Photon entanglement: why three angles?

[stevendaryl]

Let so called "Bell test angles" be known ahead of time: 0°, 45°, 22.5° and 67.5°. How that makes it any easier to come up with a single $\lambda$ function that works for each combination: E(a,b), E(a,b'), E(a',b), and E(a',b')? What time has to do with any of it?

If you know the angles ahead of time, it is easy to reproduce the predicted correlations using a local hidden-variables model.

If you know ahead of time that Alice's filter is set at angle $a$ and Bob's filter is set at angle $b$, then a model that reproduces the predictions of QM is the following:

1. With probability $\frac{1}{2} cos^2(a-b)$, send a photon to Alice that is polarized at angle $a$, and send a photon to Bob that is polarized at angle $b$.
2. With probability $\frac{1}{2} cos^2(a-b)$, send a photon to Alice that is polarized at angle $a+90^o$, and send a photon to Bob that is polarized at angle $b+90^o$.
3. With probability $\frac{1}{2} sin^2(a-b)$, send a photon to Alice that is polarized at angle $a$, and send a photon to Bob that is polarized at angle $b+90^o$.
4. With probability $\frac{1}{2} sin^2(a-b)$, send a photon to Alice that is polarized at angle $a+90^o$, and send a photon to Bob that is polarized at angle $b$.

We can independently verify that if a filter is aligned in the same direction as polarized light, then it passes 100% of the time, and if it is aligned at a 90 degree angle, relative to the polarized light, then it is blocked 100% of the time.

This trivial model reproduces exactly the predictions of QM for the twin-photon EPR experiment.

It's clear that the model could not work if you don't know Alice's and Bob's filter orientations ahead of time.

 

[johana]

We're comparing coincidence rates at various angles. Thus we need a setup that gives us a series of measurements in which everything is the same except the angles. The most practical way of doing that is to run one experiment in which only the angle varies.

What is the point of randomly switching angles and taking measurements "simultaneously" little by little, instead of to test each angle completely and separately one after another?

 

[atyy]

What is the point of randomly switching angles and taking measurements "simultaneously" little by little, instead of to test each angle completely and separately one after another?

In the Bell inequalities, it is assumed (1) that Bob's result only depends on his choice and the local hidden variables, and it is also assumed (2) that Bob's result cannot depend on Alice's choice. The idea behind making the choices randomly at each side is that the choice is only made at the "last moment" before the the result is obtained, so that if we assume that the speed of light is an upper limit to the speed of communication, Alice's choice cannot be communicated in time to Bob to affect his result.

 

[DrChinese]

What is the point of randomly switching angles and taking measurements "simultaneously" little by little, instead of to test each angle completely and separately one after another?

There is nothing wrong with that. Most Bell tests do just that.

The purpose of those that do fast switching is to demonstrate that there is no additional sub-lightspeed action in play that might affect the results. But we already know that from experiments such as Weihs et al, so that is no longer relevant. atyy's post above covers this issue.

 

[johana]

If you know the angles ahead of time, it is easy to reproduce the predicted correlations using a local hidden-variables model.

If you know ahead of time that Alice's filter is set at angle $a$ and Bob's filter is set at angle $b$, then a model that reproduces the predictions of QM is the following:

1. With probability $\frac{1}{2} cos^2(a-b)$, send a photon to Alice that is polarized at angle $a$, and send a photon to Bob that is polarized at angle $b$.
2. With probability $\frac{1}{2} cos^2(a-b)$, send a photon to Alice that is polarized at angle $a+90^o$, and send a photon to Bob that is polarized at angle $b+90^o$.
3. With probability $\frac{1}{2} sin^2(a-b)$, send a photon to Alice that is polarized at angle $a$, and send a photon to Bob that is polarized at angle $b+90^o$.
4. With probability $\frac{1}{2} sin^2(a-b)$, send a photon to Alice that is polarized at angle $a+90^o$, and send a photon to Bob that is polarized at angle $b$.

Those are four different functions. As you said the whole point is to come up with a single model, can you do that?

You can not use (a-b) for a local hidden variable, that's exactly what makes QM equations nonlocal. In local reality properties and interaction of photon B and polarizer B are of no consequence to photon A and polarizer A, and vice versa. For local reality a hidden variable must fit a single function that applies separately to each side.

For each photon pair:

$\lambda(photon_A, polarizer_A) = V|H$
$\lambda(photon_B, polarizer_B) = V|H$

...which integrated over many pairs sums up to yield:

$VV + HH - VH - HV = cos^2(A-B) - sin^2(A-B)$

 

[stevendaryl]

Those are four different functions. As you said the whole point is to come up with a single model, can you do that?

It's not 4 different functions, it's 4 different values for the hidden variable $\lambda$. If $\lambda = \lambda_1$, both Alice's and Bob's photons pass their filters. This value is chosen with probability $\frac{1}{2} cos^2(a-b)$, etc.

You can not use (a-b) for a local hidden variable, that's exactly what makes QM equations nonlocal.

That was the point I was making. If $a$ and $b$ are known ahead of time, then there is nothing nonlocal involved in taking those values into account. It only becomes nonlocal if you allow Alice and Bob to choose $a$ and $b$ while the photons are in flight.

 

[stevendaryl]

That was the point I was making. If $a$ and $b$ are known ahead of time, then there is nothing nonlocal involved in taking those values into account. It only becomes nonlocal if you allow Alice and Bob to choose $a$ and $b$ while the photons are in flight.

I should say: If the values of $a$ and $b$ are known ahead of time, it's not necessarily nonlocal.

 

[johana]

In the Bell inequalities, it is assumed (1) that Bob's result only depends on his choice and the local hidden variables, and it is also assumed (2) that Bob's result cannot depend on Alice's choice. The idea behind making the choices randomly at each side is that the choice is only made at the "last moment" before the the result is obtained, so that if we assume that the speed of light is an upper limit to the speed of communication, Alice's choice cannot be communicated in time to Bob to affect his result.

FTL question is addressed by placing the two polarizers far enough apart so the time difference between when "signal" and "idler" photon meet with their polarizers is less than the speed of light would require to go from one to the other.

Let signal photon A go vertical through polarizer A at time t= 0. Let idler photon B be one light year away from photon A and one meter away from polarizer B at time t= 0. Photon B now has to assume the same polarization as photon A before it meets with polarizer B, that's the trick regarding FTL. But what angle polarizer B is set to, and when, is of no consequence to this speed of correlation/information between photon A and photon B.

 

[johana]

It's not 4 different functions, it's 4 different values for the hidden variable $\lambda$. If $\lambda = \lambda_1$, both Alice's and Bob's photons pass their filters. This value is chosen with probability $\frac{1}{2} cos^2(a-b)$, etc.

Proposing different models for each angle is invalid to begin with. Plus, you can not use (a-b) to define a hidden local variable. It does not qualify, it does not compare.

That was the point I was making. If $a$ and $b$ are known ahead of time, then there is nothing nonlocal involved in taking those values into account. It only becomes nonlocal if you allow Alice and Bob to choose $a$ and $b$ while the photons are in flight.

Angles are always known and deliberately chosen ahead of time. Are you saying there is some difference if we randomly switch angles and take measurements "simultaneously" little by little, instead of to test each angle completely and separately one after another?

 

[atyy]

FTL question is addressed by placing the two polarizers far enough apart so the time difference between when "signal" and "idler" photon meet with their polarizers is less than the speed of light would require to go from one to the other.

Let signal photon A go vertical through polarizer A at time t= 0. Let idler photon B be one light year away from photon A and one meter away from polarizer B at time t= 0. Photon B now has to assume the same polarization as photon A before it meets with polarizer B, that's the trick regarding FTL. But what angle polarizer B is set to, and when, is of no consequence to this speed of correlation/information between photon A and photon B.

Take a look at stevendaryl's post #71. If the settings are known ahead of time, then his variable (a-b) can be a local hidden variable, ie. although (a-b) is associated with distant apparatuses, there's plenty of time to propagate the detector settings back to the source.

 

[stevendaryl]

Proposing different models for each angle is invalid to begin with. Plus, you can not use (a-b) to define a hidden local variable. It does not qualify, it does not compare.

Sigh. You're coming into the middle of a long discussion, and your points are not relevant to the particular point I was discussing. If the order of the events is the following:
(I'm saying IF it takes place in the following way---I'm not saying that it does, and I'm not
saying that Bell allowed for it, or whatever. This is simply a hypothetical explanation for a particular thought experiment.)

1. Alice chooses her filter angle $a$
2. Bob chooses his filter angle $b$
3. A hidden variable $\lambda$ is generated taking into account $a$ and $b$.
4. A pair of photons is generated that somehow encode this value
5. Whether Alice's photon passes or not is a deterministic function of $\lambda$ and $a$
6. Whether Bob's photon passes or not is a deterministic function of $\lambda$ and $b$.

My claim is that it is possible to implement the above scenario, using local hidden variables, in a way that reproduces the predictions of quantum mechanics. If the choice of $\lambda$ depends on the settings $a$ and $b$, then there is no proof of nonlocality (or nonrealism, or whatever it is that Bell's theorem talks about).

To get Bell's proof to go through, you have to assume that the hidden variable $\lambda$ is NOT dependent on Alice's and Bob's filter settings. If you don't make that assumption, then there is certainly a local hidden-variables explanation.

Angles are always known and deliberately chosen ahead of time. Are you saying there is some difference if we randomly switch angles and take measurements "simultaneously" little by little, instead of to test each angle completely and separately one after another?

If the settings are chosen ahead of time, then that leaves a loophole for local hidden variables theories. It might not be a plausible loophole, but it's a loophole.

 

[DrChinese]

johana,

Just to make sure everyone is on the same page: no one is arguing that it makes a difference if there is fast switching or not. However, that is something we know now that was not known 100% prior to experiment (Aspect, Weihs, etc.). If you want to rule out a local action so that the settings A and B are provably independent, use fast switching. So stevendaryl is simply saying that a local realistic model was technically viable prior to that because there might be time for subluminal communication from one measuring device to the other (crazy as it seems). But now we know better.

 

[stevendaryl]

johana,

Just to make sure everyone is on the same page: no one is arguing that it makes a difference if there is fast switching or not. However, that is something we know now that was not known 100% prior to experiment (Aspect, Weihs, etc.). If you want to rule out a local action so that the settings A and B are provably independent, use fast switching. So stevendaryl is simply saying that a local realistic model was technically viable prior to that because there might be time for subluminal communication from one measuring device to the other (crazy as it seems). But now we know better.

Exactly. Thanks.

 

[johana]

If the settings are chosen ahead of time, then that leaves a loophole for local hidden variables theories. It might not be a plausible loophole, but it's a loophole.

Yes, loophole. But what loophole are you talking about? FTL loophole is addressed by separating polarizers far enough apart relative to the time interval between when signal and idler photon are supposed to meet with their polarizers. It works for any individual angle when tested separately, so that's not it.

 

[johana]

Take a look at stevendaryl's post #71. If the settings are known ahead of time, then his variable (a-b) can be a local hidden variable, ie. although (a-b) is associated with distant apparatuses, there's plenty of time to propagate the detector settings back to the source.

time0: photons A & B emitted with unknown/undefined polarization

time1: photon A goes through 0° polarizer A and acquires 0° polarization

time2: photon B acquires 0° polarization, for some reason ( time1 = time2 ?? )

time3: photon B with 0° polarization interacts with 0° polarizer B, so it too goes throughNow, if polarizer B was at 90° and switched to 0° just a moment before time3, what difference does it make?

 

[Nugatory]

Now, if polarizer B was at 90° just a moment before time3, what difference does it make?

Not much. But suppose that polarizer A was at 90° until just a moment before $t1$... In fact, for such a short moment that both $t2$ and $t3$ have come and gone before a light signal from polarizer A's flip to 0 degrees could have reached photon B... That makes B's flip to zero much harder to explain.

 

[atyy]

Now, if polarizer B was at 90° and switched to 0° just a moment before time3, what difference does it make?

To add to Nugatory's point, you can read more discussion of the issue in:
http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.105.250404
http://arxiv.org/abs/1007.5518
http://arxiv.org/abs/1008.3612
http://arxiv.org/abs/1303.2849 (section B.2 "Locality loophole")

 

[johana]

Not much. But suppose that polarizer A was at 90° until just a moment before $t1$... In fact, for such a short moment that both $t2$ and $t3$ have come and gone before a light signal from polarizer A's flip to 0 degrees could have reached photon B... That makes B's flip to zero much harder to explain.

It is accepted by both sides time2 is less than time3, otherwise causality would be acting backwards in time, and that's not really kind of thing local realists are hoping for. Each subsequent time is greater or equal to previous time:

time0: photons A & B emitted with unknown/undefined polarization

->time0.7: polarizer A set to 45°
->time0.8: polarizer B set to 90°

time0.9: polarizer A set to 0°
time1: photon A goes through 0° polarizer A and acquires 0° polarization

->time1.7: polarizer A set to 15°
->time1.8: polarizer B set to 75°

time2: photon B acquires 0° polarization, for some reason ( time1 = time2 ?? )

->time2.7: polarizer A set to 90°
->time2.8: polarizer B set to 45°

time2.9: polarizer B set to 0°
time3: photon B with 0° polarization interacts with 0° polarizer B, so it too goes through


Please note those time events marked with arrows, are they anyhow relevant to what happens at time1, time2, or time3?

 

[StevieTNZ]

Isn't it when the photon is detected by an apparatus, it is then that the system takes on the polarisation of the polariser?

 

[johana]

To add to Nugatory's point, you can read more discussion of the issue in:
http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.105.250404
http://arxiv.org/abs/1007.5518
http://arxiv.org/abs/1008.3612
http://arxiv.org/abs/1303.2849 (section B.2 "Locality loophole")

They do talk about it, but they don't explain it. Try to answer the question in your own words, with a sentence or two, and maybe you'll see the same paradox I see. Or maybe you'll see an explanation, that's even better.

 

[atyy]

They do talk about it, but they don't explain it. Try to answer the question in your own words, with a sentence or two, and maybe you'll see the same paradox I see. Or maybe you'll see an explanation, that's even better.

They do. Read the papers.

 

[johana]

Isn't it when the photon is detected by an apparatus, it is then that the system takes on the polarisation of the polariser?

It is when the first photon A interacts with the polarizer A that its "wave function collapses", which we are meant to understand its, until then, "undefined" polarization becomes real and defined by the polarizer A. At the same time, or a bit later, the other photon B, whose polarization was also undefined until then, acquires the same definite polarization as the first photon A. From there on both photons are in "classic" mode of operation, having definite polarization, and so the interaction between second photon B and polarizer B goes on normally in accordance to Malus law.

 

[stevendaryl]

Yes, loophole. But what loophole are you talking about? FTL loophole is addressed by separating polarizers far enough apart relative to the time interval between when signal and idler photon are supposed to meet with their polarizers. It works for any individual angle when tested separately, so that's not it.

No, that is it. If angles $a$ and $b$ are chosen too far in advance, then you can't be certain that the hidden variable isn't influenced by those settings.

 

[stevendaryl]

They do talk about it, but they don't explain it. Try to answer the question in your own words, with a sentence or two, and maybe you'll see the same paradox I see. Or maybe you'll see an explanation, that's even better.

No, I don't know what paradox you are talking about.

 

[stevendaryl]

It is when the first photon A interacts with the polarizer A that its "wave function collapses", which we are meant to understand its, until then, "undefined" polarization becomes real and defined by the polarizer A. At the same time, or a bit later, the other photon B, whose polarization was also undefined until then, acquires the same definite polarization as the first photon A. From there on both photons are in "classic" mode of operation, having definite polarization, and so the interaction between second photon B and polarizer B goes on normally in accordance to Malus law.

That's the "collapse" interpretation of EPR, but it's explicitly nonlocal.

 

[Nugatory]

It is accepted by both sides time2 is less than time3, otherwise causality would be acting backwards in time, and that's not really kind of thing local realists are hoping for. Each subsequent time is greater or equal to previous time:

Time1 is not necessarily less than time3. You can set up the experiment in such a way that some of the people watching the experiment observe that photon A reaches polarizer A before photon B reaches polarizer B while others (who happen to be moving relative to the first group) observe that photon B reaches polarizer B before photon A reaches polarizer A. This is Einstein's "relativity of simultaneity" (google for that phrase if you're not already familiar with it) at work.

Because we cannot directly observe photon B acquiring 0° polarization, we don't know if time2 is different from time3 (it's natural to expect that it is, but that expectation comes from our day-to-day experience, which is a poor guide to how QM works). However, if time2 is different from time3, it will always be less than time3 for all observers.

Please note those time events marked with arrows, are they anyhow relevant to what happens at time1, time2, or time3?

As long as they are all greater than time0 the changes to the A setting cannot affect the result at B (and vice versa) in any local theory. Yet the quantum mechanical result is that the result at B depends on the setting at A at time0.9 - the reason for doing the fast switching experiments is to confirm that this is indeed the case.

 

[RUTA]

time0: photons A & B emitted with unknown/undefined polarization

time1: photon A goes through 0° polarizer A and acquires 0° polarization

time2: photon B acquires 0° polarization, for some reason ( time1 = time2 ?? )

time3: photon B with 0° polarization interacts with 0° polarizer B, so it too goes through


Now, if polarizer B was at 90° and switched to 0° just a moment before time3, what difference does it make?

Let's look at the Hardy state from the Mermin paper in my post #21. You have two particles, two settings 1 and 2, two outcomes R and G. Here are the facts for that state being measured in this configuration taken from p 881:

The data exhibit the following important features:

(a) In runs in which the detectors end up with different
settings, they never both flash green: 21 GG and
12 GG never occur.

(b) In runs in which both detectors end up set to 2, one
occasionally finds both flashing green: 22GG sometimes
occurs.

(c) In runs in which both detectors end up set to 1, they
never both flash red: 11RR never occurs.

Now suppose you're particle 1 and know ahead of time that you both will be measured in setting 2. You agree with your partner that you'll both give a G result, i.e., 22GG. As you approach your detector you see that it's set to 1. What do you do?

Suppose you choose to be 1R, thinking your partner will be 2G and you have to satisfy condition (a). But, what happens if your partner also encounters setting 1 and, thinking the same thing, decides to be 1R. Now you have a 11RR outcome in violation of (c). Suppose you go with 1G. Now if your partner's detector isn't changed and he thinks all is ok and goes with 2G you have a 12GG outcome in violation of (a). So, by encountering a setting that isn't what you expected you're screwed.

Is that clear enough?

 

[stevendaryl]

Let's look at the Hardy state from the Mermin paper in my post #21. You have two particles, two settings 1 and 2, two outcomes R and G.

Yes. In my opinion, even though Bell's inequality might be more amenable to experimental tests, when it comes to discussions about the weirdness of quantum mechanics and entanglement, the Hardy state is much easier to understand. The impossibility of a local, deterministic hidden-variables explanation is much clearer, since you don't really need to do any kind of mathematics involving expectation values, or probability calculations, or whatever. You have a situation which clearly is impossible, classically, yet happens, quantum mechanically.

 

[Johan0001]

Suppose you choose to be 1R, thinking your partner will be 2G and you have to satisfy condition (a). But, what happens if your partner also encounters setting 1 and, thinking the same thing, decides to be 1R. Now you have a 11RR outcome in violation of (c). Suppose you go with 1G. Now if your partner's detector isn't changed and he thinks all is ok and goes with 2G you have a 12GG outcome in violation of (a). So, by encountering a setting that isn't what you expected you're screwed.

Is that clear enough?

This is the big mystery.
Could you give a reference to which experiment best exploits this phenomena, with the least possible loopholes such as reality and locality?

 

[DrChinese]

This is the big mystery.
Could you give a reference to which experiment best exploits this phenomena, with the least possible loopholes such as reality and locality?

Some of the most important include these:

http://arxiv.org/abs/quant-ph/9810080
Violation of Bell's inequality under strict Einstein locality conditions
Gregor Weihs, Thomas Jennewein, Christoph Simon, Harald Weinfurter, Anton Zeilinger (University of Innsbruck, Austria)
(Submitted on 26 Oct 1998)

We observe strong violation of Bell's inequality in an Einstein, Podolsky and Rosen type experiment with independent observers. Our experiment definitely implements the ideas behind the well known work by Aspect et al. We for the first time fully enforce the condition of locality, a central assumption in the derivation of Bell's theorem. The necessary space-like separation of the observations is achieved by sufficient physical distance between the measurement stations, by ultra-fast and random setting of the analyzers, and by completely independent data registration.

http://www.nature.com/nature/journal/v409/n6822/full/409791a0.html
Experimental violation of a Bell's inequality with efficient detection

M. A. Rowe1, D. Kielpinski1, V. Meyer1, C. A. Sackett1, W. M. Itano1, C. Monroe2 & D. J. Wineland1


http://arxiv.org/abs/1306.5772
Detection-Loophole-Free Test of Quantum Nonlocality, and Applications
B. G. Christensen, K. T. McCusker, J. Altepeter, B. Calkins, T. Gerrits, A. Lita, A. Miller, L. K. Shalm, Y. Zhang, S. W. Nam, N. Brunner, C. C. W. Lim, N. Gisin, P. G. Kwiat
(Submitted on 24 Jun 2013 (v1), last revised 26 Sep 2013 (this version, v2))

We present a source of entangled photons that violates a Bell inequality free of the "fair-sampling" assumption, by over 7 standard deviations. This violation is the first experiment with photons to close the detection loophole, and we demonstrate enough "efficiency" overhead to eventually perform a fully loophole-free test of local realism. The entanglement quality is verified by maximally violating additional Bell tests, testing the upper limit of quantum correlations. Finally, we use the source to generate secure private quantum random numbers at rates over 4 orders of magnitude beyond previous experiments.

 

[johana]

Here's something from DrChinese younger self.

https://www.physicsforums.com/showthread.php?t=39614&page=7

Despite what you (and others) might think, you don't need to change polarizer settings in flight or otherwise vary the angles to test Bell's Theorem. You only need to calculate the correlation percentages at three particular angle settings (these can be done fully independently). Then combine a la Bell.

Varying is only necessary if you are asserting that the measurement devices are (or might be) communicating with each other so as to affect the outcome of the correlation tests. We already know from Aspect that doesn't happen, because he did the experiments both ways and there was no difference in the outcomes! Even that should be a definitive conclusion of Aspect. Further regarding the varying issue:

a. If you are a local realist, I would assume that wouldn't be much of an issue to you since you think there are classical, intuitive explanations for everything anyway - strange new types of communication between measuring devices should not be an issue.
b. If, on the other hand, you follow the Copenhagen interpretation, varying also shouldn't matter as you don't isolate out communication with other parts of the measurement apparatus for any other type of experiment (such as double slit) either.
c. Also, if you believe the correlation is non-local then the varying analyzers are superfluous.
d. And finally, if you are a local non-realist like me :) then you already believe that the only "real" component being measured is the angle between the remote polarizers anyway i.e. the measurement is fundamental to the process.

This makes perfect sense. Switching angles is unnecessary and is not a substitute for placing detectors far apart. It's about some out of this world type of theory neither nonlocalists nor local realists care to imagine even in their wildest dreams. The only thing that doesn't make sense is "local non-realist". What in the world is that?

 

[stevendaryl]

This makes perfect sense. Switching angles is unnecessary and is not a substitute for placing detectors far apart. It's about some out of this world type of theory neither nonlocalists nor local realists care to imagine even in their wildest dreams. The only thing that doesn't make sense is "local non-realist". What in the world is that?

The quote from Dr. Chinese points out that switching angles in-flight is only unnecessary because people have ALREADY showed that it makes no difference. To demonstrate that there is no local, classical explanation for EPR, you have to check out the possibility that the filter settings affect the outcome in a slower-than-light way.

 

[johana]

The quote from Dr. Chinese points out that switching angles in-flight is only unnecessary because people have ALREADY showed that it makes no difference. To demonstrate that there is no local, classical explanation for EPR, you have to check out the possibility that the filter settings affect the outcome in a slower-than-light way.

Switching angles does not relate to any local or classical explanation, it's about nonlocal correlation between polarizers, not just photons. No one is proposing that, they are just messing up measurements with additional unnecessary complexity and randomness.


DrChinese explains redundancy of it well here:

a. If you are a local realist, I would assume that wouldn't be much of an issue to you since you think there are classical, intuitive explanations for everything anyway - strange new types of communication between measuring devices should not be an issue.

b. If, on the other hand, you follow the Copenhagen interpretation, varying also shouldn't matter as you don't isolate out communication with other parts of the measurement apparatus for any other type of experiment (such as double slit) either.

c. Also, if you believe the correlation is non-local then the varying analyzers are superfluous.

d. And finally, if you are a local non-realist like me :) then you already believe that the only "real" component being measured is the angle between the remote polarizers anyway i.e. the measurement is fundamental to the process.

 

[DrChinese]

The only thing that doesn't make sense is "local non-realist". What in the world is that?

There are a number of non-realistic interpretations. Not everyone will concur with my categorization, but this is an answer to your question.

We all know that the Bohmian group (de Broglie-Bohm, Bohmian Mechanics) and several others are explicitly non-local. So I call anything that is not EXPLICITLY non-local to be "non-realistic" by definition (to comply with Bell). That would then include: Many Worlds, Relational Blockworld (ask RUTA about that), Cramer's Absorber, Aharanov's Time Symmetric QM, and a few others.

You mentioned something in an earlier post about non-causal situations (future affects the past). Before you rule those out, you might take note: there are substantial experiments that demonstrate the future affects the past. These experiments are not a rock solid proof of same, but they are definitely powerful evidence. For example:

http://arxiv.org/abs/quant-ph/0201134
Experimental Nonlocality Proof of Quantum Teleportation and Entanglement Swapping
Thomas Jennewein, Gregor Weihs, Jian-Wei Pan, Anton Zeilinger
(Submitted on 29 Jan 2002)

"Quantum teleportation strikingly underlines the peculiar features of the quantum world. We present an experimental proof of its quantum nature, teleporting an entangled photon with such high quality that the nonlocal quantum correlations with its original partner photon are preserved. This procedure is also known as entanglement swapping. The nonlocality is confirmed by observing a violation of Bell's inequality by 4.5 standard deviations. Thus, by demonstrating quantum nonlocality for photons that never interacted our results directly confirm the quantum nature of teleportation. "

See page 5. All of this is garden QM, as time ordering is not critical in many setups using entangled systems.

 

[stevendaryl]

Switching angles does not relate to any local or classical explanation, it's about nonlocal correlation between polarizers, not just photons.

I'm not sure what is the root of our communication problems, but something is not getting communicated here. Switching angles at the last possible minute does relate to local or classical explanations in the sense that it proves that there are no such explanations. If you DON'T switch, then that leads to the possibility that the settings affect the hidden variable in a local way.

No one is proposing that, they are just messing up measurements with additional unnecessary complexity and randomness.

They're closing a possible loophole. That's all. I don't understand what it is that you don't understand about it.