A Hidden Assumptions in Bell's Theorem?

[vanhees71]

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

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

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

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

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

 

[kurt101]

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

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

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

 

[DrChinese]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

-DrC

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

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

 

[LittleSchwinger]

Did I understand you correctly?

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

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

 

[lodbrok]

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

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

Here is what the papers say:

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

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

...​

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

...​

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

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

​

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

...​

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

​

 

[lodbrok]

Our decision to do nothing - or something - for step e) above CAUSES - without any ambiguity whatsoever - the statistics of the DISTANT [1 & 4] pairs to change (from Entangled State statistics to Product State statistics). That is because the entanglement swapping operation is a physical process/event/action that is essential to the outcome. It cannot be considered as a mere "selection" of a subset, as we select the exact same swap events but get different results.

This is not true as you can see from the papers themselves as quoted in my reply to SimpleQuestion. Look at the second paper, the Delayed-Choice Swapping experiment by Zeilinger's group published in 2012. Alice and Bob's measurements of [1 & 4] were done well before the BSM measurement on [2 & 3]. Now answer this simple question: Does Victor's measurement of [2 & 3] which is in the future, change anything physical about the [1 & 4] measurement? The [1 & 4] measurement was already done and the results recorded, and those particles were already destroyed long before any [2 & 3] bell-state measurement was performed.

 

[DrChinese]

1. This is not true as you can see from the papers themselves as quoted in my reply to SimpleQuestion. Look at the second paper, the Delayed-Choice Swapping experiment by Zeilinger's group published in 2012. Alice and Bob's measurements of [1 & 4] were done well before the BSM measurement on [2 & 3].

2. Now answer this simple question: Does Victor's measurement of [2 & 3] which is in the future, change anything physical about the [1 & 4] measurement? The [1 & 4] measurement was already done and the results recorded, and those particles were already destroyed long before any [2 & 3] bell-state measurement was performed.

1. Thank you for the quotes you supplied. Finally a direct answer. Of course, the successful swap is all of these: heralding, selection, and an action. So I have no disagreement whatsoever with your quotes.

But... there is no standard suitable word or phrase to label quantum "causality". I can certainly provide quotes that use swapping as a verb to indicate action, or words such as teleportation to indicate an action.

From the Hanson team: "We generate entanglement between the two distant spins by entanglement swapping in the Barrett-Kok scheme..."

From the Gisin team: Here we demonstrate quantum teleportation of the polarization state of a telecom-wavelength photon onto the state of a single collective excitation stored in a rare-earth-ion doped crystal.

So this is equal proof to your quotes, which of course none of which "prove" anything. These are simply useful descriptors/communications in a paper. The proof is in the pudding, which is to say what actually happens. 2. Of course Victor's future action changes the past. That's the entire point, my friend! Quantum mechanics does NOT respect Einsteinian causality, even in direction of causality. Victor's action to entangle the distant pair can be done anytime and anywhere, as experiments actually demonstrate. It could be in the past, present or future. It could be near or distant. Is this paradoxical? Yes! Is it consistent with QM? Yes! To reference the late great Asher Peres (see [4]):

Per the Zeilinger team here, page 5:
"A seemingly paradoxical situation arises — as suggested by Peres [4] — when Alice’s Bellstate analysis is delayed long after Bob’s measurements. This seems paradoxical, because Alice’s measurement projects photons 0 and 3 into an entangled state after they have been measured. Nevertheless, quantum mechanics predicts the same correlations. Remarkably, Alice is even free to choose the kind of measurement she wants to perform on photons 1 and 2. Instead of a Bell-state measurement she could also measure the polarizations of these photons individually. Thus depending on Alice’s later measurement, Bob’s earlier results either indicate that photons 0 and 3 were entangled or photons 0 and 1 and photons 2 and 3. This means that the physical interpretation of his results depends on Alice’s later decision. Such a delayed-choice experiment was performed by including two 10 m optical fiber delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore, this result indicate that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."

So to you it is a paradox (because it goes against your preferred interpretation, presumably). To me, it's strange and magical QM (which is contextual). Just because you think a cause must precede an effect, does not make it so. Regardless of when the swap occurs - even in the future - it is a necessary action to CAUSE the swap. Of course, there are other necessary elements as well, which is normal for any experiment. If you select what you call a "subset" - I can later change the outcome... even after the Bell test is performed.

 

[PeterDonis]

I think Peter deleted it with the reasoning that I was promoting my own theory, but it is not a theory just a toy model

A toy model you made up yourself to support a claim you are making that is not mainstream, it is your own personal claim. That is out of bounds here. If you get your model and your claim published in a peer-reviewed paper, then you can use it as a reference here. But not until then.

 

[vanhees71]

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

No, I think that QT is the correct description of Nature and not local hidden variable theories, because all Bell tests disprove realistic local hidden-variable theories and confirm the predictions of QT.

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

The only problem is that there is no such model discovered yet.

 

[PeterDonis]

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

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

Only one of these "sources" is a valid reference for your claim.

#1 is your own personal work and is out of bounds here; see post #108 just now.

#3 does not contradict @DrChinese, as I have already explained earlier in this thread.

#2 is a peer-reviewed paper that we have discussed before; it makes the non-mainstream claim that a "collider bias" loophole is responsible for the results. This paper's claims cannot be asserted as fact since this is still an open area of research and the majority of the scientific community in question does not accept its claims.

 

[vanhees71]

However it's indeed @DrChinese 's personal claim that local relativistic QFT is not respecting causality. The contrary is true. As all physical theories it's causal, i.e., given the initial state the dynamical equations uniquely determine the state at any later time!

 

[PeterDonis]

Of course Victor's future action changes the past.

You have no way of demonstrating this by experiment. There is no way to measure "what the past would have been" if the other action had been taken in that particular experiment. You have to do a separate experiment where the other action is taken. That separate experiment doesn't "change the past" of the first one; it just has its own past with different statistics.

The very fact that you have already distinguished "quantum causality" from "classical causality" should make you very wary of using classical causal terminology like "changes the past" in this context.

 

[PeterDonis]

given the initial state the dynamical equations uniquely determine the state at any later time!

Not once you add probabilistic measurements (or more generally decoherence events) to the mix. Which you have to do unless you are going to adopt an interpretation like the MWI.

 

[vanhees71]

Of course that's right. For me the minimal statistical interpretation is all that's needed to interpret QT as a natural science.

 

[Simple question]

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

No. This is not even wrong. Preparation in QM is the procedure that allow you to define the initial state. This has nothing to do with measurement, or anything downstream.

Does Victor's measurement of [2 & 3] which is in the future, change anything physical about the [1 & 4] measurement? The [1 & 4] measurement was already done and the results recorded, and those particles were already destroyed long before any [2 & 3] bell-state measurement was performed.

That question has been answered add nauseam in this thread. So again, succinctly:
Victor's measurement between 2&3 change the quantum state 1&4. Either if done in the past, the future or even in a space-like region (think another galaxy). There is no causality to be seen or observed.
It changes it from fully non-entangled, to partially-entangled. Nothing in QM, allows for such "state change" to randomly happens between two unrelated particle. Except by swapping entanglement.
Even if you cannot observe locally any differences by experiment, someone on the other side of the universe can measure information that will allow him to pick-up exactly those "changed to entangled" pair (again, changed because they have NOT been prepared in such a state).

That's definitely a step up because with only one pair, you could always pretend to explain the correlation, by saying is created during that "preparation-procedure", which is causal (in the past) of the two measured particles. The problem comes with hand waving away the concerns about how probabilities at space-like site get correlated (Realistic assumption)

But swapping (that actually happens, and is measured) "create correlation" between particle NOT created at the same place.
It seem that some people have very difficult time accepting such non-causal "interaction" is part of NATURE, despite the experiment. The truth is NATURE does spooky things.
Other accept those non-causal properties but then contradict themselves by claiming that QM is complete and microcausal.

 

[DrChinese]

You have no way of demonstrating this by experiment...

And I am saying we can distinguish it from "selection" and "updating of knowledge" and conditional probabilities. Again, quoting from a groundbreaking experiment about delayed choice entanglement swapping:

"A seemingly paradoxical situation arises — as suggested by Peres [4] ... Therefore, this [experimental] result indicate that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."

Back to you... where is a single experiment that contradicts the Zeilinger team quoted above? The point being made is simple: a future context is as important to the quantum mechanical prediction as any context elements in the past. Time ordering is irrelevant, and there are absolutely no theoretical or experimental results that indicate otherwise.

Which is exactly why entanglement swapping experiments are - excuse the pun - a quantum leap in distinguishing interpretations. Any interpretation featuring Einsteinian causality must be excluded by theory and experiment.

-DrC

 

[Simple question]

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

So I suppose that there is an infinity of such configuration ? And/or no way to compute the ratio of those that would match (entanglement swapping) against the complete set of those in quantum equilibrium ?
Or similarly, if this ratio would change depending on the number of swap in the "network" ?
Bohmian's computation is interesting in that sense, if it theoretically could maybe make interesting and testable prediction for swapping "efficiency" beyond standard QM ?

 

[PeterDonis]

I am saying we can distinguish it from "selection" and "updating of knowledge" and conditional probabilities.

None of this allows you to experimentally detect "changing the past", which is the phrase I was objecting to. It's not like you measure one past, then you do something in the future and now you measure a different past. You measure one past, one future, and they have correlations that can't be explained by a local hidden variable model and that do not obey the usual classical notion of causality. But that last fact, as I have said, means that language like "changing the past" is not appropriate; that language is classical causal language and you have already agreed that classical causality does not apply.

There simply is no ordinary language phrase that appropriately describes what is going on in these experiments. Our ordinary language simply did not evolve with this kind of thing in mind. Eventually we might develop some, once knowledge and acceptance of these results becomes common enough.

where is a single experiment that contradicts the Zeilinger team quoted above?

I have never claimed that there was one. See above.

 

[lodbrok]

Is this paradoxical? Yes! Is it consistent with QM? Yes! To reference the late great Asher Peres (see [4]):

There's nothing paradoxical about these results, the subset of already recorded results selected will change based on which measurement victor did. Nothing physical changes about the particles in the stream [1 & 4].

The quotes I provided were to demonstrate the falsity of your earlier claim that there is some influence from [2&3] that is changing the physical situation at [1 & 4]. That's all.

 

[kurt101]

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

Our decision to do nothing - or something - for step e) above CAUSES - without any ambiguity whatsoever - the statistics of the DISTANT [1 & 4] pairs to change (from Entangled State statistics to Product State statistics).

This is a very compelling argument against reality and causality if true. Inserting the polarizer at step e can't change the measurement at distant 1 & 4 when the measurement at 1 & 4 has already been made, but at the same time inserting the polarizer should not change the measurement outcome at the BSM (though I suppose that would be an unexpected possibility since we can't see what is actually going on). So assuming what you have told me is true, I would accept your argument.

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

However, given that the experiment has not actually been performed, that leaves a lot of doubt in my mind. Maybe you are not thinking of the problem correctly? So I guess I just have to reserve judgement either way until the actual experiment is performed or I understand the insertion of the polarizer better.

Do other credible physicists agree with your expectation?

If this experiment has so many implications to quantum mechanics, why hasn't this experiment been performed?

 

[Demystifier]

So I suppose that there is an infinity of such configuration ?

Yes. Moreover, it's an uncountable (continuous) infinity.

And/or no way to compute the ratio of those that would match (entanglement swapping) against the complete set of those in quantum equilibrium ?

Since it's uncountable, you need to define a measure on the continuous set. If the measure is proportional to $|\psi|^2$, then the set of non-equilibrium configurations should be negligible.

Bohmian's computation is interesting in that sense, if it theoretically could maybe make interesting and testable prediction for swapping "efficiency" beyond standard QM ?

In equilibrium, no.

 

[vanhees71]

No. This is not even wrong. Preparation in QM is the procedure that allow you to define the initial state. This has nothing to do with measurement, or anything downstream.That question has been answered add nauseam in this thread. So again, succinctly:
Victor's measurement between 2&3 change the quantum state 1&4. Either if done in the past, the future or even in a space-like region (think another galaxy). There is no causality to be seen or observed.
It changes it from fully non-entangled, to partially-entangled. Nothing in QM, allows for such "state change" to randomly happens between two unrelated particle. Except by swapping entanglement.
Even if you cannot observe locally any differences by experiment, someone on the other side of the universe can measure information that will allow him to pick-up exactly those "changed to entangled" pair (again, changed because they have NOT been prepared in such a state).

That's definitely a step up because with only one pair, you could always pretend to explain the correlation, by saying is created during that "preparation-procedure", which is causal (in the past) of the two measured particles. The problem comes with hand waving away the concerns about how probabilities at space-like site get correlated (Realistic assumption)

But swapping (that actually happens, and is measured) "create correlation" between particle NOT created at the same place.
It seem that some people have very difficult time accepting such non-causal "interaction" is part of NATURE, despite the experiment. The truth is NATURE does spooky things.
Other accept those non-causal properties but then contradict themselves by claiming that QM is complete and microcausal.

But it's not non-causal. The correlations were already there in the preparation, i.e., projecting out to a Bell state doing a local manipulation with the pair (23) implies that also the pair (14) for the corresponding sub-ensemble is in a Bell state although in the initial state the photon pairs (14) and (23) were not entangled. This can indeed only be verified by exchanging the results of the corresponding measurements at the three sites (photon 1, photon pair (23), and photon 3), and thus there's no possibility for faster-than-light signalling.

 

[vanhees71]

If this experiment has so many implications to quantum mechanics, why hasn't this experiment been performed?

Entanglement swapping has been performed for more then 20 years. Of course you can only do experiments that can be done in Nature. Fictitious would-be experiments, which cannot be done even in principle, of course, lead to inconsistencies. QT only describes experiments which really can be done, not fictitious constructs which are impossible in principle.

 

[gentzen]

Just my usual lamento: Please define, what you mean by "local" or "non-local".

The meaning of "local" there was implicitly defined by "if decomposition into „local“ pieces is rejected". Of course, the intention is that the decomposition be reasonable for the problem at hand, and the "local pieces" be easy to understand, or at least easier than the full non-decomposed problem.

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

On the one hand, you make a very similar argument as I did above: QED is a formalism that allows you to make calculations and ultimately predictions, and its microcausality condition is crucial for its working and its success. So how can you use that formalism (i.e. "decomposition" in my abstract sense) without also accepting microcausality condition (i.e. "locality" in my abstract sense).

On the other had, you talk about "causal connections" as if it would be clear how DrC understands those words, while I commented to DrC before that those words induce more ambiguity than he admits:

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

I'd say entanglement swapping is one specific kind of teleportation. I don't understand, what you have doubts about: What's done ...

I had in mind a derivation of quantum teleportation like "Fig 6.5 A circuit-theoretic derivation of the quantum teleportation protocol." in section "6.5 Teleportation" in "Quantum Computer Science. An Introduction" by David Mermin. And now I was faced with the question, how I can get from the knowledge that the "complete protocol" works to the knowledge that the "poor man" protocol works too. Because if I believe that something "magical" happens during that protocol, then how can I be sure that the "magic" still happens for my "simplified" protocol?
There are many possible answers, for example I could just derive quantum teleportation directly for the "simplified" protocol. One can even argue that such a derivation is implicitly present in the text of section "6.5 Teleportation". But I wondered instead what I would have to "check" to confirm that the simplified protocol works, and what I have to "check" in the end are statistics of measurement results.

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

Also in this case, what I have to "check" to confirm that two classical bits can describe a single qubit "in context" are statistics of measurement results. ... In the end, this means that I work in the minimal statistical interpretation, and use it as my criterion for "checking" correctness of proposed "protocols".

 

[Simple question]

There's nothing paradoxical about these results

Indeed, this is just spooky correlation at a distance. It is paradoxical only for people invoking causality and locality to explain the phenomenon.

, the subset of already recorded results selected will change based on which measurement victor did.

See ? What if Victor forget to pay its bills, and did/is/will do nothing ?

Nothing physical changes about the particles in the stream [1 & 4]

Make-up you mind, you've just described a paradox. If nothing changes about 1&4, than QM say that there is a zero sub ensemble of entangled 1&4 pair. Yet, Victor did/is/will find one.

 

[Simple question]

But it's not non-causal.

It clearly is, because you cannot establish an ordering. Causality is irrelevant.

The correlations were already there in the preparation, i.e.,

The preparation is two space-like crystals doing Spontaneous Parametric Down-Conversion. So 1 & 4 are clearly uncorrelated.

projecting out to a Bell state doing a local manipulation with the pair (23)

Is not a preparation, not local to 1&4 nor can be causally ordered with it.

implies that also the pair (14) for the corresponding sub-ensemble is in a Bell state although in the initial state the photon pairs (14) and (23) were not entangled.

That "implication" or "projection" occurs between non-local and non-causal sites.

This can indeed only be verified by exchanging the results of the corresponding measurements at the three sites (photon 1, photon pair (23), and photon 3), and thus there's no possibility for faster-than-light signalling.

Correct. Nobody question this.

 

[kurt101]

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

I thought it was a requirement of entanglement swapping for the photons to be indistinguishable. If after the beam splitter you add the polarizers for step e, and that makes the photons distinguishable, doesn't that imply that the photons were never indistinguishable in the first place?

 

[DrChinese]

I thought it was a requirement of entanglement swapping for the photons to be indistinguishable. If after the beam splitter you add the polarizers for step e, and that makes the photons distinguishable, doesn't that imply that the photons were never indistinguishable in the first place?

I am reached out to one of the Delft experiment authors to get more information on this point. Generally, I agree with what you are saying about being indistinguishable of course. However, there are some nuances here.

One of the issues is that there is no such thing as "indistinguishable" in the classical world - the one where you are "selecting" a subset during the BSM. If you are merely "selecting", you simply make a list of characteristics that you want to match (color, size, shape, texture, whatever) and both things have these in common or they don't. There is no action, and certainly no remote change to statistics elsewhere.

My point is that once you narrow the set down to the ones with matching characteristics - a subset in which the related photons will show Bell correlations (including perfect correlations) - you can do things that will cause the BSM subset to be distinguishable even though they previously possessed the matching characteristics. If swapping is an action, an event, then there is action at a distance. If it is "selecting", there can be no distant change when you distinguish. Something has to give, and I believe that can be physically demonstrated.

 

[Fra]

It clearly is, because you cannot establish an ordering. Causality is irrelevant.

Does it help it we instead say that the correlation is explained by the preparation + post-selection, rather than "caused by"? Causality without order makes no sense, but explanation without order does, it does not necessarily need an order at least not in the same sense.

To just say nature is non-causal and spooky sound like a pathology-declaration, it is not constructive.

/Fredrik

 

[lodbrok]

See ? What if Victor forget to pay its bills, and did/is/will do nothing ?

Nothing to see. If Victor does a different BSM, then he gets a different subset of particle pairs [2&3] and therefore the subset of [1&4] would be different. This is the only sense in which Victor's results affect the analysis. Remember all of the [1&4] data already exists in the spreadsheet. We are just asking Victor for which rows to consider.

Make-up you mind, you've just described a paradox. If nothing changes about 1&4, than QM say that there is a zero sub ensemble of entangled 1&4 pair. Yet, Victor did/is/will find one.

There is nothing paradoxical about the fact that Victor gets a different set of rows based on which experiments he does on [2&3]. QM tells you that the full [1&4] stream shows no correlation and also that you can "prepare" a sub-ensemble of [1&4] that shows a correlation. This is what these experiments demonstrate. But you are probably hung up on the word "prepare". You think it means something physical is happening to [1&4] when Victor does his measurement, but you are wrong. These experiments disprove such an idea, especially the delayed-choice one which I already quoted. The entangled sub-ensemble of [1&4] was "prepared" by post-selection of data, long after the particles were already measured and destroyed.
It's only a paradox for you if you continue to believe "preparation" in QM must be a physical process that happens to the particles, in which case you would have no choice but to believe in absurd notions like retro-causality as was hinted at earlier.

 

[vanhees71]

The meaning of "local" there was implicitly defined by "if decomposition into „local“ pieces is rejected". Of course, the intention is that the decomposition be reasonable for the problem at hand, and the "local pieces" be easy to understand, or at least easier than the full non-decomposed problem.On the one hand, you make a very similar argument as I did above: QED is a formalism that allows you to make calculations and ultimately predictions, and its microcausality condition is crucial for its working and its success. So how can you use that formalism (i.e. "decomposition" in my abstract sense) without also accepting microcausality condition (i.e. "locality" in my abstract sense).

But that's precisely what I'm trying to argue for the whole time! The microcausality condition rules out causal connections between space-like separated events, and thus if the projection-measurement event on (23) is space like to the meausurement-registration events at photons 1 and photons 2, then the projection measurement on (23) cannot be the cause for the pair (14) in the corresponding selected subensemble are entangled. For me it's simply the correlation implied by the preparation of the pairs (12) and (34) in the entangled states, which ensures that a selection of the pair (23) to be in a Bell state implies that then also the pair (14) is in a Bell state, although the measurement on (23) enabling the projection, is done at a place arbitrarily far from both places the measurements on 1 and 4 where done. You need the measurement protocols for all these three measurements to post-select the corresponding subensemble.

 

[Simple question]

Does it help it we instead say that the correlation is explained by the preparation + post-selection, rather than "caused by"?

Well, sort of, I suppose. For people that think that Nature have some explaining to do.
I myself prefer to keep the explaining part in the epistemology, not the lab, I like my theories to be logical and consistent.
So the preparation+post selection caused completely disjoint space-like events to be correlated ... in the lab.
While in the theory, the explanation cannot be local and realistic (microcausality is realistic). Bell' proved that age ago.

Causality without order makes no sense, but explanation without order does, it does not necessarily need an order at least not in the same sense.

We are on the same page then.

To just say nature is non-causal and spooky sound like a pathology-declaration, it is not constructive.

And yet I don't find your previous quote "non constructive". I am not worried about "words". I use spooky, you use "explanation without order".

 

[Simple question]

Nothing to see. If Victor does a different BSM, then he gets a different subset of particle pairs [2&3] and therefore the subset of [1&4] would be different.

Or not be, so there is nothing to see. See ?
The point is that the subset can be empty.

This is the only sense in which Victor's results affect the analysis. Remember all of the [1&4] data already exists in the spreadsheet. We are just asking Victor for which rows to consider.

Which is a contradiction. You do not understand that those spreadsheet can NOT contains entanglement traces. You cannot violate Bell's inequalities with socks an spread sheet. You have to entangle quantum objects first by preparation.

There is nothing paradoxical about the fact that Victor gets a different set of rows based on which experiments he does on [2&3].

Correct. Victor is just measuring things, as Bod and Alice. The paradox is in your thinking about the final analysis.

QM tells you that the full [1&4] stream shows no correlation

So far so good

and also that you can "prepare" a sub-ensemble of [1&4] that shows a correlation.

Paradox. There is no such thing in QM. The initial state of 1&4 is clear-cut.

But you are probably hung up on the word "prepare". You think it means something physical is happening to [1&4] when Victor does his measurement, but you are wrong.

Quite the opposite. I am not hung-up on preparation. QM is. The analysis is based on the initial state. That's how it works.

These experiments disprove such an idea, especially the delayed-choice one which I already quoted. The entangled sub-ensemble of [1&4] was "prepared" by post-selection of data, long after the particles were already measured and destroyed.

Those experiment confirmed basic QM AND Bell's theorem. QM does not contain "post-selection" to hand-wave explanation.

It's only a paradox for you if you continue to believe "preparation" in QM must be a physical process

No, it's only a paradox for those not understanding that QM contains non-causal correlation, as Bell's theorized, and then was measured many time since. This experiment is just another type of test, involving swapping.

physical process that happens to the particles, in which case you would have no choice but to believe in absurd notions like retro-causality as was hinted at earlier.

Nope, retro-causality will be invoked by people thinking that NATURE is entirely causal, they are even ready to make it negative. Again paradoxical thinking.

Yet, despite you claims, entanglement is a physical process. As no pair 1&4 are entangled, and then later some pair are measure to be entangle. The only non-paradoxical conclusion is that their entanglement has been(is/ will be) swapped (changed)

That is what the experiment is telling us.

 

[gentzen]

This is the only sense in which Victor's results affect the analysis. Remember all of the [1&4] data already exists in the spreadsheet. We are just asking Victor for which rows to consider.

Which is a contradiction. You do not understand that those spreadsheet can NOT contains entanglement traces. You cannot violate Bell's inequalities with socks an spread sheet. You have to entangle quantum objects first by preparation.

Sorry, but you simply don't understand what lodbrok is trying to "explain". He didn't use socks in that attempt to try to get his point across. And you certainly can use a spread sheet to tabulate your measurement results.

In quantum mechanics, measurement results are more important than the specific formalism used to predict their statistics. That you prefer to use a formalism which includes collapse of the wavefunction doesn't mean that there must be anything physical corresponding to the collapse. Or at least not something physical involving three independent real parameters.

There might be "non-local randomness" concerning two classical bits. (Or maybe four, or some other small number, depending on the specific experiment, and the number of entangled qubits involved.) But such classical bits can be described by classical analogies, especially if you postpone discussion of the "non-local randomness" part.

 

[Simple question]

Sorry, but you simply don't understand what lodbrok is trying to "explain".

That is possible. I understand that he says that nothing change in 1&4. That's simply false, they've build 3 labs and more to measure that change.
I also think he do not understand what DrChinese is saying, which is pretty straightforward.

He didn't use socks in that attempt to try to get his point across. And you certainly can use a spread sheet to tabulate your measurement results.

But not if that spread sheet contains data about socks or coin. Those spread sheet cannot contains entanglement correlation either.
You know DrChinese does not need those analogies, and that they can be misleading.

The rest of you post, I agree completely with.

 

[martinbn]

That is possible. I understand that he says that nothing change in 1&4. That's simply false, they've build 3 labs and more to measure that change.

What has changed in 1&4? What change exactly was measured at 1&4? To be more specific: suppose we perform the experiment many times, say 2000. I get 2&3 you get 1&4. In the first 1000 i do nothing to 2&3, in the second 1000 I perform the measurement. You can do whatever you want to all 2000 1&4's. Can you see the change in the second half compair to the first half? (Without any additional information from me.)

 

[martinbn]

I also think he do not understand what DrChinese is saying, which is pretty straightforward.

I don't understand what @DrChinese says. At first I thought that by nonlocality he means violation of Bell's inequality. Then it seems to me, that he means more. It seems that he insists on an instantaneous action at a distance. In fact more than that, because he wants that instantaneous action to affect different instances at the same time. I know it sounds contradictory, but somehow that is what he says. At least this is the impression I get.

 

[Lord Jestocost]

Maybe, it might be of help to read “Can relativity be considered complete? From Newtonian nonlocality to quantum nonlocality and beyond” by Nicolas Gisin: "Actually, the situation is even more interesting: Not
only does God play dice, but he plays with nonlocal dice!"

https://arxiv.org/abs/quant-ph/0512168

 

[DrChinese]

Maybe, it might be of help to read “Can relativity be considered complete? From Newtonian nonlocality to quantum nonlocality and beyond” by Nicolas Gisin: "Actually, the situation is even more interesting: Not
only does God play dice, but he plays with nonlocal dice!"

https://arxiv.org/abs/quant-ph/0512168

1. Wow, this is a treasure trove for me! LOL... from the reference:

"ENTANGLEMENT AS A CAUSE OF CORRELATION...
Quantum physics predicts the existence of a totally new kind of correlation that will never have any kind of mechanical explanation. And experiments confirm this: Nature is able to produce the same randomness at several locations, possibly space-like separated. The standard explanation is ”entanglement”, but this is just a word, with a precise technical definition. ... Quantum correlations simply happen, as other things happen (fire burns, hitting a wall hurts, etc). Entanglement appears at the same conceptual level as local causes and effects. It is a primitive concept, not reducible to local causes and effects. ... In other worlds, a quantum correlation is not a correlation between 2 events, but a single event that manifests itself at 2 locations."

Gisin is one of the top experimentalists and theoreticians in the area of entanglement/QM, having authored or co-authored 400+ papers. He has viewed these experiments first hand, and contemplated them from many angles. I am sure his views are similar to many others in the field: take the predictions of QM as accurate, without attempting to reconcile the underlying mechanics with classical elements (such as Einsteinian causality). Nature doesn't work like that... obviously.

2. I don't understand what @DrChinese says. At first I thought that by nonlocality he means violation of Bell's inequality. Then it seems to me, that he means more. It seems that he insists on an instantaneous action at a distance. In fact more than that, because he wants that instantaneous action to affect different instances at the same time. I know it sounds contradictory, but somehow that is what he says. At least this is the impression I get.

2. A violation of a Bell Inequality is always an experimental demonstration of the correctness of the predictions of QM, and a repudiation of Local Realistic theories. Most experimentalists conclude such is evidence of "quantum nonlocality*" without strictly specifying whether it is Locality and/or Realism which must be rejected. It is therefore not a proof (by itself) of "instantaneous action at a distance".

However... The invention of experiments in which entangled pairs (as evidenced by violation of Bell Inequalities) are created from distant** independent sources that have never interacted takes things to an entirely new level. There is no meaningful way to describe the events which occur in a manner consistent with Einsteinian causality. To adapt the words of Gisin: quantum nonlocality is not a correlation between distant events; it is a single event that manifests itself at distant spacetime locations.

The more common term for the above "distant spacetime locations" is "context". Quantum Mechanics is contextual. A quantum context violates the limits of Einsteinian spacetime action as defined by c, and it violates the limits imposed by causality which requires ordering to indicate causes and effects (i.e. cause must precede effect). We know from experiment that we can entangle particles after the fact. That violates classical causality, but is consistent with context. We know from experiment that distant systems that have never interacted can be made to violate Bell inequalities, showing they are not separable (regardless of distance).

What is interesting is that a quantum context does have limits, even though those limits do not respect direction in time (local causality). The context is itself created from individual connections that respect +/-c. In a standard Bell test, the spacetime diagram looks something like a "V". In a standard swapping arrangement, the spacetime diagram looks something like a "VV". Here is an experiment in which the spacetime diagram looks something like a "VVV" (6 photons from 3 initial entangled pairs, 2 BSMs and 2 swaps!):

https://arxiv.org/abs/0808.2972
Multistage Entanglement Swapping (2008)*Einstein called this "spooky action at a distance." Of course he never knew about Bell, so his rejection of the idea is understandable. Personally, I say: Quantum nonlocality and spooky action at a distance are the same thing.
** Distant meaning separated in spacetime, not just space.

3. What has changed in 1&4? What change exactly was measured at 1&4? To be more specific: suppose we perform the experiment many times, say 2000. I get 2&3 you get 1&4. In the first 1000 i do nothing to 2&3, in the second 1000 I perform the measurement. You can do whatever you want to all 2000 1&4's. Can you see the change in the second half compared to the first half? (Without any additional information from me.)

3. Of course, you must send the "event ready" (heralding) indicator so I know which [1 & 4] pairs to select - since I am looking only for pairs in a particular Bell state (such as Psi-***). You execute the usual swap on one set of those (as you say). But do nothing (no swap) on the other set except identify the same basic indistinguishable characteristics but without executing a swap. Assume for the sake of this discussion I can describe how that can be made to occur.

Then yes, one group has perfect correlation, the other has random correlation.

How to select [2 & 3] pairs without executing a swap:
Keep in mind: the rule is that the [2] and [3] photons have appropriate matching characteristics that make them indistinguishable, and then distill those into the desired Bell State. You say you are simply selecting those pairs from a larger group, and the swap is not a physical action. I say it is a physical action that changes the remote state of the [1 & 4] pairs. If you are correct, then simply identifying those same indistinguishable characteristics on the [2] and [3] photons should enough, no precise overlap is required. I say only sufficient overlap will lead to a swap. (After all, you say the overlap does nothing because it is not physical.)

We can write out a list of those selection characteristics for the Psi- Bell state: a) same time window; b) same wavelength; c) opposite polarization; d) both reflect (R) or both transmit (T) at the same beamsplitter. You herald those [2 & 3] pairs, I'll look at the [1 & 4] pairs that go with those.***Psi-occurs in about 25% of all suitable pairs. The [1 & 4] pairs in this state will be polarization entangled with anti-correlation. The other Bell states are Psi+, Phi-, and Phi+.

 

[Simple question]

What has changed in 1&4? What change exactly was measured at 1&4?

Entanglement. Some pair have been swapped (form none to full). I don't think you follow the conversation.

To be more specific: suppose we perform the experiment many times, say 2000. I get 2&3 you get 1&4. In the first 1000 i do nothing to 2&3, in the second 1000 I perform the measurement. You can do whatever you want to all 2000 1&4's. Can you see the change in the second half compair to the first half? (Without any additional information from me.)

Nobody disagree with that.

I don't understand what @DrChinese says.

Why ? All is very straightforward and standard QM

At first I thought that by nonlocality he means violation of Bell's inequality.

Why ? Non-locality means non-locality. I am pretty sure you understand what space-like events means.

Then it seems to me, that he means more. It seems that he insists on an instantaneous action at a distance.

No, nobody said that. In entanglement it is the correlation that get's "action'ed" at a distance. They change

In fact more than that, because he wants that instantaneous action to affect different instances at the same time. I know it sounds contradictory, but somehow that is what he says. At least this is the impression I get.

No, less that than, more like zero. You cannot quote him saying that. In the tons of text he was kind enough to write to explains things to local absolutist, only a few half sentence could be misconstrued, and PeterDonis fixed them.

My impression is that you say that nature is entirely local and causal. It's not. Nor is QM btw. The article in post #138 is quite good, it may help you.

 

[kurt101]

You think it means something physical is happening to [1&4] when Victor does his measurement, but you are wrong. These experiments disprove such an idea, especially the delayed-choice one which I already quoted.

But in the case where Victor (2 & 3) measurement is done before measurement of 1 & 4, would you say something physically is happening? At least in the context of reality where we have time ordered cause and effect, doing the measurement 2 & 3 does something to the 1 & 4 measurement. I am not sure if this would qualify under your definition of physical given that the cause and effect is non-local.

I read through some of your previous comments on this thread and it was not clear to me that you were distinguishing between the two cases in your arguments. But if you are taking some kind of realist viewpoint of this experiment where cause and effect matter, I don't see how you can deny that 1 & 4 do not directly affect each other in the case where the BSM test is done before the measurement of 1 & 4 since you could change their angles of their measurement well you after you obtained the knowledge that these pairs are entangled at Victor (2 & 3) (same logic you used in one of your arguments for the opposite case) and you would get the correct statistics which could not happen if this was some post selection phenomena. If 1 & 4 are not truly entangled (i.e. not a post selection phenomena) in this case, you would not get the right statistics for some angles of measurement you decided to use at the very last moments before measurement of 1 & 4 no matter what the BSM had previously told you you about 1 & 4.

 

[kurt101]

1. Wow, this is a treasure trove for me! LOL... from the reference:

"ENTANGLEMENT AS A CAUSE OF CORRELATION...
Quantum physics predicts the existence of a totally new kind of correlation that will never have any kind of mechanical explanation. And experiments confirm this: Nature is able to produce the same randomness at several locations, possibly space-like separated. The standard explanation is ”entanglement”, but this is just a word, with a precise technical definition. ... Quantum correlations simply happen, as other things happen (fire burns, hitting a wall hurts, etc). Entanglement appears at the same conceptual level as local causes and effects. It is a primitive concept, not reducible to local causes and effects. ... In other worlds, a quantum correlation is not a correlation between 2 events, but a single event that manifests itself at 2 locations."

By "mechanical explanation" does the author mean "local causes and effects" and not non-local causes and effects. In other words is the author ruling out mechanical explanations but not mechanistic explanations? It is hard to tell what he means because he throws in randomness which is not mechanistic, but he uses "a single event that manifests itself at 2 locations" which does sound mechanistic. And by randomness it is not clear if he means fundamental randomness or apparent randomness (because we can't see what the starting state is). So to me his statement is as clear as mud.

 

[Cthugha]

2. Of course Victor's future action changes the past. That's the entire point, my friend! Quantum mechanics does NOT respect Einsteinian causality, even in direction of causality. Victor's action to entangle the distant pair can be done anytime and anywhere, as experiments actually demonstrate. It could be in the past, present or future. It could be near or distant. Is this paradoxical? Yes! Is it consistent with QM? Yes! To reference the late great Asher Peres (see [4]):

Per the Zeilinger team here, page 5:
"A seemingly paradoxical situation arises — as suggested by Peres [4] — when Alice’s Bellstate analysis is delayed long after Bob’s measurements. This seems paradoxical, because Alice’s measurement projects photons 0 and 3 into an entangled state after they have been measured. Nevertheless, quantum mechanics predicts the same correlations. Remarkably, Alice is even free to choose the kind of measurement she wants to perform on photons 1 and 2. Instead of a Bell-state measurement she could also measure the polarizations of these photons individually. Thus depending on Alice’s later measurement, Bob’s earlier results either indicate that photons 0 and 3 were entangled or photons 0 and 1 and photons 2 and 3. This means that the physical interpretation of his results depends on Alice’s later decision. Such a delayed-choice experiment was performed by including two 10 m optical fiber delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore, this result indicate that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."

I do not intend to enter into discussions of interpretations, but I would like to comment on the state of the community. I had the pleasure of cohosting Gregor Weihs last week in the colloquium of our department and besides funny/tragic stories about what discussions with Joy Christian are like, he also speaks quite clearly about Zeilinger's take on interpretations and Zeilinger's view is a modern information-theoretic version of Copenhagen which is heavily on the update-of-information side (https://arxiv.org/abs/quant-ph/0005084 and follow-up papers). It goes by the name Brukner-Zeilinger interpretation and a (non-Shannon) information vector is fundamental. The idea of finiteness of information is a golden thread through much of Zeilinger's work.

Quoting Zeilinger in support of a statement claiming that a future action changes the past in entanglement swapping experiments misrepresents Zeilinger's point of view. This becomes clear when reading Zeilinger's own papers on delayed choice entanglement swapping (https://arxiv.org/abs/1203.4834).

He closes stating:
" If one views the quantum state as a real physical object, one could get the seemingly paradoxical situation that future actions appear as having an influence on past and already irrevocably recorded events. However, there is never a paradox if the quantum state is viewed as to be no more than a “catalogue of our knowledge”. Then the state is a probability list for all possible measurement outcomes, the relative temporal order of the three observer’s events is irrelevant and no physical interactions whatsoever between these events, especially into the past, are necessary to explain the delayed-choice entanglement swapping."

and goes on to explain:
"What, however, is important is to relate the lists of Alice, Bob and Victor’s measurement results. On the basis of Victor’s measurement settings and results, Alice and Bob can group their earlier and locally totally random results into subsets which each have a different meaning and interpretation. This formation of subsets is independent of the temporal order of the measurements. According to Wheeler, Bohr said: “No elementary phenomenon is a phenomenon until it is a registered
phenomenon.”We would like to extend this by saying: “Some registered phenomena do not have a meaning unless they are put in relationship with other registered phenomena.”"

Whether or not one considers this convincing is everone's own decision. However, quoting Zeilinger to support interpretations where the future influences the past is going against Zeilinger's views and intentions.

 

[PeterDonis]

The microcausality condition rules out causal connections between space-like separated events

For experiments that test the Bell inequalities or entanglement swapping, this is actually a red herring, because the results of the experiments are actually the same regardless of whether the individual measurement events are spacelike, null, or timelike separated!

 

[lodbrok]

But in the case where Victor (2 & 3) measurement is done before measurement of 1 & 4, would you say something physically is happening? At least in the context of reality where we have time ordered cause and effect, doing the measurement 2 & 3 does something to the 1 & 4 measurement. I am not sure if this would qualify under your definition of physical given that the cause and effect is non-local.

In these experiments, time ordering makes no difference. Nothing that happens to particles (2&3) physically affects anything that happens to particles (1&4). It doesn't matter whether it is done before or after. It's all about how experimental results are handled after the experiments are long complete.

Chtugha's quote from Zeilinger himself says it more clearly:

"What, however, is important is to relate the lists of Alice, Bob and Victor’s measurement results. On the basis of Victor’s measurement settings and results, Alice and Bob can group their earlier and locally totally random results into subsets which each have a different meaning and interpretation. This formation of subsets is independent of the temporal order of the measurements."

 

[kurt101]

In these experiments, time ordering makes no difference.

Nobody on this forum is disputing the order makes any difference to the statistical results.

Nothing that happens to particles (2&3) physically affects anything that happens to particles (1&4). It doesn't matter whether it is done before or after. It's all about how experimental results are handled after the experiments are long complete.

That is your opinion based on some non-realistic interpretation that lacks cause and effect that you have chosen. If you choose a realistic interpretation with cause and effect then what happens at (2&3) does affect (1&4), but only in the case where (2&3) is done first. To the best of my knowledge nobody has come up with an experiment that disproves realism with cause and effect. The delayed choice experiments are no different.

 

[PeterDonis]

If you choose a realistic interpretation with cause and effect then what happens at (2&3) does affect (1&4), but only in the case where (2&3) is done first.

This doesn't make sense, though, given the fact that the actual results are the same regardless of the spacetime relationship between the measurements. Since the results are the same, whatever is going on "underneath" should be the same as well. But your claim here says that's not true; that whatever is going on "underneath" has to be different depending on the spacetime relationship between the measurements.

To my knowledge nobody in the QM community has proposed any such interpretation.

 

[kurt101]

Since the results are the same, whatever is going on "underneath" should be the same as well.

And that is your potential error. I am not saying that you are wrong, but it is possible that you are wrong. In other words what is going on underneath does need to be the same in both cases. That may seem strange, but I don't think it is strange at all. After all I wrote a simulation that tells me it is not really strange at all. I have tried to explain it in words and maybe I will try again, but once you understand it it is just symmetry.

To my knowledge nobody in the QM community has proposed any such interpretation.

I am just assuming a generic realistic cause and effect interpretation. Didn't Einstein, Bell, and others use such an interpretation to make progress in understanding quantum mechanics?

 

[PeterDonis]

In other words what is going on underneath does need to be the same in both cases. That may seem strange, but I don't think it is strange at all.

I don't think you'll find many people who share your opinion. In particular, if there is anyone in the QM community who has published a claim along these lines, you should reference it. I doubt that there is, but you are welcome to post a reference if you have one.

After all I wrote a simulation that tells me it is not really strange at all.

As you have already been told multiple times, your simulation is off topic here. If you mention it again you will receive a warning.

I am just assuming a generic realistic cause and effect interpretation.

I'm not sure what you mean by this; certainly, given the extensive debate in the literature, you can't just help yourself to this term as though its meaning were self-evident. You need to give a specific reference that describes the interpretation you are using.

Didn't Einstein, Bell, and others use such an interpretation to make progress in understanding quantum mechanics?

Einstein believed that such an interpretation that would preserve locality as well as realism was possible, but Einstein didn't know about Bell's Theorem.

Bell himself was well aware that his theorem ruled out any interpretation along the lines Einstein was looking for. In at least one of his papers (collected in Speakable and Unspeakable in Quantum Mechanics), he notes that the Bohmian interpretation is an obvious, simple interpretation that accounts for violations of the Bell inequalities by being a nonlocal hidden variable model--and then remarks that this is the sort of resolution that Einstein would have liked least.

 

[kurt101]

I don't think you'll find many people who share your opinion. In particular, if there is anyone in the QM community who has published a claim along these lines, you should reference it. I doubt that there is, but you are welcome to post a reference if you have one.

I don't have any reference. I can explain it in the language of cause and effect and realism. I will never understand why such language would be off limits since that is the basis we all come from and is the common language we used to understand each other. When something does not seem to follow realism and cause and effect we explain how it differs from this basis of understanding.

Bell himself was well aware that his theorem ruled out any interpretation along the lines Einstein was looking for. In at least one of his papers (collected in Speakable and Unspeakable in Quantum Mechanics), he notes that the Bohmian interpretation is an obvious, simple interpretation that accounts for violations of the Bell inequalities by being a nonlocal hidden variable model--and then remarks that this is the sort of resolution that Einstein would have liked least.

Bell also discussed interpretations in the generic sense. He clearly did not rule out realistic non-local interpretations in general. And while Einstein might not have been an advocate for non-locality he definitely discussed it in the context of reality.