I A new realistic stochastic interpretation of Quantum Mechanics

[PeterDonis]

Maybe you can try to clarify.

The clarification is simple: the QM math is the same regardless of the ordering of the measurements, and so are the observed correlations, which violate the Bell inequalities and related inequalities. And that, by Bell's and other theorems, rules out any model of the type you are calling "classical", for any ordering of the measurements.

So any purported "explanation" that says a "classical" model works for some orderings of measurement but not for others, can't be right. The correlations are not "classical" for any ordering of the measurements.

I am not saying your wrong about what @DrChinese means, but as far as I can tell you are saying different things.

@DrChinese is of course welcome to correct me, but I think the concept of "classical" he is using is intended to be ruled out by both the QM math and actual experimental results for any ordering of the measurements, for the basic reason I gave above. Indeed, he has made a similar argument to the one I made above in other threads on this general topic.

In other words, @DrChinese did not offer to bet you because he thought you had any chance at all of winning the bet. He believes (and I do too) that you are doomed to lose no matter what you do, because for you to win would have to mean that proven mathematical theorems are not correct.

 

[PeterDonis]

I disagree with @Morbert statement in general, but I think he correct for the case that I am discussing.

You would be well advised to read my post #44.

 

[Herman Trivilino]

Don't we have enough interpretations already? Unless something is making testable predictions then what is the point? Mathematically I'm sure you can cook things an infinite number of ways.

I think the hope is that an interpretation will gain an overwhelming majority of support from physicists. Other branches of physics enjoy this feature, and it facilitates explaining the theory to students and the public at large.

 

[DrChinese]

And I am trying to find a classical correlation swapping example that @DrChinese is wagering the bet on, but where I differ from @Morbert is I only think it happens in the case where the 2 & 3 swap is done last.

@PeterDonis I don't think you are helping as you seem to be giving your own spin on this conversation. I think I am best off hearing it from the horses mouth @DrChinese. I am not saying your wrong about what @DrChinese means, but as far as I can tell you are saying different things.

If you have any kind of communication between entangled particles (1 & 2 or 3 & 4), that violates locality. That's what @PeterDonis is saying... so it's already not classical. Classical ideas strictly obey locality.

When you get into FTL mechanisms - which the Remote Entanglement Swapping (RES) experiments seem to require - you quickly realize that even those have issues. In the RES type, there are 3 parties (Alice, Bob, Chris) distant to each other and there would need to be some kind of communication/coordination between all 3 to make things work out.

I personally have no idea how that would work, but it is easy to see that all of the "simple" explanations quickly fall to the wayside. That's because there are too many rules that must be obeyed all at once with the 3 parties. Alice and Bob must see perfect correlations when Chris decides to execute a swap, but they must see NO correlations when Chris does not execute the swap. And like any Bell test, a Bell inequality (such as CHSH) must be violated when Alice and Bob's respective entangled photons are measured at specific angle settings. Meanwhile, the photons sent to Alice and Bob cannot have ANY correlation initially, because they are entangled with other particles.

This is why I use GHZ and RES as a measuring stick. If the author won't tackle these head on, it's nearly a certainty that there is hand-waving occurring somewhere - although it may be hidden. If the author does tackle these experiments explicitly, at least you have a chance to agree or disagree with the line of thinking. And oftentimes, the author simply says "the math is the same so the predictions are the same" - and you know you have a true interpretation - but it actually doesn't shed much new light on things. But it may still be of benefit for some people. Anything that helps me grasp what is happening a little better is... good.

 

[martinbn]

2. Whoa! Another big statement, and yet no peer-reviewed reference supporting your statement. You are basically denying the quantum nature of entanglement. Hmmm. I'll pay you $10 (I'm a cheap bettor, but I'll give you decent odds) if you can find a classical "correlation swapping" example with the following attributes, which are demonstrated in quantum experiments such as this or this.

• a. The photons (or whatever classical objects you prefer) detected by Alice and Bob never exist/interact in a common light cone. Let's call these objects 1 and 4 to match my experimental references.
• b. 1 and 4 cannot be entangled or otherwise made identical in their initial states, because the decision to entangle them (or not) will be made in a remote (FTL distant) place by Chris. So Alice, Bob and Chris are spacelike separated at the time that 1 and 4 become entangled - or correlated, or whatever you care to call it. They are also all spacelike separated when Alice and Bob perform their chosen measurements.
• c. Alice and Bob can choose to measure either i) on any same basis (in which case we must see perfect correlation); or ii) on different bases (a la CHSH, and violating a Bell inequality). I'll be impressed if you can do this for even just case i).
• d. Chris can choose to entangle - or not - the 1 and 4 objects. The observed Alice/Bob correlations must change along with this choice. No correlation if Chris chooses not to classically correlate.

This is impossible in any classical scenario, as it should be obvious - which is why the Remote Entanglement Swapping experiments are critical to interpretation analysis. I certainly have never seen a concrete example that could even remotely (pun intended) pull this off.

I'll give this one a go. On each trial prepare 1 and 4 in any way you want. They can be spacelike and at different times and in arbitrary states. Entirely up to you. All measurements are made along the same axis. Alice measures 1 and records the result. Bob measures 4 and records the result. This is done on all trials. Then the spread sheets are sent to Chris. Now he can do one of two things. He can do nothing then the data shows no correlation whatsoever between the results of measurements on 1 and 4. Or he can go through the results trial by trial and keep only those that are opposite i.e. if Alice's is "up" and Bob's is "down" he keeps the result, same if they were the other way round, and discards all others. Then the data shows perfect anti-correlation. If the measurements are done along different axis, then Chris discards all trials that are not along the same axis and then proceeds as above. This way the data for 1 and 4 shows exactly the same as if 1 and 4 were in the usual Bell state. Conclusion the act of Chris, choosing to do one or the other thing, entangles photons 1 and 4. Which of course can be chosen so that they have never coexisted in whatever sense you want.

 

[PeterDonis]

Conclusion the act of Chris, choosing to do one or the other thing, entangles photons 1 and 4.

This is interpretation dependent. On an ensemble interpretation this viewpoint would be reasonable (although I'm not sure that even all versions of the ensemble interpretation would agree). But on an interpretation where the quantum state is taken to represent the state of individual systems, the viewpoint you describe does not work. @DrChinese, in threads on this topic, has always adopted the latter type of interpretation.

 

[martinbn]

This is interpretation dependent. On an ensemble interpretation this viewpoint would be reasonable (although I'm not sure that even all versions of the ensemble interpretation would agree). But on an interpretation where the quantum state is taken to represent the state of individual systems, the viewpoint you describe does not work. @DrChinese, in threads on this topic, has always adopted the latter type of interpretation.

I am playing the devil's advocate. I don't agree with what I wrote. But I also don't agree with some of the things @DrChinese writes about entanglement swaping. I have tried to write it the same way as he does to see if he thinks it is different and if yes why?

 

[PeterDonis]

I am playing the devil's advocate. I don't agree with what I wrote. But I also don't agree with some of the things @DrChinese writes about entanglement swaping.

We should expect disagreement on any issue that involves QM interpretations, since different interpretations say different and mutually contradictory things. There is no way to resolve such disagreements since all QM interpretations make the same experimental predictions. The guidelines for this subforum spell that out.

 

[martinbn]

We should expect disagreement on any issue that involves QM interpretations, since different interpretations say different and mutually contradictory things. There is no way to resolve such disagreements since all QM interpretations make the same experimental predictions. The guidelines for this subforum spell that out.

I disagree with the statement that Chris causes the entanglement no matter what the interpretation is. In any case i want to hear if i get the 10$ or at least a pint of lone star.

 

[DrChinese]

I'll give this one a go. On each trial prepare 1 and 4 in any way you want. They can be spacelike and at different times and in arbitrary states. Entirely up to you. All measurements are made along the same axis. Alice measures 1 and records the result. Bob measures 4 and records the result. This is done on all trials. Then the spread sheets are sent to Chris. Now he can do one of two things. He can do nothing then the data shows no correlation whatsoever between the results of measurements on 1 and 4. Or he can go through the results trial by trial and keep only those that are opposite i.e. if Alice's is "up" and Bob's is "down" he keeps the result, same if they were the other way round, and discards all others. Then the data shows perfect anti-correlation. If the measurements are done along different axis, then Chris discards all trials that are not along the same axis and then proceeds as above. This way the data for 1 and 4 shows exactly the same as if 1 and 4 were in the usual Bell state. Conclusion the act of Chris, choosing to do one or the other thing, entangles photons 1 and 4. Which of course can be chosen so that they have never coexisted in whatever sense you want.

None of this fits the requirements I presented. So you don't get the $10, but as consolation prize: I'll buy you that Lone Star anytime you are in Dallas.

And I agree that Chris keeping a spreadsheet can be considered classical.

But that is not what Chris' role is. He is independently choosing to entangle photons 1 & 4 by doing something to photons 2 & 3. And when Chris chooses to entangle, the final stats should show the perfect correlations (or anti-correlations); and when Chris chooses not to entangle, there can be no correlation. Alice, Bob and Chris are sufficiently distant that their results will be independent. They all send their independent results to some other party for summarizing. We can call that person Dave. Dave buys the beer, by the way.

For those that don't see this kind of experiment as a diehard proof of (quantum) nonlocality, seriously, how do you propose to explain it? There is clearly a dependency on the results obtained a 3 distant locations regarding photons from independent sources that have never overlapped in a common light cone.

And just to strike home a point that seems to get lost at times: in ALL cases in which there is indistinguishable overlap of photons 2 & 3 at Chris' beamsplitter, there is a swap. There are 4 possible Bell states, current technology only allows 2 (max) of these to be distinguished. But it is possible to look at the 1 & 4 photons' arrival times, and therefore determine which trials there *could* have been overlapping (of 2 & 3) at Chris' beamsplitter. But you need info from Chris to learn if that overlap was allowed - or blocked - by Chris's decision. The frequency of the 2 identifiable Bell states (all of which occur randomly) will constitute very nearly 2/4 of all possible overlaps (since half of the cases cannot be identified as to Bell state). That you only identify 2 of the 4 Bell states in no way affects the conclusion. So in those 2 identifiable Bell states, there is a four-fold coincidence (based on relative arrival times, adjusted for travel distance). One click at Alice's station (photon 1), one click at Bob's station (photon 4), and two clicks at Chris' station (2 & 3).

So @martinbn: You said you don't agree with what I say about this type of remote entanglement swapping. What specifically do you not agree with? I am describing actual experiments, there's not much to imagine. As @PeterDonis says, different interpretations tend to take different views as to whether Chris' decision is the "cause" of the swap. So I get that. Also, some disagree that anything nonlocal was transmitted, even a quantum state. Nonetheless, Chris's decision is undeniably a part of the overall experimental context in which the final quantum state of 1 & 4 is different than the initial quantum states of 1 & 4. Which were always nonlocal to each other.

 

[martinbn]

None of this fits the requirements I presented. So you don't get the $10, but as consolation prize: I'll buy you that Lone Star anytime you are in Dallas.

And I agree that Chris keeping a spreadsheet can be considered classical.

I don't understand! You wanted photons 1 and 4, which are initially unrelated in any way (arbitrary states and arbitrary far way in space and time from each other) to be entangled or not depending on what Chris does. My example does exactly that! If you disagree you need to tell me why.

But that is not what Chris' role is. He is independently choosing to entangle photons 1 & 4 by doing something to photons 2 & 3. And when Chris chooses to entangle, the final stats should show the perfect correlations (or anti-correlations); and when Chris chooses not to entangle, there can be no correlation. Alice, Bob and Chris are sufficiently distant that their results will be independent. They all send their independent results to some other party for summarizing. We can call that person Dave. Dave buys the beer, by the way.

There is no 2&3 in the task for the bet. I think I stratified all your requirements. Here they are:

• a. The photons (or whatever classical objects you prefer) detected by Alice and Bob never exist/interact in a common light cone. Let's call these objects 1 and 4 to match my experimental references.
• b. 1 and 4 cannot be entangled or otherwise made identical in their initial states, because the decision to entangle them (or not) will be made in a remote (FTL distant) place by Chris. So Alice, Bob and Chris are spacelike separated at the time that 1 and 4 become entangled - or correlated, or whatever you care to call it. They are also all spacelike separated when Alice and Bob perform their chosen measurements.
• c. Alice and Bob can choose to measure either i) on any same basis (in which case we must see perfect correlation); or ii) on different bases (a la CHSH, and violating a Bell inequality). I'll be impressed if you can do this for even just case i).
• d. Chris can choose to entangle - or not - the 1 and 4 objects. The observed Alice/Bob correlations must change along with this choice. No correlation if Chris chooses not to classically correlate.

Why do you not accept my example?

PS: I will replay to the rest of your post in a minute.

 

[martinbn]

So @martinbn: You said you don't agree with what I say about this type of remote entanglement swapping. What specifically do you not agree with? I am describing actual experiments, there's not much to imagine. As @PeterDonis says, different interpretations tend to take different views as to whether Chris' decision is the "cause" of the swap. So I get that. Also, some disagree that anything nonlocal was transmitted, even a quantum state. Nonetheless, Chris's decision is undeniably a part of the overall experimental context in which the final quantum state of 1 & 4 is different than the initial quantum states of 1 & 4. Which were always nonlocal to each other.

This was addressed by others in the other threads. But here is what I disagree with:

1. You insists on the statement that 1&4 have an entangled state at the end. But since there isn't any notion of them existing at the same time to say that they (the system consisting of the two) have any state (entangled or not) is erroneous. And this is interpretation independent.

2. You always neglect the fact that the results that show the correlations happen only on a subset of the trials. In statistical terms you could only claim that a sub ensemble is entangled. And this would be interpretation dependent. But you claim that every single pair 1&4 are entangled.

3. You always point out that the order of the measurements is irrelevant. So the set up could be that the measurements on 1&4 are done and recorded, written on paper well before Chris decides to do anything. Then you claim that his decision changes something about 1&4. How!? If I am looking at the paper with the results, will they magically change before my eyes?

 

[PeterDonis]

There is no 2&3 in the task for the bet.

They're in the papers that were referenced in the post where the bet was originally proposed.

 

[PeterDonis]

you claim that every single pair 1&4 are entangled.

No, he doesn't. He is only claiming that the 1 & 4 pairs in runs of the experiment in which Chris performs the "entanglement swap" operation on photons 2 & 3 are entangled. The experimental data for each run contains an indication of whether or not the "entanglement swap" operation was performed, so it contains sufficient data to post-select the subensemble of runs in which 1 & 4 are entangled.

The interpretation difference comes in when you state what all that means. In the interpretation @DrChinese is using, where the wave function describes each individual set of photons in each individual run, the runs in which 1 & 4 are entangled have those individual photons being entangled, and the "entanglement swap" operation done in those runs is a physical process that physically swaps the entanglement from the pairs 1&2, 3&4 to the pairs 1&4, 2&3.

In an ensemble interpretation, which is what you appear to be implicitly using, the wave function only describes an abstract ensemble of systems that are all prepared by the same process. Post-selecting a subensemble by picking out only the runs whose data contains the "entanglement swap" indicator amounts to including that indicator being present in your definition of the preparation process. On this interpretation, the wave function says nothing about individual photons or combinations of photons in individual runs; it only allows you to make predictions about the statistics of the ensemble, which should be approximated reasonably well by the statistics from your actual experimental data if the number of runs is sufficiently large. In particular, this interpretation makes no claim that the "entanglement swap" operation has to do something physical to each individual set of photons in each individual run; it only claims that that operation, being part of the preparation process, affects the statistics of the ensemble you end up selecting.

 

[PeterDonis]

the order of the measurements is irrelevant.

Yes. See further comments below.

So the set up could be that the measurements on 1&4 are done and recorded, written on paper well before Chris decides to do anything. Then you claim that his decision changes something about 1&4. How!?

Obviously this kind of interpretation requires any "change" involved to not respect the usual rules of causality. Either your description of which way the causality goes, i.e., which event is the "cause" and which is the "effect", has to change depending on the order (so if 1 & 4 are measured first, then it is their measurements that cause whatever happens at 2 & 3 to happen), or you have to expand your definition of "causality" to allow events to be causally connected even if those events commute, i.e., what happens at them is independent of the order in which they occur, so that it is impossible to pick out which is the "cause" and which is the "effect".

 

[PAllen]

Yes. See further comments below.


Obviously this kind of interpretation requires any "change" involved to not respect the usual rules of causality. Either your description of which way the causality goes, i.e., which event is the "cause" and which is the "effect", has to change depending on the order (so if 1 & 4 are measured first, then it is their measurements that cause whatever happens at 2 & 3 to happen), or you have to expand your definition of "causality" to allow events to be causally connected even if those events commute, i.e., what happens at them is independent of the order in which they occur, so that it is impossible to pick out which is the "cause" and which is the "effect".

Suppose no action on 2 and 3 is done until they are in causal future (SR sense) of both measurements being made on 1 and 4? Saying time ordering doesn't matter of spacelike separated events is rather like saying time ordering doesn't matter when time ordering is meaningless (per SR).

 

[DrChinese]

I don't understand! You wanted photons 1 and 4, which are initially unrelated in any way (arbitrary states and arbitrary far way in space and time from each other) to be entangled or not depending on what Chris does. My example does exactly that! If you disagree you need to tell me why.

There is no 2&3 in the task for the bet. .

Sorry, I thought you were following the remote entanglement swapping experiment I referenced:

High-fidelity entanglement swapping with fully independent sources
"Entanglement swapping allows to establish entanglement between independent particles that never interacted nor share any common past. This feature makes it an integral constituent of quantum repeaters. Here, we demonstrate entanglement swapping with time-synchronized independent sources with a fidelity high enough to violate a Clauser-Horne-Shimony-Holt inequality by more than four standard deviations."

See Figure 1, which shows the labeling of the photons by number. Alice sees 1, Bob sees 4, Chris chooses to execute (or not) the remote swap by overlapping 2 & 3 and detecting a Bell state (which also requires a 4-fold coincidence). In this particular setup, I assigned the names Alice/Bob/Chris just for clarity/discussion purposes. Also in this particular setup, the Alice and Bob locations are far enough separated that photons 1 & 4 do not share any backward light cone. However, as I read the specs, the Chris location is not specifically distant from Alice & Bob. (Other similar experiments I have referenced make this clear.)

So Chris can choose to execute the entanglement of 1 & 4 together by choosing (or not) to allow photons 2 & 3 to overlap. That is Chris's role. Alice, Bob, and Chris can be arbitrarily far apart, the order of their detections can be made as desired, and they can be made as close to simultaneous as desired - without changing the observed results. Those being: observation of HOM dip, and violation of CHSH inequalities. Note that violation of CHSH (in agreement with the predictions of QM) always implies that perfect correlations will be observed as well.

So yeah, you gotta have Chris doing something to 2 & 3 to get entanglement between 1 & 4. And Chris is too far away for a classical signal to arrive at Alice or Bob's stations before 1 and 4 arrive. So the results occur in mutually remote (in terms of light speed distance) locations.

 

[PeterDonis]

Suppose no action on 2 and 3 is done until they are in causal future (SR sense) of both measurements being made on 1 and 4? Saying time ordering doesn't matter of spacelike separated events is rather like saying time ordering doesn't matter when time ordering is meaningless (per SR).

In this case the measurements commute regardless of whether they are timelike, spacelike, or null separated. Their commutation is unrelated to the QFT condition that spacelike separated measurements must always commute.

That, in itself, already makes it clear that whatever kind of "connection" there is between the measurement events, it can't be an ordinary causal connection of the kind we are familiar with.

 

[PAllen]

In this case the measurements commute regardless of whether they are timelike, spacelike, or null separated. Their commutation is unrelated to the QFT condition that spacelike separated measurements must always commute.

That, in itself, already makes it clear that whatever kind of "connection" there is between the measurement events, it can't be an ordinary causal connection of the kind we are familiar with.

Suppose an action on 2 and 3 is made in the causal future of all of the following:

Measurements are taken on 1 and 4 AND these results are communicate to e.g. experimenter at 5. In the causal future of all of this, decisions are made about about actions taken on 2 and 3, by some different experimenter.

 

[DrChinese]

This was addressed by others in the other threads. But here is what I disagree with:

1. You insists on the statement that 1&4 have an entangled state at the end. But since there isn't any notion of them existing at the same time to say that they (the system consisting of the two) have any state (entangled or not) is erroneous. And this is interpretation independent.

2. You always neglect the fact that the results that show the correlations happen only on a subset of the trials. In statistical terms you could only claim that a sub ensemble is entangled. And this would be interpretation dependent. But you claim that every single pair 1&4 are entangled.

3. You always point out that the order of the measurements is irrelevant. So the set up could be that the measurements on 1&4 are done and recorded, written on paper well before Chris decides to do anything. Then you claim that his decision changes something about 1&4. How!? If I am looking at the paper with the results, will they magically change before my eyes?

1. There is no requirement that 1 & 4 ever co-exist, but in the references I am supplying they do. They just don't co-exist in a common backward light cone because they were never close to each other. Upon creation, they head out in opposite directions. As far as I know, there is no interpretation that disputes this point.

2. There is no subensemble that is neglected. All cases in which a 4 fold coincidence occurs are considered.

3. My "claim" on this is quite simple. This experiment has already been performed (although by the same team, it is a different paper per below). It is 100% consistent with the predictions of QM, which again AFAIK no interpretation disputes. Different interpretations do explain it differently though. So explain it as you will, but denying experimental results is never a good look. The bet is to attempt to explain these results using a classical explanation with local realism. In fact, even an acceptable classical local explanation would be enough for me to lose the bet.

Delayed-choice gedanken experiments and their realizations (2016)

See particularly section 5.A. starting on page 22. Page 23: "The diagram of the temporal order of the relevant events is shown in Fig. 33. For each successful run (a 4-fold coincidence count), both Victor’s [Victor is my "Chris"] measurement event and his choice were in the time-like future of Alice’s and Bob’s measurements."

 

[DrChinese]

Suppose no action on 2 and 3 is done until they are in causal future (SR sense) of both measurements being made on 1 and 4? Saying time ordering doesn't matter of spacelike separated events is rather like saying time ordering doesn't matter when time ordering is meaningless (per SR).

This is more fully explained in my post #80, but this version has been explicitly performed years ago. Time ordering (with a * for a specific single particle exception per @PeterDonis) doesn't change the observable results. (Note that I didn't say it doesn't change the results, I just say that there is no observable change.)

Delayed-choice gedanken experiments and their realizations (2016)

See particularly section 5.A. starting on page 22. Page 23: "The diagram of the temporal order of the relevant events is shown in Fig. 33. For each successful run (a 4-fold coincidence count), both Victor’s [Victor is my "Chris"] measurement event and his choice were in the time-like future of Alice’s and Bob’s measurements."

 

[PeterDonis]

Suppose an action on 2 and 3 is made in the causal future of all of the following:

Measurements are taken on 1 and 4 AND these results are communicate to e.g. experimenter at 5. In the causal future of all of this, decisions are made about about actions taken on 2 and 3, by some different experimenter.

This wouldn't change anything. Once a measurement is made, communicating the result to others doesn't change anything further.

 

[PAllen]

This wouldn't change anything. Once a measurement is made, communicating the result to others doesn't change anything further

I am confused here. Suppose all measurements at 1 and 4, when compared, imply 2 and 3 were not made to interact. None the less, in the causal future of all this, the decision is made to have 2 and 3 interact.

 

[DrChinese]

Suppose an action on 2 and 3 is made in the causal future of all of the following:

Measurements are taken on 1 and 4 AND these results are communicate to e.g. experimenter at 5. In the causal future of all of this, decisions are made about about actions taken on 2 and 3, by some different experimenter.

Adding experimenter 5 changes nothing.

I am confused here. Suppose all measurements at 1 and 4, when compared, imply 2 and 3 were not made to interact. None the less, in the causal future of all this, the decision is made to have 2 and 3 interact.

I think what you are missing is that the 1 & 4 pairs are randomly thrown into 1 of 4 Bell states in very nearly equal measure. 2 of these can be identified, because they result in 4 fold coincidences*. The two identifiable states are ψ+ and ψ-. One yields correlation, the other yields anti-correlation. So until you see the results from Chris (a/k/a Victor), you won't be able to confirm the type of entanglement of 1 & 4.


*In the 2 that cannot be identified, there are only 3 fold coincidences. That happens because the 2 & 3 photons end up in the same detector so close together in time that only a single detection is registered. Current technology for photon avalanche detectors requires too large a time interval between trigger events to yield 2 detections in such cases. This 3 fold coincidence happens 50% of the time, while the other 50% result in 4 fold coincidences.

 

[PeterDonis]

Suppose all measurements at 1 and 4, when compared, imply 2 and 3 were not made to interact. None the less, in the causal future of all this, the decision is made to have 2 and 3 interact.

The "decision" to have 2 & 3 interact is not fully under the experimenter's control. The 2 & 3 photons have to arrive at the beam splitter that could execute the entanglement swap within a certain time window. If they don't, the experimenter cannot make 2 & 3 interact no matter what.

The QM prediction is then that, in any case where 1 & 4's measurement results indicate no entanglement, the 2 & 3 photons will not arrive within the required time window and will be unable to interact no matter what the experimenter does.

I believe this alternative has actually been realized in at least one of the experiments described in the papers @DrChinese has referenced, and the QM prediction is confirmed.

Of course it is hard to imagine how this would work; welcome to how counterintuitive QM is.

 

[DrChinese]

1. The "decision" to have 2 & 3 interact is not fully under the experimenter's control. The 2 & 3 photons have to arrive at the beam splitter that could execute the entanglement swap within a certain time window. If they don't, the experimenter cannot make 2 & 3 interact no matter what.

2. The QM prediction is then that, in any case where 1 & 4's measurement results indicate no entanglement, the 2 & 3 photons will not arrive within the required time window and will be unable to interact no matter what the experimenter does.

3. I believe this alternative has actually been realized in at least one of the experiments described in the papers @DrChinese has referenced, and the QM prediction is confirmed.

Of course it is hard to imagine how this would work; welcome to how counterintuitive QM is.

Just clarifying a couple of things, everything you say is correct but might not be intuitively obvious to some.

1. The experimenter (Chris in my example) can definitely prevent 1 & 4 from becoming entangled. This is accomplished by making the 2 & 3 photons distinguishable. This is accomplished by delaying the 2 photon but not the 3 photon (or vice versa).

And of course there are cases in which the 2 & 3 photons naturally do NOT arrive within the same time window, because they are created at random times by independent sources. (I will explain this more in a separate post for those interested.)


2. If you were looking only at 1 & 4 coincidence outcomes at the same angles (perfect correlation is ψ+, perfect anti-correlation is ψ-): You could rule out one - but not the other - of whether there could be entanglement or not. If you got HH> for example, you certainly cannot have ψ- as a result for photons 2 & 3. If you instead got VH>, you certainly cannot have ψ+ as a result for photons 2 & 3.


3. Yes, the QM prediction is always confirmed! At least so far.

At least one specific experiment the decision to interact (swap) or not was specifically randomized and analyzed. The results showed no correlation when the entangled swap was prevented via an intentionally introduced time delay.

 

[PAllen]

Just clarifying a couple of things, everything you say is correct but might not be intuitively obvious to some.

1. The experimenter (Chris in my example) can definitely prevent 1 & 4 from becoming entangled. This is accomplished by making the 2 & 3 photons distinguishable. This is accomplished by delaying the 2 photon but not the 3 photon (or vice versa).

And of course there are cases in which the 2 & 3 photons naturally do NOT arrive within the same time window, because they are created at random times by independent sources. (I will explain this more in a separate post for those interested.)


2. If you were looking only at 1 & 4 coincidence outcomes at the same angles (perfect correlation is ψ+, perfect anti-correlation is ψ-): You could rule out one - but not the other - of whether there could be entanglement or not. If you got HH> for example, you certainly cannot have ψ- as a result for photons 2 & 3. If you instead got VH>, you certainly cannot have ψ+ as a result for photons 2 & 3.


3. Yes, the QM prediction is always confirmed! At least so far.

At least one specific experiment the decision to interact (swap) or not was specifically randomized and analyzed. The results showed no correlation when the entangled swap was prevented via an intentionally introduced time delay.

So if you arrange the experiment so 2 and 3 never get close to each other until the causal future of measurements of both 1 and 4, then no action on 2 and 3 will make 1 and 4 entangled?

 

[PeterDonis]

The experimenter (Chris in my example) can definitely prevent 1 & 4 from becoming entangled.

Yes, agreed. He just can't make them entangled with 100% certainly, because he does not control whether or not photons 2 & 3 arrive at the BSM device within the required time window.

Also, a scenario in which, whenever Chris is informed that the 1 & 4 measurement results do not show entanglement, he explicitly prevents the swap from occurring, would not, I assume, be the kind of thing we want to discuss. I would assume we want to discuss the case where Chris never explicitly takes action to prevent the swap, so that the only factors involved are the arrival times of photons 2 & 3 at the BSM and whatever other "natural" factors contribute to whether or not a swap occurs.

 

[PeterDonis]

So if you arrange the experiment so 2 and 3 never get close to each other until the causal future of measurements of both 1 and 4, then no action on 2 and 3 will make 1 and 4 entangled?

No. A swap happening at the BSM when 2 & 3 arrive can be in the causal future of the 1 & 4 measurements. It's just that, in any case where the swap does happen at 2 & 3, it must also have been the case that the 1 & 4 measurement results were consistent with entanglement.

 

[PAllen]

No. A swap happening at the BSM when 2 & 3 arrive can be in the causal future of the 1 & 4 measurements. It's just that, in any case where the swap does happen at 2 & 3, it must also have been the case that the 1 & 4 measurement results were consistent with entanglement.

But this would mean that 1 and 4 can communicate their results to where and when 2 and 3 arrive (that's what causal future means), at which point someone at 2&3 future interaction point is free to perform an action supposedly inconsistent with this result.