration process, they are essentially selecting a subset of the streams based on the result of the interaction.

Therefore, the experiment did not demonstrate what is often claimed. The B and C streams were used to select a subset of the A and D streams. It is this coincidence counting/filtration process that transfers the entanglement.

Imagine the photons being black/white beads, and the AB (or CD) is always alternate colors but with no correlation between the AB stream of beads and the CD stream of beads. We could do a coincidence experiment in which we compare the B & C beads. If they are the same we select the iteration of (A, B, C, D) and if they are not the same we discard the iteration. We are only ever comparing B & C, as A & D could be far away from the interacting point. At the end of this experiment, if we look only at the subset of beads which were selected in the experiment, A & D will be perfectly correlated!

The keys to understanding it is:
1. Recognize that the experiment deals with streams of particles not single particles
2. Recognize that coincidence counting is involved
3. Recognize that the A & D particles which are entangled are only a subset of the original streams
4. Recognize that the reduction of the original A & D streams to the subset, involved the use of information from the interaction between B & C.

 

[DrChinese]

We have to be very careful in interpreting those types of results in this way. Firstly, these experiments (eg, https://www.nature.com/articles/srep09333) do not measure just 4 photons (A, B, C, D). They measure 4 streams where streams A & B (C & D) consist of entangled pairs. Secondly, the interaction between B & C is not simply an interaction between one photon from the B stream and one photon from the C stream. Rather it is a coincidence experiment between the two streams. A coincidence counting experiment is akin to a filtration process, they are essentially selecting a subset of the streams based on the result of the interaction.

Therefore, the experiment did not demonstrate what is often claimed. The B and C streams were used to select a subset of the A and D streams. It is this coincidence counting/filtration process that transfers the entanglement.

Imagine the photons being black/white beads, and the AB (or CD) is always alternate colors but with no correlation between the AB stream of beads and the CD stream of beads. We could do a coincidence experiment in which we compare the B & C beads. If they are the same we select the iteration of (A, B, C, D) and if they are not the same we discard the iteration. We are only ever comparing B & C, as A & D could be far away from the interacting point. At the end of this experiment, if we look only at the subset of beads which were selected in the experiment, A & D will be perfectly correlated!

4. Recognize that the reduction of the original A & D streams to the subset, involved the use of information from the interaction between B & C.

I agree that a system consisting of A & B interacts with a system consisting of C & D, but that's where we part ways.

There is no "coincidence" filtration occurring as you describe. Yes, there are coincidence counts occurring, but that is to identify if and when entanglement occurs and which kind. It should be obvious that when the A & D pair is entangled, you have so-called perfect correlations in polarization tests. If I set both measuring apparati to 282 degrees, you always get perfect correlation (or anti-correlation as the case may be). That absolutely does not happen by "coincidence" as you say. That's why your analogy with white/black beads is invalid; you'd need hundreds of colors of beads so that the ones match up for when the settings are the same. In other words: there are no more than 4 combinations of B & C pairs, but there are hundreds of possible A & D pairs.

You can also have repeaters, where you have pair A & B, pair C & D, and pair E & F: A & F can be fully entangled even though the system of A & B never interacts with the system of E & F. And in fact the choices to entangle them can be performed both before AND after A & F are detected (i.e. within the same run, by choice of teleportation sequence).

You would be hard-pressed to support your idea by any reputable source I am aware; if you have a representative quote from a solid scientific source about swapping, I'd like to see something that describes it as you do. Because Quantum teleportation is not an "accident" that results from predetermined local hidden variables, any more than that is a viable explanation for a standard Bell test. Teleportation defies classical norms of locality and causality, and should be considered another nail in the coffin for one or both of those concepts. If you needed (more) experimental proof of the existence of (quantum) nonlocal action, this is it.

 

[kurt101]

I agree that a system consisting of A & B interacts with a system consisting of C & D, but that's where we part ways.

There is no "coincidence" filtration occurring as you describe. Yes, there are coincidence counts occurring, but that is to identify if and when entanglement occurs and which kind. It should be obvious that when the A & D pair is entangled, you have so-called perfect correlations in polarization tests. If I set both measuring apparati to 282 degrees, you always get perfect correlation (or anti-correlation as the case may be). That absolutely does not happen by "coincidence" as you say. That's why your analogy with white/black beads is invalid; you'd need hundreds of colors of beads so that the ones match up for when the settings are the same. In other words: there are no more than 4 combinations of B & C pairs, but there are hundreds of possible A & D pairs.

I think you are mistaken that this rules out hidden variables and realism.

Lets just take the stranger of the cases where you first measure A & D and then do the entanglement swapping with B & C.

When you measure A, this influences B. When you measure D, this influences C. Then when you do the swapping with B & C, they are already tainted by the respective polarizer you used when measuring A and B.

So you can't make the argument that you are making that because you ran one experiment like this with the same polarizer for A & D that always showed A & D were 100% corelated that this proves when you run the Bell violation version of this experiment using different polarizers that you are comparing apples to apples. The post selection is going to be different between these experiments because of the taint I explained above. So you are comparing apples to oranges and this invalidates your premise that hidden variables can't exist.

 

[PeterDonis]

I think you are mistaken that this rules out hidden variables and realism.

While this might be your personal opinion, it certainly appears to contradict Bell's Theorem. You can't just wave your hands with terms like "hidden variables" and "realism". You need to either reference a specific mathematical model that you think has those properties, and explain what mathematical properties in the model correspond to those terms, and show how that model makes predictions that match experiment--or you need to stop making claims that you cannot support. The fact that you really, really want to find an interpretation of QM that matches the title of this thread does not mean that one exists.

 

[Nullstein]

We have to be very careful in interpreting those types of results in this way. Firstly, these experiments (eg, https://www.nature.com/articles/srep09333) do not measure just 4 photons (A, B, C, D). They measure 4 streams where streams A & B (C & D) consist of entangled pairs. Secondly, the interaction between B & C is not simply an interaction between one photon from the B stream and one photon from the C stream. Rather it is a coincidence experiment between the two streams. A coincidence counting experiment is akin to a filtration process, they are essentially selecting a subset of the streams based on the result of the interaction.

Therefore, the experiment did not demonstrate what is often claimed. The B and C streams were used to select a subset of the A and D streams. It is this coincidence counting/filtration process that transfers the entanglement.

Imagine the photons being black/white beads, and the AB (or CD) is always alternate colors but with no correlation between the AB stream of beads and the CD stream of beads. We could do a coincidence experiment in which we compare the B & C beads. If they are the same we select the iteration of (A, B, C, D) and if they are not the same we discard the iteration. We are only ever comparing B & C, as A & D could be far away from the interacting point. At the end of this experiment, if we look only at the subset of beads which were selected in the experiment, A & D will be perfectly correlated!

The keys to understanding it is:
1. Recognize that the experiment deals with streams of particles not single particles
2. Recognize that coincidence counting is involved
3. Recognize that the A & D particles which are entangled are only a subset of the original streams
4. Recognize that the reduction of the original A & D streams to the subset, involved the use of information from the interaction between B & C.

This is the exactly the correct explanation for this phenomenon and it's well understood. It's just the effect of conditioning on a Collider. I tried to explain this to DrChinese in another thread already without success. His central misunderstanding is to believe that A & D are entangled when really they are in a perfectly uncorrelated product state. Only the conditional subensembles are entangled, but that doesn't entail any causal relationship, because conditioning on a collider generally generates fake correlations. Entanglement swapping indeed adds nothing new to the mystery of entanglement. Those willing to learn can read more about this e.g. in Pearl's famous book titled "Causality."

 

[kurt101]

While this might be your personal opinion, it certainly appears to contradict Bell's Theorem. You can't just wave your hands with terms like "hidden variables" and "realism". You need to either reference a specific mathematical model that you think has those properties, and explain what mathematical properties in the model correspond to those terms, and show how that model makes predictions that match experiment--or you need to stop making claims that you cannot support. The fact that you really, really want to find an interpretation of QM that matches the title of this thread does not mean that one exists.

I never said anything about entanglement swapping experiments not violating Bell's theorem. I believe the experimental results that @DrChinese claims. I am just pointing out the flaw in @DrChinese claim that these experiments says anything meaningful about reality or hidden variables. There is nothing novel about entanglement swapping that he seems to be claiming there is. It is just like the pop science explanations of delay quantum eraser and claims of retro causality.

And this is a perfect example of why we need a realist interpretation. So we are not fooled into thinking crazy ideas that we actually don't observe in the real world.

 

[gentzen]

Only the conditional subensembles are entangled, but that doesn't entail any causal relationship, because conditioning on a collider generally generates fake correlations. Entanglement swapping indeed adds nothing new to the mystery of entanglement. Those willing to learn can read more about this e.g. in Pearl's famous book titled "Causality."

Assuming I am willing to learn, what would I have to read specifically? I assume that Pearl won't explicitly mention entanglement, and even less entanglement swapping (or "fake correlations"). I see that he does mention quantum mechanics from time to time, like in:

Second, the Laplacian conception is more in tune with human intuition. The few esoteric quantum mechanical experiments that conflict with the predictions of the Laplacian conception evoke surprise and disbelief, and they demand that physicists give up deeply entrenched intuitions about locality and causality (Maudlin 1994). Our objective is to preserve, explicate, and satisfy – not destroy – those intuitions.

or:

The ideal of remaining compatible with the teachings of nondeterministic physics seems to be the only viable aspect remaining in the program of probabilistic causation, and this section questions whether maintaining this ideal justifies the sacrifices. ... If the program is an exercise in modern physics, then the word “causality” is nonessential – quantum-level causality follows its own rules and intuitions, and another name (perhaps “qua-sality”) might be more befitting.

Teleportation defies classical norms of locality and causality, and should be considered another nail in the coffin for one or both of those concepts.

I guess I prefer saying that quantum-level locality and causality follows its own rules and intuitions (like Pearl effectively did in the quotes above), instead of talking of a coffin for those concepts.

 

[Nullstein]

Assuming I am willing to learn, what would I have to read specifically?

You would have to read about how conditioning on colliders produces fake correlations. If you are bothered by the words "fake correlations", here's how Wikipedia puts it: "In the terminology of causal graphs, conditioning on the collider opens the path between X and Y. This will introduce bias when estimating the causal association between X and Y, potentially introducing associations where there are none." But again, all of this is explained in full detail in Pearls book.

I assume that Pearl won't explicitly mention entanglement, and even less entanglement swapping (or "fake correlations"). I see that he does mention quantum mechanics from time to time, like in:

or:
I guess I prefer saying that quantum-level locality and causality follows its own rules and intuitions (like Pearl effectively did in the quotes above), instead of talking of a coffin for those concepts.

I too, like Pearl, think that quantum-level causality follows its own rules. It's just that entanglement swapping adds nothing to the mystery, because particles A & D are actually not entangled. Only subensembles are entangled, but that's just a well understood statistical effect resulting from conditioning on a collider. What's left to explain is really just why A & B and C & D are entangled. This is indeed a mystery.

 

[kurt101]

When you measure A, this influences B. When you measure D, this influences C. Then when you do the swapping with B & C, they are already tainted by the respective polarizer you used when measuring A and B.

I meant to say "Then when you do the swapping with B & C, they are already tainted by the respective polarizer used when measuring A and D", but I must have edited the comment to many times already to change it.

 

[PeterDonis]

I am just pointing out the flaw in @DrChinese claim that these experiments says anything meaningful about reality or hidden variables.

If by this you mean that "reality" and "hidden variables" are things that vary from interpretation to interpretation, so no experiment can tell you things about them directly, because no experiment can distinguish between different QM interpretations (since they all make the same experimental predictions), that's fine.

But you appear to be saying more than that: you appear to be saying that some interpretation must exist that has all the properties you like. And you can't just claim that. You have to actually show us the interpretation--and not just as your personal theory but as something in the actual literature. So far, the main message of the discussion in this thread appears to me to be that there is no interpretation in the actual literature that has all the properties you like. For you to just keep insisting that there must be, in the absence of any support for that claim from the literature, is pointless and this thread will simply be closed if that is where we are.

 

[kurt101]

If by this you mean that "reality" and "hidden variables" are things that vary from interpretation to interpretation, so no experiment can tell you things about them directly, because no experiment can distinguish between different QM interpretations (since they all make the same experimental predictions), that's fine.

But you appear to be saying more than that: you appear to be saying that some interpretation must exist that has all the properties you like. And you can't just claim that. You have to actually show us the interpretation--and not just as your personal theory but as something in the actual literature. So far, the main message of the discussion in this thread appears to me to be that there is no interpretation in the actual literature that has all the properties you like. For you to just keep insisting that there must be, in the absence of any support for that claim from the literature, is pointless and this thread will simply be closed if that is where we are.

I don't want the thread closed. I found it useful to have the discussion with @DrChinese about entanglement swapping. Maybe it is too early to conclude this, but I have learned that the A & D photons in the entanglement swapping are not entangled but correlated. For a realist, this is an extremely important to know.

For me, the value of an interpretation is to aid in understanding of the math and the experimental results.

I see General Relativity, Quantum Mechanics, Bell non-locality, as improvements on understanding a Newtonian Universe, not as invalidating it. I think this is a very reasonable interpretation to hold to, especially since we know these improvements have issues and don't integrate well with each other. Also, these improvements rely heavily on classical logic in their formulation and so I don't see any reason to abandon the classical logic they were founded on.

I see Quantum Mechanics as a mathematical framework to make statistical predictions. Though, no doubt it also tells us a great deal about reality, but as a realist, I don't think all of its principles are true outside the context of the statistical mathematical framework.

I imagine I will be able to find some acceptable peer review papers that support different aspects of my form of a realist interpretation, but is there no actual name for such an interpretation? It seems to be used all through the history of formulating Quantum Mechanics in some form or another. Sometimes I see it referred to as hidden variable theory, but it is clearly there, even if it is not described in as much detail as say a Bohmian Mechanics interpretation is.

I like aspects of the Bohmian Mechanics interpretation, but I think it goes too far in making assumptions that don't fit well with reality.

I will try to find an interpretation I can use for discussions on realism that is acceptable to both myself and this forum. I appreciate any constructive suggestions.

 

[PeroK]

Given the experimental evidence of the 20th and 21st centuries, I'd say it's highly unrealistic to be a realist and interpret modern physics as a addendum to Newtonian mechanics!

 

[kurt101]

Given the experimental evidence of the 20th and 21st centuries, I'd say it's highly unrealistic to be a realist and interpret modern physics as a addendum to Newtonian mechanics!

Ok, why? What specific experiments? And obviously we have to be specific about what aspects of a Newtonian universe are kept and what are abandoned in such a discussion. I would like to be able to discuss this, but I am not sure how to do so and stay within the confines of the rules. I know in at least other forums mentioning terms such as absolute time and space are not allowed even if they were for the purpose of understanding why they are invalid or incompatible.

 

[PeterDonis]

I don't want the thread closed.

I understand that, but we still have to have some basis for discussion other than your personal opinion.

I like aspects of the Bohmian Mechanics interpretation, but I think it goes too far in making assumptions that don't fit well with reality.

I will try to find an interpretation I can use for discussions on realism that is acceptable to both myself and this forum.

Unfortunately, the most "realist" interpretation of QM we have is the Bohmian interpretation. If you don't like that one, you're going to like all the others even less.

 

[PeroK]

Ok, why? What specific experiments? And obviously we have to be specific about what aspects of a Newtonian universe are kept and what are abandoned in such a discussion.

Newton's laws don't extend to SR, GR or QM. In some cases they are good approximations, but only in special cases.

 

[PeterDonis]

I see General Relativity, Quantum Mechanics, Bell non-locality, as improvements on understanding a Newtonian Universe, not as invalidating it. I think this is a very reasonable interpretation to hold to

No, it isn't, it's an extremely unreasonable interpretation to hold to. See below.

Ok, why? What specific experiments?

The fact that you have to ask this question tells me that you need to spend some serious time becoming familiar with the vast body of experimental evidence that has forced physicists to abandon Newtonian mechanics and adopt relativity and QM. Richard Feynman once said that "quantum physics was not wished upon us by theorists". The same is true of relativity, although it's harder to see historically because Einstein was able to come up with a working theoretical framework for both special and general relativity based on a fairly small body of evidence. But pretty much every other physicist had to be dragged to relativity kicking and screaming, just as they all had to be dragged to QM. But now that that process has taken place, it is obvious that Newtonian mechanics, considered as a viewpoint on "how the universe works", is simply wrong. The only viable view of Newtonian mechanics now is that it is an approximation to relativity and QM that is valid under certain very restricted conditions. It just so happens that our everyday experience here on Earth, outside of scientific experiments, meets those very restricted conditions.

There is an excellent Living Reviews article on experimental tests of general relativity here:

https://link.springer.com/article/10.12942/lrr-2014-4

Unfortunately I don't know of a similar single source for experimental tests of QM. But some obvious pivotal experimental tests historically were:

Black body radiation spectrum

Photoelectric effect

Compton effect

Davisson-Germer experiment

Stern-Gerlach experiment

Lamb shift

 

[DrChinese]

This is the exactly the correct explanation for this phenomenon and it's well understood. It's just the effect of conditioning on a Collider. I tried to explain this to DrChinese in another thread already without success. His central misunderstanding is to believe that A & D are entangled when really they are in a perfectly uncorrelated product state. Only the conditional subensembles are entangled, but that doesn't entail any causal relationship, because conditioning on a collider generally generates fake correlations. Entanglement swapping indeed adds nothing new to the mystery of entanglement.

The experiment is performed on the entangled sub-ensemble, the other pairs are not of any interest. You never consider B & C pairs that do not arrive together within a suitably small time window. The other pairs appear equally often, separated into 4 combinations. For all intents and purposes, all of those are considered.

-----------------------------

Obviously, the problem with your idea is that you want it both ways... the A & D pairs are in product states to begin with, but they become entangled by your purported distillation process. Were that true, then you would be agreeing with me and be inconsistent. But I will try to work through your reasoning as best possible anyway.

As I mention, your distillation process - "entanglement swapping" or "quantum teleportation" is what everyone else calls it - doesn't have enough outcome options to match up photons with leading to perfect correlations for ultimately entangled A & D pairs. There are essentially a large (or perhaps infinite) number of angle settings at which they would need to matched for your idea to work. Your idea being that A & D happen to have identical predetermined orientations (which are reflected in their respective twins, B & C).

So let's say we have A & B entangled, and C & D entangled. We ALREADY know from Bell that the A & B (or C & D) entanglement cannot itself be explained by any local hidden variables. So basically, your idea is already failed (as you are attempting to push a local realistic explanation for entanglement swapping in which Bell's Inequality is violated).

Regardless, and still going down your path: We compare B & C, considering only those pairs in which B & C arrive together at a single polarizing beam splitter within a small time window; and are therefore indistinguishable. There can be only 4 outcomes corresponding to the four maximally entangled Bell states |Φ+>, |Ψ+>, |Ψ−>, and |Φ −>. If your idea is correct, there is no "process" occurring - since there is nothing happening when B & C interact other than to "reveal" some pre-existing correlation. But what would that pre-existing correlation be IF there are only 4 possibilities? That implies that there exist exactly 2 and only 2 different streams possible from a PDC source (since combining those 2 types you'd get 2x2=4). That doesn't sound too crazy at first blush, but it begs the question: why do you need the compared streams to be *indistinguishable* to reveal their matching state? You could equally well run the B & C photons into *different* polarizing beam splitters such that their arrival times were within the coincidence window - just as they would be if they appeared in the same PBS. They would now be distinguishable (remember there are still only 4 outcome possibilities, as you need there to be only 2 types of PDC streams), but there would be no interaction and no swapping process. This shouldn't make any difference if your idea is correct. A & D do NOT end up entangled, unless B & C are indistinguishable. Which is the opposite of your idea.

-------------------------

The ideas you push are wrong, and it is more fully debunked in a generally accepted reference I gave in post #36. In their words:

https://arxiv.org/abs/0911.1314

"Starting from two independent pairs of entangled particles, one can measure jointly one particle from each pair, so that the two other particles become entangled, even though they have no common past history. The resulting pair is a genuine entangled pair in every aspect, and can in particular violate Bell inequalities. Intuitively, it seems that such entanglement swapping experiments exhibit nonlocal effects even stronger than those of usual Bell tests."

"In our scenario, however, they are two separate sources S1 and S2. It it thus natural to assume that the local model assigns two different states λ1 and λ2, one to each source..." [Just as I describe above.]

"Even though the local variables λ1 and λ2 are initially independent, once conditioned on the joint measurement result of Bob they will bear enough correlations to reproduce 2 non-trivial correlations between Alice’s and Charles’s system. These correlations, however, are much weaker than those that can be established through joint measurements in quantum theory. We introduce below a (quadratic) Bell inequality that is satisfied by all bilocal correlations, but which is violated by quantum correlations." [I.e. you can't get perfect correlations from your assumptions.]

I could quote as many additional seminal papers on the subject as one would desire. None of them will agree with your analysis, and as of yet no one here has bothered to present anything remotely suitable to rebut the above references or anything I have said on the subject.

This is the Interpretations subforum and the rules are more relaxed here; but generally accepted science is still generally accepted science. The facts are: entanglement swapping is a quantum process, and it itself violates local realism. I say it is a physical process, and if you follow Bohmian Mechanics or MWI you should agree with me. I don't see how it can be viewed as OTHER than a physical process, but I would acknowledge there are plenty of other interpretations that might see things differently. But it is not a local realistic phenomena.

 

[kurt101]

@DrChinese here is a paper that refutes your claim. https://link.springer.com/article/10.1007/s10701-021-00511-3

 

[Nullstein]

The experiment is performed on the entangled sub-ensemble, the other pairs are not of any interest. You never consider B & C pairs that do not arrive together within a suitably small time window. The other pairs appear equally often, separated into 4 combinations. For all intents and purposes, all of those are considered.

Sure, that makes them subensembles, which are entangled. It's just that this kind of entanglement is entirely non-mysterious, since it is a statistical effect.

Obviously, the problem with your idea is that you want it both ways... the A & D pairs are in product states to begin with, but they become entangled by your purported distillation process. Were that true, then you would be agreeing with me and be inconsistent.

First of all, it's not my idea. It's well understood statistics, I provided a reference with thousands of citations. Also, A and D don't become entangled. The full system is in a product state, but the subensembles are entangled. It's just that this entanglement is entirely non-mysterious. What's mysterious still, is that A&B are entangled and that C&D are entangled.

As I mention, your distillation process - "entanglement swapping" or "quantum teleportation" is what everyone else calls it - doesn't have enough outcome options to match up photons with leading to perfect correlations for ultimately entangled A & D pairs. There are essentially a large (or perhaps infinite) number of angle settings at which they would need to matched for your idea to work. Your idea being that A & D happen to have identical predetermined orientations (which are reflected in their respective twins, B & C).

I never claim that A & D have identical predetermined orientations, so that's a non-starter and shows that you haven't understood the argument.

So let's say we have A & B entangled, and C & D entangled. We ALREADY know from Bell that the A & B (or C & D) entanglement cannot itself be explained by any local hidden variables. So basically, your idea is already failed (as you are attempting to push a local realistic explanation for entanglement swapping in which Bell's Inequality is violated).

Again, you misrepresent "my idea". I don't want to explain anything by local hidden variables. I don't believe in hidden variables to begin with. I totally agree that the entanglement between A & B (and C & D respectively) cannot be explained by a local realistic model. The entanglement between A & B is mysterious. It's just that the entanglement of a subensemble of A & D is not mysterious given that the initial pairs are entangled for whatever mysterious reason.

Regardless, and still going down your path: We compare B & C, considering only those pairs in which B & C arrive together at a single polarizing beam splitter within a small time window; and are therefore indistinguishable. There can be only 4 outcomes corresponding to the four maximally entangled Bell states |Φ+>, |Ψ+>, |Ψ−>, and |Φ −>. If your idea is correct, there is no "process" occurring - since there is nothing happening when B & C interact other than to "reveal" some pre-existing correlation.

Not true. There is no "pre-existing correlation" in A & B to begin with, so propagating this correlation by entanglement swapping won't suddenly make it "pre-existing." So you have completely misunderstood the argument.

But what would that pre-existing correlation be IF there are only 4 possibilities?

There is no pre-existing correlation and I didn't claim there was one.

The ideas you push are wrong, and it is more fully debunked in a generally accepted reference I gave in post #36. In their words:

https://arxiv.org/abs/0911.1314

"Starting from two independent pairs of entangled particles, one can measure jointly one particle from each pair, so that the two other particles become entangled, even though they have no common past history. The resulting pair is a genuine entangled pair in every aspect, and can in particular violate Bell inequalities. Intuitively, it seems that such entanglement swapping experiments exhibit nonlocal effects even stronger than those of usual Bell tests."

I fully agree that the subensembles are fully entangled and violate Bell's inequality and that there is no local realistic explanation for this. I'm just saying that entanglement swapping does not add any additional mystery on top of the initial entanglement of the particle pairs A&B and C&D. The swapping "process" itself is just a statistical artifact.

This is the Interpretations subforum and the rules are more relaxed here; but generally accepted science is still generally accepted science. The facts are: entanglement swapping is a quantum process, and it itself violates local realism. I say it is a physical process, and if you follow Bohmian Mechanics or MWI you should agree with me. I don't see how it can be viewed as OTHER than a physical process, but I would acknowledge there are plenty of other interpretations that might see things differently. But it is not a local realistic phenomena.

I don't disagree with any accepted science. It's just that you don't understand the argument I'm making. It is in full agreement with generally accepted science.

 

[PeterDonis]

The full system is in a product state, but the subensembles are entangled.

This doesn't make sense. "Subensembles" don't even have states. In each individual run of the experiment, the full system, consisting of A, B, C, D, is entangled, because pairs of subsystems, A&B, C&D, are entangled.

 

[Nullstein]

This doesn't make sense. "Subensembles" don't even have states. In each individual run of the experiment, the full system, consisting of A, B, C, D, is entangled, because pairs of subsystems, A&B, C&D, are entangled.

Subensembles do have states in the sense that the statistics of the subensemble is perfectly well described by a density matrix. Of course there is entanglement between A&B and C&D, but what I meant to say was that the composite system A&B is not entangled with the composite system C&D and that the subsystem A&D is in a product state.

 

[DrChinese]

1. Sure, that makes them subensembles, which are entangled. It's just that this kind of entanglement is entirely non-mysterious, since it is a statistical effect.

2. First of all, it's not my idea. It's well understood statistics, I provided a reference with thousands of citations. Also, A and D don't become entangled. The full system is in a product state, but the subensembles are entangled. It's just that this entanglement is entirely non-mysterious. What's mysterious still, is that A&B are entangled and that C&D are entangled.

3. I never claim that A & D have identical predetermined orientations, so that's a non-starter and shows that you haven't understood the argument.

4, The entanglement between A & B is mysterious. It's just that the entanglement of a subensemble of A & D is not mysterious given that the initial pairs are entangled for whatever mysterious reason.

1. And how do those sub-ensembles become entangled into 1 of 4 Bell states? By the swapping process, of course! In principle, there are no sub-ensembles that do not cast entangled A & D into an entangled state. So the idea you have - that only the "correlated" sub-ensembles are being selected - is totally wrong. You completely skipped over my explanation about how A & D come to be entangled. Again, there are 4 Bell states that result from a Bell State Analyzer, and they are not entangled unless they are indistinguishable. This idea does not relate to anything you describe, and of course, it can't.

2. It IS your idea... try quoting a source. You place a URL here, and then you extract a relevant quote that directly refutes my argument or directly supports yours. It's easy for me to request, because this is a lop-sided exercise. And please, don't tell me to Google something. You have it, or you don't.

3. Well, you claimed that there is no interaction between B & C that affects A & D. So... how do A & D magically become perfectly correlated?

4. The entire point of entanglement swapping is that it is equally "mysterious". Gisin et al say: "entanglement swapping experiments exhibit nonlocal effects even stronger than those of usual Bell tests." Maybe you have Gisin (or Zeilinger, or Pan) saying something different?

 

[PeterDonis]

what I meant to say was that the composite system A&B is not entangled with the composite system C&D

Yes.

and that the subsystem A&D is in a product state.

No. Since the subsystems A&B are entangled and the subsystems C&D are entangled, you cannot factor out a product state for the subsystem A&D.

 

[Nullstein]

1. And how do those sub-ensembles become entangled into 1 of 4 Bell states? By the swapping process, of course!

It's not a physical process, it's an artificial selection. Nothing becomes entangled in a physical sense.

In principle, there are no sub-ensembles that do not cast entangled A & D into an entangled state. So the idea you have - that only the "correlated" sub-ensembles are being selected - is totally wrong.

That's not the idea that I have. You are just putting words in my mouth.

You completely skipped over my explanation about how A & D come to be entangled. Again, there are 4 Bell states that result from a Bell State Analyzer, and they are not entangled unless they are indistinguishable. This idea does not relate to anything you describe, and of course, it can't.

Your explanation is based on invalid premises as I have pointed out.

2. It IS your idea... try quoting a source. You place a URL here, and then you extract a relevant quote that directly refutes my argument or directly supports yours. It's easy for me to request, because this is a lop-sided exercise. And please, don't tell me to Google something. You have it, or you don't.

I gave you a reference, just scroll up.

3. Well, you claimed that there is no interaction between B & C that affects A & D. So... how do A & D magically become perfectly correlated?

They don't become perfectly correlated, they are completely uncorrelated. Only a subensemble is perfectly correlated much like a random ensemble of heads and tails becomes perfectly deterministic if you choose only the subensemble of events consisting of only heads.

4. The entire point of entanglement swapping is that it is equally "mysterious". Gisin et al say: "entanglement swapping experiments exhibit nonlocal effects even stronger than those of usual Bell tests." Maybe you have Gisin (or Zeilinger, or Pan) saying something different?

No, there is no "whole point of entanglement swapping" in the first place, it's just a phenomenon with many applications. Entanglement swapping is a classical statistical phenomenon that happens on top of a quantum mechanical description involving two pairs of entangled particles. I gave you a reference that explains it (with several thousand citations), you just need to read if you are willing to learn.

By the way, nonlocal in the foundations community is just a term that means "violates Bell's inequality." It doesn't entail anything about causality and some people only keep using it for historic reasons. The less interpretation-laden contemporary term would be inseparable.

 

[Nullstein]

No. Since the subsystems A&B are entangled and the subsystems C&D are entangled, you cannot factor out a product state for the subsystem A&D.

That's not correct. First of all, you don't factor anything out. You get the state of a subsystem by taking partial traces. If the full system is in the state $\rho_{AB}\otimes\rho_{CD}$, then the partial trace with respect to B and C will just be $\mathrm{Tr}_B(\rho_{AB})\otimes\mathrm{Tr}_C(\rho_{CD})$, which is a product state.

 

[DrChinese]

1. I gave you a reference, just scroll up.

2. No, there is no "whole point of entanglement swapping" in the first place, it's just a phenomenon with many applications. Entanglement swapping is a classical statistical phenomenon that happens on top of a quantum mechanical description involving two pairs of entangled particles. I gave you a reference that explains it (with several thousand citations), you just need to read if you are willing to learn.

3. By the way, nonlocal in the foundations community is just a term that means "violates Bell's inequality." It doesn't entail anything about causality and some people only keep using it for historic reasons. The less interpretation-laden contemporary term would be inseparable.

1. LOL. Not surprising you can't produce anything to counter:

"Starting from two independent pairs of entangled particles, one can measure jointly one particle from each pair, so that the two other particles become entangled, even though they have no common past history. The resulting pair is a genuine entangled pair in every aspect, and can in particular violate Bell inequalities. Intuitively, it seems that such entanglement swapping experiments exhibit nonlocal effects even stronger than those of usual Bell tests."-Gisin et al.

If they are entangled, they are entangled. It is not "revealing a statistical coincidence". Please, maybe you have something different from Zeilinger, Tittel, Pan, Kwiat, Zurek, Weihs, Branciard, or any of the other pioneers in this area.2. The whole point is the whole point. That's why it's a thing. Otherwise, it wouldn't be a thing.3. I agree with this. Quantum nonlocality is what Bell's Theorem demonstrates. It doesn't require a rejection of locality, it could also be explained by a rejection of causal direction, by MWI, and any of a number of viable interpretations. Of course, swapping is a demonstration of a much deeper level of quantum nonlocality... see 2 above.

 

[johnsankey]

All I know is that when I was trapping single ions of barium or strontium, and playing Maxwell's daemon with one electron at a time, I saw quanta that were as clear and definite as golf balls. And when I use observations as the basis for inference with entangled states, I see an equally clear result; cf. [Link to personal website redacted by the Mentors]
Never forget the gap between mathematics based upon consistency, and physics based on observables.