I A new realistic stochastic interpretation of Quantum Mechanics

[DrChinese]

For those of you interested in some of the more specific ideas behind the general Entanglement Swapping protocol, I will run through a rough example.

1. Keep in mind that the time/distance traveled can be relatively lengthened or shortened by adding fiber to one part of the overall setup or the other.

a) So the independent sources I and II can be kilometers apart.
b) Alice (observing photon 1 from PDC source I) and Bob (observing photon 4 from PDC source II) can be kilometers apart.
c) Each of those can be kilometers away from their source.
d) And Chris can be located such that Chris' decision to enable entanglement (via the swap mechanism, called a Bell State Measurement or BSM) can occur before or after the detectors of Alice and Bob click. Let's keep it simple, and have the BSM/swap positioned to occur before Alice and Bob see their clicks.


2. What needs to be synchronized?

a) The laser sources driving their respective PDC crystals must be phase locked.
b) The difference in the photon travel time from source I for photon 1 as compared to the photon travel time from source II for photon 4 should be known. This is to match an Alice click with a Bob click, indicating 2 of the 4 fold coincidence clicks we are looking for. That time difference is used to compensate for relative travel distance. The 1 & 4 photons don't need to actually arrive at their respective detectors at exactly the same time if this difference is known.
c) The difference in the photon travel time from source I for photon 1 as compared to the photon travel time from source I for photon 2 should be known. This allows for 3 fold coincidence detection.
d) The difference in the clicks of Chris's detectors (there are 4) must be known. If these do not occur within a sufficiently small time window, there can be no swap.


3. A single PDC source might randomly emit entangled pairs at the rate of perhaps 100,000 per second, or about 1 every 10,000 nanoseconds. We need the 2 & 3 photons - coming at random times from the two phase locked sources I and II to arrive within a narrow time window, such that the arrival time of its initial entangled partner (1 or 4) would not give us a clue as to which is which (they must be indistinguishable). Let's pretend that window is 5 nanoseconds. That means that on the average, sources I and II emit pairs that will ultimately overlap (at random intervals) within our desired time window of about 1 in 2,000 of each of those 100,000. That would be about 5,000 per second (this number is not particularly accurate, we are just for this example).

a) We would expect 2 fold coincidences (for 1 & 2, or 3 & 4) of about 200,000 per second, as most of the time the source I and source II pairs would not fire close enough together to be within the 5 nanosecond window.
b) We would expect 3 or 4 fold coincidences - for 1 & (2 or 3) & 4 - of about 5,000 per second, whenever the the source I and source II pairs do fire close enough together to be within the 5 nanosecond window.
c) We would expect 4 fold coincidences (for 1 & 2 & 3 & 4) of about 2,500 per second, whenever the the source I and source II pairs do fire close enough together to be within the 5 nanosecond window... AND the Bell state is identifiable. This occurs half of the time of the b) group above.


4. Keep in mind that ALL of the identifiable c) cases are considered in our results. Half of the b) cases are not used because the Bell state cannot be identified. That does NOT mean those cases were not entangled - all 5,000 per second are entangled. But half of those are φ+ or φ- entangled, and we can't distinguish between those - because both the 2 & 3 photons end up in the same detector (yielding only 1 click for the two photons). Only the ψ+ and ψ- can be identified (because the 2 & 3 photons end up in different detectors, yielding 2 clicks).

It is essential that the clicks from the 2 & 3 photons cannot provide any information as to which click is photon 2, versus which click is photon 3. That is why close arrival time is needed. And when I say close arrival time - as indicated by detector clicks: I really mean that photon 2 and photon 3 travel through the beam splitter portion of the BSM swapping mechanism close enough in time that they are allowed to interact. Because if photon 3 is delayed sufficiently such that there is no close overlap, photons 2 & 3 become distinguishable. That would be evident by one click alone arriving late, identifying photon 3. Then no swap will occur.

I hope these details will help some readers understand exactly what these experiments are demonstrating. Everything presented is orthodox QM.

 

[DrChinese]

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?

If Chris allows the 2 & 3 swap to occur, 1 & 4 will be entangled. This is true regardless of when Chris executes the swap. Distance does not matter, and timing of Chris' decision - future relative to 1 & 4 or past relative to the 1 & 4 measurements - does not matter.

So you call it the causal future, but I might simply call it the future. I would also say that the entire context of the experimental setup should be considered, and that there is no classical causality to demonstrate or consider. However, that statement is interpretation dependent.

 

[DrChinese]

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.

They cannot communicate anything to the future (assuming you interpret it that way) other than a random bit of information. The outcomes Alice and Bob see now can be consistent with 2 of 4 Bell states, only one of which can be identified. In other words: Yes, I can see from the time stamps of the Alice and Bob detections whether or not the 2 & 3 photons would have overlapped in Chris's beam splitter. And I can predict what will happen IF they are allowed to overlap. But there is no opportunity here to send a message to Chris in the future - except by a classical signal.

 

[PAllen]

They cannot communicate anything to the future (assuming you interpret it that way) other than a random bit of information. The outcomes Alice and Bob see now can be consistent with 2 of 4 Bell states, only one of which can be identified. In other words: Yes, I can see from the time stamps of the Alice and Bob detections whether or not the 2 & 3 photons would have overlapped in Chris's beam splitter. And I can predict what will happen IF they are allowed to overlap. But there is no opportunity here to send a message to Chris in the future - except by a classical signal.

I think we are not understanding each other. It is claimed that photon 2 and 3 can arrive somewhere such that an action can be taken to make them interact or not, and that this arrival event is in the causal future of measurements at 1 and 4. Causal future means people at 1 and 4 can send a classical signal to experimenter where 2 and 3 arrive. This signal can contain clock readings and measurement results. Experimenter at 2 and 3 is then free to perform actions that should be inconsistent with those measurements.

 

[DrChinese]

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, ...

Agreed, except for this point. There can be no correlation whatsoever between 1 & 4 when Chris chooses to prevent the swap. And that is what is observed in this situation: no swap, no correlation.

Keep in mind that when the 2 & 3 photons do NOT overlap, they still exhibit any prior/previously determined attributes they might have had. If, for example, you follow a realistic type interpretation. If you did, the problem you have is that you are saying that the overlap does NOT "cause" (or otherwise contribute) any physical change in photons 1 & 4. In other words: If you believe the 2 & 3 overlap is simply revealing information that pre-existed, then 2 & 3 indistinguishability is not really a requirement for 1 & 4 entanglement, is it? That means the 2 & 3 clicks simply reveal a pre-existing Bell state.

Again, all of this is interpretation to varying extent. But the entire purpose of this thread is (as far as I am concerned): If someone is selling a new interpretation, does it pass the sniff test? My sniffing, of course, being these newer* state of the art experiments.


*Newer meaning: last 25 years... LOL.

 

[DrChinese]

I think we are not understanding each other. It is claimed that photon 2 and 3 can arrive somewhere such that an action can be taken to make them interact or not, and that this arrival event is in the causal future of measurements at 1 and 4. Causal future means people at 1 and 4 can send a classical signal to experimenter where 2 and 3 arrive. This signal can contain clock readings and measurement results. Experimenter at 2 and 3 is then free to perform actions that should be inconsistent with those measurements.

That's simply not true. These experiments are not ideas: they have been performed. There is no opportunity to send a signal to the future any more than there is an opportunity to send a signal faster than light.

The problem is the word "causal". Yes, perhaps we are changing the future - or the future is changing the past. I certainly don't know anything more than what the experiments (which match theory) tell us. But all that anyone ever sees as a result is a random outcome. So not much to get out of random bits, which is why the word "causal" is a problem. If I can't make a specific outcome occur, how do you attach the word "cause" to an action? Or send a signal anywhere?

 

[PAllen]

That's simply not true. These experiments are not ideas: they have been performed. There is no opportunity to send a signal to the future any more than there is an opportunity to send a signal faster than light.

Nonsense. All signals travel in space and time in the future direction. What you can't do is send a signal to anything other than the causal future. Claiming the arrival event of 2 and 3 is in the causal future of 1 and 4 MEANS that 1 and 4 measurement results can be sent via a classical signal to this event. If they cannot, then the 2 and 3 arrival point is at spacelike separation not timelike, and is NOT in the causal future of 1 and 4.

The problem is the word "causal". Yes, perhaps we are changing the future - or the future is changing the past. I certainly don't know anything more than what the experiments (which match theory) tell us. But all that anyone ever sees as a result is a random outcome. So not much to get out of random bits, which is why the word "causal" is a problem. If I can't make a specific outcome occur, how do you attach the word "cause" to an action?

Causal in SR/GR has a rigid, fixed meaning everyone accepts.

 

[PeterDonis]

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.

No, they can't, because there is no such action that anyone is free to perform.

There is no single 1 & 4 measurement result that requires entanglement (entanglement can only be shown by appropriate statistics on a sufficient number of runs to confirm the required correlations), so it is impossible for the experimenter at 2 & 3 to freely choose to prevent entanglement and be inconsistent with the 1 & 4 results. The experimenter will simply find that the subensemble of runs for which they freely chose to prevent 1 & 4 entanglement will in fact show no 1 & 4 entanglement.

For the case where a single 1 & 4 measurement result rules out entanglement (i.e., a combination of results that cannot occur in an entangled state), the experimenter cannot freely choose to force the entanglement swap to occur at 2 & 3, because they do not control all of the necessary factors. They can only freely choose to not do anything to prevent the entanglemetn swap. They cannot make it occur. And, again, they will find that the subensemble of runs for which the 1 & 4 measurement results rule out entanglement, will also lack the indicator at the 2 & 3 operation that would indicate that a swap occurred--i.e., even though the experimenter did not prevent the swap from occurring, it nevertheless did not occur.

 

[lodbrok]

There is no opportunity to send a signal to the future any more than there is an opportunity to send a signal faster than light.

We are so out of topic at this point but aren't all signals sent to the future by definition in general relativity?

 

[DrChinese]

We are so out of topic at this point but aren't all signals sent to the future by definition in general relativity?

Sure, GR, QM, all signals go from past to future alone (at a speed not in excess of c). No one is saying otherwise AFAIK.

What I am saying is: A decision by Chris can entangle - or not - 2 distant photons. That decision can be made at any time (or distance) relative to the measurements of those photons by Alice and Bob. The photons they observe need not have ever existed in a common backward light cone. This much is pretty much standard and supported by experiment.

What is interpretation dependent is whether Chris' decision is the cause of the entanglement in those cases where it is run in the Delayed Choice version. In that case, some might say Chris' future decision caused the entanglement of the previously observed photons. Were that true, you might interpret that as retrocausality.

Of course, there are dozens of Delayed Choice experiments that all have this similar feature. What makes this unique is that the entangled photons were never local to each other, and need never have been local to Chris.

 

[DrChinese]

1. Nonsense. All signals travel in space and time in the future direction. What you can't do is send a signal to anything other than the causal future.

2. Claiming the arrival event of 2 and 3 is in the causal future of 1 and 4 MEANS that 1 and 4 measurement results can be sent via a classical signal to this event. If they cannot, then the 2 and 3 arrival point is at spacelike separation not timelike, and is NOT in the causal future of 1 and 4.

1. Yes, all signals travel in the future direction at speeds not to exceed c. I didn't make that sufficiently clear, simply because I am not saying any signal can be sent FTL (and I have never implied otherwise).

2. No it does not. There is absolutely no requirement that Chris be close (enough for a classical signal) to Alice and/or Bob when the swap occurs. That's what I keep trying to tell you. This experiment has already been performed. Alice, Bob and Chris can be arbitrarily far apart, and the ordering of the events can be arbitrary without any observable difference. This is exactly as predicted by QM.

Chris can execute the swap in the future of Alice and Bob, while also being far enough away that no signals can be exchanged between any of the 3 before their observations are completed. At the same time, the photons of Alice and Bob need not have ever co-existed in a common light cone.

 

[PeterDonis]

A decision by Chris can entangle - or not - 2 distant photons.

As I have already noted, though, Chris does not have control over all of the relevant factors involved, so his freedom to make a "decision" is limited. Chris can choose to prevent the entanglement swap with certainty, by delaying one of the photons; but Chris cannot choose to make the entanglement swap happen with certainty, because Chris cannot guarantee that, if he does not delay either of the photons, they will both arrive within the required time window and cause a swap. The best Chris can do, if he wants a swap to happen, is to not choose to prevent it.

 

[DrChinese]

As I have already noted, though, Chris does not have control over all of the relevant factors involved, so his freedom to make a "decision" is limited. Chris can choose to prevent the entanglement swap with certainty, by delaying one of the photons; but Chris cannot choose to make the entanglement swap happen with certainty, because Chris cannot guarantee that, if he does not delay either of the photons, they will both arrive within the required time window and cause a swap. The best Chris can do, if he wants a swap to happen, is to not choose to prevent it.

Certainly this is true. Chris could just turn off his station too. So we agree.

But there is a nuance here I’m trying to make. With a specific delay, Chris can insure there is no swap but also could affirmatively say a swap would have occurred without that delay. IFF the timing aligned otherwise. I.e. 3 fold coincidence with the 4th running just behind.

There is another odd twist. Adding the delay does not actually reduce the total number of swaps! It changes the line-up of timings such that differently timed pairs match up. For a swap to occur, there must be overlap in Chris’ beam splitter. Even if there is extra fiber added, the number of overlapping 2 & 3 photons stays the same, on average. In such case, the 4 fold timing would look a bit different, but a swap would occur.

This is easiest to see if you imagine that a “normal” 4 fold detection has all 4 photons traveling the same distance (within experimental accuracy). All 4 time stamps would be within the same coincidence window without adjustment. The 1 &2 photons are therefore created nearly simultaneously as the 3 & 4 photons. But each source creates pairs randomly and independently. The swap is strictly based on the indistinguishable timing of the overlap of 2 & 3, nothing else.

 

[PeterDonis]

With a specific delay, Chris can insure there is no swap but also could affirmatively say a swap would have occurred without that delay.

Is that true? As I understand it, even if there is no artificially imposed delay, it is still possible that the photons do not arrive within the required time window to cause a swap; that is not under the experimenter's control. So there is no way to affirmatively say that a swap must occur if there is no artificially imposed delay.

 

[PeterDonis]

Adding the delay does not actually reduce the total number of swaps! It changes the line-up of timings such that differently timed pairs match up.

Wouldn't this only be true for some very precisely chosen values of the delay timing?

 

[DrChinese]

1. Is that true? As I understand it, even if there is no artificially imposed delay, it is still possible that the photons do not arrive within the required time window to cause a swap; that is not under the experimenter's control. So there is no way to affirmatively say that a swap must occur if there is no artificially imposed delay.

1. If they would have arrived within the BSM's time window without the delay, there would affirmatively be a swap. Suppose as an example, we had the following timings (not realistic) with exactly equal path lengths (also not particularly realistic). Assume no delay added to the photon 3 path unless specified. Check out especially a. versus d.

a. 1 arrives at .200 ms; 2 & 3 (indistinguishably) arrive at .200 ms (i.e. 2 clicks); 4 arrives at .200 ms. A swap occurs. The 1 & 4 times are the same.

b. 1 arrives at .200 ms; 2 arrives at .200 ms; 3 arrives at .400ms; 4 arrives at .400 ms. No swap occurs since 2 & 3 are distinguishable. This is the most common case that Chris sees for 2 & 3, because the creation times for 2 & 3 aren't nearly close enough together. This b. variation might occur 1000 times more often than a.

c. 1 arrives at .200 ms; 2 arrives at .200 ms; 3 arrives at .201ms; 4 arrives at .201 ms. No swap occurs since 2 & 3 are distinguishable even though the difference in arrival times is small. The creation times for 2 & 3 aren't quite close enough together.

Now Chris adds a .001 ms delay to the photon 3 path, sufficient to insure no swap occurs in case d.

d. 1 arrives at .200 ms; 2 arrives at .200 ms; 3 arrives at .201ms (.200 + .001 delay); 4 arrives at .200 ms. No swap occurs since 2 & 3 are distinguishable even though they are close. But note that the 1 & 4 times are the same! That means without the .001 delay of photon 3, there affirmatively would have been a swap. Because there would have been overlap, and proper overlap always leads to a swap.

e. 1 arrives at .201 ms; 2 & 3 (indistinguishably) arrive at .201 ms; 4 arrives at .200 ms. A swap occurs. Notice that the 1 & 4 photons traveled the same length as always, but their arrival times were different. No problem, because them arriving simultaneously is not a requirement. This counts as a 4 fold coincidence.

2. Wouldn't this only be true for some very precisely chosen values of the delay timing?

2. Not at all! The precision timing is the overlap at Chris' beamsplitter (photons 2 & 3). It doesn't matter at all when photon 2 was created relative to photon 3. And the overlap is simply randomly occurring, with the majority of Chris' clicks being a lone 2 or a lone 3 - easily distinguished because there are only 2 of 4 total possible clicks within the time window. When Chris does get 2 clicks within the small time window, the next step will be to associate the click that Alice gets with Chris' double click. Ditto for Bob. Then you have the 4 fold results. From the reference below: "In the BSM, it is critical that the signal photons sent by Alice and Bob arrive at the 50:50 beam splitter (BS) simultaneously."

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Below is from Field test of entanglement swapping over 100-km optical fiber with independent 1-GHz-clock sequential time-bin entangled photon-pair sources
It's just another permutation of these remote setups whereby Alice, Bob and Chris (here named Charlie) are all distant from each other when their respective measurements are performed. Here Alice is located next to source I and Bob is near source II but are delayed by the addition of fiber, so technically the photons each observe are in each others' light cones. Nonetheless, you can see that the positioning (and distances) are arbitrary; Alice, Bob and Charlie (my Chris) can be located further away from each other simply by placing them physically further away from each other with less coiled fiber.

 

[PeterDonis]

If they would have arrived within the BSM's time window without the delay, there would affirmatively be a swap.

But whether they arrive in the time window is not completely controllable by the experimenter, correct? The experimenter can force them not to by imposing a delay, but if the experimenter doesn't do that, it's still not guaranteed that they will arrive within the time window, as I understand it.

The reason I keep harping on this is that, if it is possible for the experimenter (Chris) to guarantee that a swap does happen, then we have the problem @PAllen brought up earlier: in a setup where the 1 & 4 measurement results are in the past light cone of Chris making the decision of what, if anything, to do to photons 2 & 3 before they arrive at the BSM, Chris could wait until he sees a pair of 1 & 4 results that are inconsistent with entanglement (for example, a combination of results that is impossible in the entangled state), and then force a swap to happen at the BSM--which would contradict the predictions of QM, since QM predicts that if the 1 & 4 results show no entanglement, a swap cannot happen. The only way to avoid this contradiction is to not have Chris be able to force a swap to happen--i.e., to have at least some factors involved in determining whether a swap happens be out of Chris's control.

 

[PAllen]

But whether they arrive in the time window is not completely controllable by the experimenter, correct? The experimenter can force them not to by imposing a delay, but if the experimenter doesn't do that, it's still not guaranteed that they will arrive within the time window, as I understand it.

The reason I keep harping on this is that, if it is possible for the experimenter (Chris) to guarantee that a swap does happen, then we have the problem @PeroK brought up earlier: in a setup where the 1 & 4 measurement results are in the past light cone of Chris making the decision of what, if anything, to do to photons 2 & 3 before they arrive at the BSM, Chris could wait until he sees a pair of 1 & 4 results that are inconsistent with entanglement (for example, a combination of results that is impossible in the entangled state), and then force a swap to happen at the BSM--which would contradict the predictions of QM, since QM predicts that if the 1 & 4 results show no entanglement, a swap cannot happen. The only way to avoid this contradiction is to not have Chris be able to force a swap to happen--i.e., to have at least some factors involved in determining whether a swap happens be out of Chris's control.

That is exactly the point I was raising as well. Maybe you meant me, not @PeroK ?

 

[PeroK]

That is exactly the point I was raising as well. Maybe you meant me, not @PeroK ?

It must be. I shot my bolt as far as this thread is concerned many posts ago.

 

[PeterDonis]

That is exactly the point I was raising as well. Maybe you meant me, not @PeroK ?

Oops, yes, fixed now.

 

[DrChinese]

But whether they arrive in the time window is not completely controllable by the experimenter, correct? The experimenter can force them not to by imposing a delay, but if the experimenter doesn't do that, it's still not guaranteed that they will arrive within the time window, as I understand it.

The reason I keep harping on this is that, if it is possible for the experimenter (Chris) to guarantee that a swap does happen, then we have the problem @PeroK brought up earlier: in a setup where the 1 & 4 measurement results are in the past light cone of Chris making the decision of what, if anything, to do to photons 2 & 3 before they arrive at the BSM, Chris could wait until he sees a pair of 1 & 4 results that are inconsistent with entanglement (for example, a combination of results that is impossible in the entangled state), and then force a swap to happen at the BSM--which would contradict the predictions of QM, since QM predicts that if the 1 & 4 results show no entanglement, a swap cannot happen. The only way to avoid this contradiction is to not have Chris be able to force a swap to happen--i.e., to have at least some factors involved in determining whether a swap happens be out of Chris's control.

1. You are correct in that the experimenter has no control over which 2 & 3 pairs will arrive at the BSM such that they overlap indistinguishably - that's strictly random. When the 1 & 4 photons arrive is of course related to the 2 & 3 arrival time(s) at the BSM. But... there aren't any 1 & 4 results that are inconsistent with an entangled state - at least not without considering the 2 & 3 results.

Remember that when it comes to timing and time windows, everything must be adjusted/normalized relative to everything else. And likewise, the 2 independent sources and all observers can be located as close or as far apart relative to each other as desired. No combination of locations/distance (ideal case) changes the observed outcomes.

If you didn't follow the permutations I presented in post #106 (lol), then consider this one instead. All of the sources, detectors and observers are in the same room. The distance to the 1 & 4 detectors from their respective sources is exactly the same. The distance to the 2 & 3 BSM (beamsplitter portion) is the same, plus there is an extra 20 km of fiber rolled up on the floor that the 2 & 3 photons must travel through. The distance from the beamsplitter to the BSM detectors is exactly equal. So Alice and Bob will see their mutual results together, a little before Chris decides to add a bit of delay (or not) to the photon 3 line.

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2. We only consider cases where Alice (photon 1) and Bob (photon 4) both see simultaneous clicks (defined as being within the specified time window, of course). As mentioned, this is simply something that occurs randomly. The basic scenario is as follows with Chris adding no delay, and ideal lossless conditions (and very fast observers LOL):

a) Every outcome has photons 1 & 4 entangled. Every, single one. If 1 & 4 arrive at their detectors simultaneously, so will 2 & 3. That's because 1 & 4 traveled the same distance relative to each other, and so did 2 & 3 relative to each other. So they will all appear in the same adjusted time window.


b) If Alice and Bob are checking photons 1 & 4 at the same angle settings: They can tell Chris the information needed (match or no match) to predict - with certainty - which of 2 Bell states will occur at the BSM. The BSM being in the future of what Alice and Bob just witnessed. If Alice and Bob matched (at same angles), then the BSM can only yield 2 possible outcomes: ψ+ or φ+ entanglement. If Chris got 2 clicks, it's ψ+; if only one click, it's φ+. Which of those occurs is simply random. The total number of clicks is 4 if ψ+/-; or 3 if φ+/-.

Nothing Alice or Bob do can any way affect whether the outcome is ψ+ or φ+, so there is no way to send a signal from Alice and Bob to Chris using the entanglement channel. Even though Alice and Bob can pass information on their results to Chris before Chris knows how many clicks show up at the BSM from the 2 & 3 photons. Keeping in mind that φ+/- outcomes only yield a single click at the BSM - but if Chris knows that Alice and Bob simultaneously matched at the same angles, then the φ- case can be ruled out.

Importantly: When Chris does see see 2 clicks, it will ALWAYS be a BSM combination that indicates ψ+ (if Alice and Bob matched) or ψ- (if Alice and Bob did not match). There are multiple permutations of clicks at the BSM that indicate this. On the other hand: if Chris sees only a single click at the BSM, that is neither proof nor disproof that the appropriate φ+ or φ- case occurred. Because a single click at the BSM cannot distinguish these.


c) If Alice and Bob are checking photons 1 & 4 at the different angle settings (such as for CHSH): They cannot tell Chris the information needed to predict which of 4 Bell states will occur at the BSM. That's because any combination of outcomes that Alice and Bob see is consistent with any of the 4 Bell states. Alice and Bob could provide information to Chris as to what is likely to occur though. Because there are outcomes that are more likely by far. (Typically a bit less than 75% accurate and a bit more than 25% inaccurate in the CHSH angle settings.)

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3. Hopefully everything presented so far makes sense. Now, everything the same as above, but instead: Chris chooses to implement the delay feature before knowing what Alice and Bob saw (match or no match).

d) Every outcome has 1 & 4 NOT entangled. There is no correlation at all with anything that happens at the BSM. Of course, Alice and Bob don't know this. In each and every case, the 2 & 3 photons arrived at the BSM far enough apart in time that Chris can pick out the 2 photon as the one arriving earlier, and the 3 photon as the one arriving later. Since they are distinguishable, there cannot be a swap.

Regardless of the clicks that Chris sees, there will be absolutely 0 correlation between those clicks (i.e. what they would otherwise indicates as to which Bell state occurred - and whether Alice and Bob matched or not.

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4. I would say that Chris' decision to add the delay time (or not) to photon 3's travel time - which occurs AFTER photons 1 & 4 outcomes are registered - was the "causal factor" in whether or not entanglement was swapped. There is absolutely no other factor involved, as you can clearly see, and the outcome is certain. Chris makes that decision while ignorant of the outcomes that Alice and Bob see, and in fact it wouldn't matter anyway.

Now, most anyone is going to say that Chris' decision to entangle or not - which is 100% demonstrably* a result of that decision - cannot change something that occurred in the past. And certainly, regardless, how to properly explain what happens is interpretation dependent. But what is not interpretation dependent is that Alice, Bob and Chris can literally be anywhere relative to each other - distant in terms of spacetime - and the outcomes will be exactly as I describe. And of course in no known situation can anyone send a signal that would violate signal locality, and no one can cause anything to occur in the past (or future) that would be inconsistent with the present.

-DrC

*If you are unsure about this, compare again cases a) and d) above. Note that the absolute proof can ONLY be seen in cases in which Chris sees 2 clicks at the BSM. 2 BSM clicks, no delay -> 100% correlation. 2 BSM clicks, with delay -> 0% correlation.

 

[PeterDonis]

You are correct in that the experimenter has no control over which 2 & 3 pairs will arrive at the BSM such that they overlap indistinguishably - that's strictly random.

Ok, good. That's what I thought.

When the 1 & 4 photons arrive is of course related to the 2 & 3 arrival time(s) at the BSM.

"Related" only in a very weak sense. Obviously if the 1 & 4 measurements are going to be in the past light cone of the BSM operation, there has to be a significant delay involved with the 2 & 3 photons. But the precise length of that delay is not exactly controllable by the experimenter. So the experimenter also has no exact control over the relationship between the 1 & 4 photon measurement times and the time window required for the BSM to do a swap.

there aren't any 1 & 4 results that are inconsistent with an entangled state - at least not without considering the 2 & 3 results.

Hm. I guess this is because all 4 of the entangled Bell states are possible outcomes of the BSM swap operation--it's just that we don't (at least currently) have any experimental setups that can distinguish all 4 of them. In experiments I remember being discussed in previous threads, the only entangled state that could be distinguished was the singlet state, and of course any pair of 1 & 4 results that give the same outcome for measurements in the same direction is inconsistent with that state. But from what you're saying, that would not mean 1 & 4 could not be entangled at all; it would just mean that if they are entangled, they're not in the singlet state. So as long as whatever 2 & 3 outcomes were measured were consistent with them not being in the singlet state, that would be ok. And the experimenter can't control the outcome of the 2 & 3 operation: even if it is a swap, the experimenter can't dictate that it is a swap into a particular entangled state.

the 2 independent sources and all observers can be located as close or as far apart relative to each other as desired. No combination of locations/distance (ideal case) changes the observed outcomes.

Yes, agreed.

 

[DrChinese]

1. Ok, good. That's what I thought.


2. "Related" only in a very weak sense. Obviously if the 1 & 4 measurements are going to be in the past light cone of the BSM operation, there has to be a significant delay involved with the 2 & 3 photons. But the precise length of that delay is not exactly controllable by the experimenter. So the experimenter also has no exact control over the relationship between the 1 & 4 photon measurement times and the time window required for the BSM to do a swap.


3. Hm. I guess this is because all 4 of the entangled Bell states are possible outcomes of the BSM swap operation--it's just that we don't (at least currently) have any experimental setups that can distinguish all 4 of them. In experiments I remember being discussed in previous threads, the only entangled state that could be distinguished was the singlet state, and of course any pair of 1 & 4 results that give the same outcome for measurements in the same direction is inconsistent with that state. But from what you're saying, that would not mean 1 & 4 could not be entangled at all; it would just mean that if they are entangled, they're not in the singlet state. So as long as whatever 2 & 3 outcomes were measured were consistent with them not being in the singlet state, that would be ok. And the experimenter can't control the outcome of the 2 & 3 operation: even if it is a swap, the experimenter can't dictate that it is a swap into a particular entangled state.


Yes, agreed.

1. Yay!


2. Peter, this is not correct. The relationship of the relative timings is extremely precise. And is completely under control of the experimenter - from millimeters apart to kilometers apart. This is in fact demonstrated in almost all swapping experiments, as well as many Bell type experiments. You already know this, but it may have gotten lost with all the back and forth.

Below is the HOM dip, which is measured in picoseconds; while the window is usually measured in nanoseconds. These are 4 fold coincidences. Obviously, the experiments I am citing are typically 10-15 years old. Recent advances are moving timings into much greater resolution, such as this from 2018: Attosecond-Resolution Hong-Ou-Mandel Interferometry.

In delayed choice variations, where the BSM is performed in the future of photons 1 & 4: usually that delay to the future is relatively short - exactly as you imagine - on the order of 100 meters (about 500 ns) as in here. After all, there is no specific theoretical difference between 100 meters and 100 times that. Of course, swapping itself has been demonstrated over distances of as much as 100 km (here and here*) without the delayed choice version. Since there is no particular open question related to the many delayed choice quantum experimental variations - all confirming the predictions of QM - I am not sure there is much experimental interest in setting new records for one particular version.

*From the reference: "As shown in Fig. 2, the typical peak-to-peak delays between arrival times of photons from Alice and Bob changes are 200 ps, 500 ps, and 1000 ps in rainy days, cloudy days, and sunny days, respectively, which are much larger than the coherent time of signal photons (∼ 110 ps). We use the difference between the arrival times of signal photons from Alice and Bob as error signals and feed them into a delay line to suppress the relative delay to 6 ps under all weather conditions, which is <<∼ 110 ps to ensure high interference visibility."


3. Yes and no. Assuming 1 & 4 are measured at the same angles when entangled, the results (match or mismatch) reduces the possible Bell states to 2 (which occur randomly). But only 1 of those can be discerned explicitly by the BSM. When doing the CHSH inequality, you are correct: any of the 4 Bell states would be compatible with any individual 1 & 4 result. And only 2 of those 4 can be discerned.

In some experiments, it is easier for the experimenter to look for a single specific Bell state with the BSM (what you can the singlet). So it depends what the experimenter is looking to do (in the particular paper) that determines whether to consider 1 or 2 of the 4 Bell states. If they are looking for 2 Bell states: it is possible to discern ψ+ or ψ- (most common), or φ+ or φ- (less common). But in no scenario does it make any difference to the scientific conclusion. Keep in mind that the experimenters know perfectly well that each Bell state occurs randomly and very nearly equally. If there were any hint otherwise, that would be a major issue.


4. Yay again

 

[PeterDonis]

he typical peak-to-peak delays between arrival times of photons from Alice and Bob changes are 200 ps, 500 ps, and 1000 ps in rainy days, cloudy days, and sunny days, respectively, which are much larger than the coherent time of signal photons (∼ 110 ps)

This was the sort of thing I was thinking of in what you labeled as item 2.

We use the difference between the arrival times of signal photons from Alice and Bob as error signals and feed them into a delay line to suppress the relative delay to 6 ps under all weather conditions

So this looks like the experimenters are correcting for the issue above. That looks like a key factor that I was missing.

 

[AndreasC]

Don't we have enough interpretations already?

Well it's not like we pay tax on them.

 

[Fra]

The point is Conceptual clarity.

At first I would agree but this is apparently one of the most relative notions ever invented. What is clear to you may well be hazy to someone else. So I think the discriminator is still whatever makes progress on open questions easier, clear or not.

/Fredrik

 

[Fra]

I skimmed some of

https://arxiv.org/abs/2302.10778
https://arxiv.org/abs/2309.03085
https://arxiv.org/abs/2402.16935

I skimmed these but I lack any for me preferred new conceptual grip, it seems mainly descriptive as I find no references to the observer or the context.

In the last paper he writes...

"At the level of dynamics, the microphysical laws consist of conditional or transition probabilities of the form Γij(t) ≡ p(i, t|j, 0) [for i, j = 1, . . .N], (18) each of which supplies the probability for the system to be in its ith configuration at a continuously variable time t..."

Sounds reasonable and these obviously encode the corresponding hamiltonian details, but the question is, what is the process whereby these laws (transition probabilities) are inferred by a real observer. Without this, this seems to be out of taste for me. In principle I can imagine some elaborations where these transition amplities are constructed, but I see no traces of that in this thikning from skimming the papers. without this, this remains pursely descriptive, treating the "observer" as an implicit non-interacting context, just like most other interpretations.

/Fredrik

 

[RUTA]

TL;DR Summary: I attended a lecture that discussed the approach in the 3 papers listed below. It seems to be a genuinely new interpretation with some interesting features and claims.

These papers claim to present a realistic stochastic interpretation of quantum mechanics that obeys a stochastic form of local causality. (A lecture I recently attended mentioned these papers). It also claims the Born rule as a natural consequence rather than an assumption. This appears to me to be a genuinely new interpretation. I have not delved into the papers in detail, but figured some people here may be interested.

https://arxiv.org/abs/2302.10778
https://arxiv.org/abs/2309.03085
https://arxiv.org/abs/2402.16935

Here is a talk he gave this month

 

[RUTA]

Well, that's kinda the issue, isn't it? He says here: "one can reformulate quantum theory in terms of old-fashioned configuration spaces together with 'unistochastic' laws. These unistochastic laws take the form of directed conditional probabilities, which turn out to provide a hospitable foundation for encoding microphysical causal relationships. This unistochastic reformulation provides quantum theory with a simpler and more transparent axiomatic foundation, plausibly resolves the measurement problem, and deflates various exotic claims about superposition, interference, and entanglement."

That abstract sounds exotic to me! Superposition and interference are merely "claims? Measurement problem: solved! And entanglement... well I think it is very clear entanglement is a great big target on the back of this formulation. No, you cannot define/redefine the phrase "causal locality" to be different than "local causality", and then expect to dodge GHZ, advanced entanglement issues and the latest no-go's.

That's a far cry from agreeing with the idea that there is signal locality - which as far as I know is disputed by essentially no one. And if in fact you are correct, he has a new mathematical representation: so is it in fact exactly identical (since he drops the standard mathematical methods entirely) ? How would a reader understand that either way? His abstract contains some big claims, and I certainly missed the elements where he convinces of the abstract's claims.

Here is the last sentence of his conclusion, you tell me if he thinks he is onto something different and important. Because it certainly reads to me that the Bell conclusion* (along with GHZ etc.) is being thrown out.

"Remarkably, one therefore arrives at what appears to be a causally local hidden-variables formulation of quantum theory, despite many decades of skepticism that such a theory could exist."


*Which is: "No physical theory of local Hidden Variables can ever reproduce all of the predictions of Quantum Mechanics."-DrC

His last statement is correct, given his redefinition of local causality. But, as many of you have pointed out, changing the semantics will not solve the mystery of quantum entanglement for most people.

 

[Fra]

While I see other problems, not at all addressed by this "new interpretation" (such as constructing the transition matrixes rather than just assume their existence), I appreciate the attempt to clarify the meaning and definition of causality as I think the "nature of causation" is indeed at the heart of the matter, so it might be a more compatible with bayesian interpretations.

As the various traditional bell defintions of terms based on what violates bell inequalities are conceptually entangled with assumptions going into Bells ansatz. But those assumptions I see as a outdate legacy.

His last statement is correct, given his redefinition of local causality. But, as many of you have pointed out, changing the semantics will not solve the mystery of quantum entanglement for most people.

quantum nonlocality are probabilistic, and therefore do not constitute evidence of what might be labeled as "causal" anyway.

I think as long as one refuse to include the observers actions into the physics, we will likely not make progress.

The merit I find in defining causality in terms of conditional probabilities instead of single events is that they are what would be expected to causally influence an agents actions towards it's environment, rather than single events.

The only for me at least conceptually meaningful notion of local causality principle, is that local decisions are influence only by local information. Thus "causality" would then not be a statement of future correlations, but a statement about present actions. It is in this sense I also envision (but maybe differently Barandes) that "hidden variables" CAN explain the correlations in entanglement as per Reichenbach's Principle, while violating bell inequality, because the causal mechanism is not on "outcomes" but on "actions"; so the Reichenbach's keys is still hidden, so the ignorance anzats of Bell cant' be valid. This is the confusion that I always felt is built into the legacty anzats of Bell, as it implies a "ignorance interpretation".

But this conceptual understanding still needs to be realized in reconstructing the interactions (transition matrixes) that would essentiall encode the actions of matter (linking to unification). So a massive task! But I think some simply would see this conceptual possibility until the full theory is explicit. While others(me included) find it a guiding principle - I find no clues of this in his papers though. Which is why I appreciate attacking the nature of causality, but don't yet understand what else it adds.

/Fredrik