Third Loophole Against Entanglement Eliminated

[Charles Wilson]

"Third Loophole" Against Entanglement Eliminated

The third and final loophole in an important test of the quantum nature of the photon has been closed by an international team of physicists. The researchers have shut what is called the "fair sampling" loophole, which says that classical – rather than quantum – effects could be responsible for measured correlations between entangled pairs of photons. The photon is now the first system in which the violation of "Bell's inequality" has been unambiguously established. While few physicists will be surprised that all three loopholes have now been closed, doing so could be an important step towards developing failsafe quantum cryptography.

http://physicsworld.com/cws/article/news/2013/apr/23/third-bell-loophole-closed-for-photons

CW

 

[ZapperZ]

Please note that Cthugha has highlighted this paper a week ago in our Recent Noteworthy Physics Papers thread:

https://www.physicsforums.com/showpost.php?p=4351057&postcount=157

Zz.

 

[Demystifier]

I don't think this result is extremely important, given that they say (my bolding):
"There is one catch, however. To ensure that more than 67% of the pairs were detected, the experiment was done in the lab with Alice, Bob and the source near to each other. As a result, this particular experiment did not simultaneously rule out the other two loopholes. According to Giustina, closing all three in a Canary Islands experiment would be extremely difficult."

 

[ZapperZ]

I don't think this result is extremely important, given that they say (my bolding):
"There is one catch, however. To ensure that more than 67% of the pairs were detected, the experiment was done in the lab with Alice, Bob and the source near to each other. As a result, this particular experiment did not simultaneously rule out the other two loopholes. According to Giustina, closing all three in a Canary Islands experiment would be extremely difficult."

But I think you are missing the point of the experiment. The fair-sampling loophole (which is the target of the experiment) is the only one left that hasn't been closed in ANY experiment till now. The detection loophole, and the locality loophole, all have been closed in different experiments (but not simultaneously). So this was the last one.

Of course, now, the target is to have an experiment that closes all 3 loopholes simultaneously.

Zz.

 

[DrChinese]

But I think you are missing the point of the experiment. The fair-sampling loophole (which is the target of the experiment) is the only one left that hasn't been closed in ANY experiment till now. The detection loophole, and the locality loophole, all have been closed in different experiments (but not simultaneously). So this was the last one.

Of course, now, the target is to have an experiment that closes all 3 loopholes simultaneously.

Zz.

I always thought the fair sampling loophole and the detection loophole were essentially the same. Per Wineland et al (2001):

"Early experiments to test Bell's inequalities were subject to two primary, although seemingly implausible, loopholes. The first might be termed the locality or ‘lightcone’ loophole, in which the correlations of apparently separate events could result from unknown subluminal signals propagating between different regions of the apparatus. Aspect has given a brief history of this issue, starting with the experiments of ref. 8 and highlighting the strict relativistic separation between measurements reported by the Innsbruck group. Similar results have also been reported for the Geneva experiment. The second loophole is usually referred to as the detection loophole. All experiments up to now have had detection efficiencies low enough to allow the possibility that the subensemble of detected events agrees with quantum mechanics even though the entire ensemble satisfies Bell's inequalities. Therefore it must be assumed that the detected events represent the entire ensemble; a fair-sampling hypothesis. Several proposals for closing this loophole have been made; we believe the experiment that we report here is the first to do so."

So I understood the Zeilinger et al experiment to be the first to close this loophole using photons. The significance of this (in my puny mind) is that one can envision extending this in a future experiment such that the locality loophole is closed simultaneously. That could not be accomplished with the massive particles used in the Wineland et al study.

 

[DrChinese]

In the referenced paper, they say:

"The two other main assumptions include 'locality' and 'freedom of choice'."

I don't consider freedom of choice to be a loophole in this type of experiment. Freedom of choice could be equally invoked for ANY scientific experiment as a loophole. So I don't consider it "scientific" at all. But that is just my opinion.

 

[Demystifier]

The fair-sampling loophole (which is the target of the experiment) is the only one left that hasn't been closed in ANY experiment till now.

You mean any experiment with PHOTONS, right? Because, I think, it has been closed with charged particles.

 

[ZapperZ]

You mean any experiment with PHOTONS, right? Because, I think, it has been closed with charged particles.

Sorry, yes, with photons. Obviously the fair-sampling issue with charged particles makes no sense since we are not faced with the detection issue there as we do with photons.

Zz.

 

[Cthugha]

As a result, this particular experiment did not simultaneously rule out the other two loopholes. According to Giustina, closing all three in a Canary Islands experiment would be extremely difficult."

No, but at least now all of the loopholes have been closed in a single system, not in different systems. Importance is always somewhat in the eye of the beholder, but I can see why some people will consider that an important result.

 

[harrylin]

http://physicsworld.com/cws/article/news/2013/apr/23/third-bell-loophole-closed-for-photons

CW

Thanks that looks very interesting!

 

[Samshorn]

In the referenced paper, they say:

"The two other main assumptions include 'locality' and 'freedom of choice'."

I don't consider freedom of choice to be a loophole in this type of experiment. Freedom of choice could be equally invoked for ANY scientific experiment as a loophole. So I don't consider it "scientific" at all. But that is just my opinion.

I think the article's comments about those other "two" loopholes must be garbled. They refer to a 2010 experiment of Zeilinger, apparently the one described in this article:

phys.org/news/2010-11-physicists-loopholes-violating-local-realism.html

After explaining how they closed the "locality" loophole, this article goes on to address what they call the "freedom of choice" loophole. It says "To close the freedom-of-choice loophole, the scientists spatially separated the setting choice and the photon emission, which ensured that the setting choice and photon emission occurred at distant locations and nearly simultaneously... The scientists also added a delay to Bob's random setting choice. These combined measures eliminated the possibility of the setting choice or photon emission events influencing each other."

Maybe I'm mis-reading it, but this seems like just more closure of the "locality" loophole. It doesn't really seem to address the freedom-of-choice loophole at all, which most people (including Bell) have always considered to be uncloseable, since (among other reasons) the setting choices will always share a common causal past. They must define the freedom-of-choice loophole differently than it has traditionally been defined, i.e., different than the "free choice" loophole that Bell described.

 

[Charles Wilson]

In the referenced paper, they say:

"The two other main assumptions include 'locality' and 'freedom of choice'."

I don't consider freedom of choice to be a loophole in this type of experiment. Freedom of choice could be equally invoked for ANY scientific experiment as a loophole. So I don't consider it "scientific" at all. But that is just my opinion.

Dr. Chinese:

"Not to criticize but merely to understand..." (I HATE quoting Bohr but it's funny.)

I'm going to try to walk a fine line here and not violate Forum Rules. In the Clauser interview ( http://www.aip.org/history/ohilist/25096.html ):

"Clauser:

Well, I'm throwing out locality. Keeping realism and objectivity. The cornerstones are locality and realism. So chuck one, take your pick. So I'm still a realist, and what do I have to give up if I chuck locality. Well, I have to somehow propagate signals faster than the speed of light. As soon as I do that, I automatically create the possibility of causal loops. Now, in a causal loop, A sends a signal to B— And it's all in the back of Bohm's textbook on special relativity. He has a very nice appendix in there where he describes all of this. But A sends a signal to B, B to C, C to D, D sends a signal back to A. And all of the observers are moving relative to each other, and I think that at least two of the four transmissions have to be super-luminal. And then just applying standard special relativity, A gets the answer from D before he sends the signal to B. So he's reversed the time order of these events. So he doesn't like the answer he gets from D, so he doesn't send a signal to A. So it doesn't arrive at B, so it doesn't arrive at D. So he didn't get it, so therefore he can't dislike it, so he does send it. So what does this say. Well, it says, the naive question is, "Well, does he or doesn't he send the signal." He can make the decision, "I will send the signal if I don't receive a signal from D," since that occurs in the other order. So, yes, he does, and no, he doesn't. And naively, I want to say, "Well, this is clearly absurd and impossible. It cannot happen, therefore one of our assumptions must be wrong. The only new assumption was that we could propagate super-luminal signals, therefore that must be wrong." That's the standard logic. Now, let's look at this for a second. What do we have. We have yes, he did, and no, he didn't simultaneously true. History is multi-valued! Where else did we encounter a very similar dilemma. The particle could go through the first slit, or the particle could go through the second slit, but the two are mutually exclusive, but both do occur. Well, let's wake up and smell the physics for a second. Where did we get these. We got one from quantum mechanics. That was the fact that history could have gone both ways, and in fact, must have gone both ways. The other we got from special relativity, which we got without knowing a lick about quantum mechanics. These are very different sources of exactly the same dilemma..."

I caught Hell for quoting a position I did not believe (to prove a point) but here we are again. What is it about "Choice"? the Tension here is that GenRel rules in a manifestly local manner. What counts for evidence that there is more than Locality and Realism? Clauser shows that "History is Multivalued". Doesn't he?

The Loophole discussions show that there are arguments that will support Locality Stubbornly! As I said in an earlier post, all an "Einstein" needs is, "Suppose we have an electron...". As soon as this is asserted, the EPR gang has an entrance to claim QM is "Incomplete". This is so because if it is an "Object" in Positive Space, it MUST be there when it is not observed. "And Positive Space is all we have, right?"

My position is, "Well, no. We have more than that. That's where QM comes in." ('N before you go off on me here, this is what is asserted in Born's Probabilistic Normalization (At least in ONE Physics Textbook I have...): "But the electron has to be SOMEWHERE..." and all of these possibilities sum to "1".)I think Clauser's Multi-Valued Histories argument may not be "air-tight" but his thoughts and the Loophole article cited above are pointing to the solution: "How many ways do Super-Luminal signals map onto Positive Spacetime?"

The Loophole Closures, as well as articles such as, " http://physicsworld.com/cws/article/news/2013/apr/22/spin-waves-carry-energy-from-cold-to-hot " are important! They are pointing us to consider a "something", a Symmetry Break perhaps, that will show that the Unity of the Early Universe was perhaps Super-Luminal and after the Symmetry Break, the handshake between two points to exchange information still occurs Super-Luminally, but the information exchanges occur, not Super-Luminally, but at the Speed of Light.

But I'm now into Kook Land, math notwithstanding, and for that I apologize - "Not to criticize, but merely to understand".

CW

PS: To ZapperZ and Dr. Chinese: If I have stepped way over the line here, I'll edit out the offending passages as before.

 

[DrChinese]

History is multi-valued! Where else did we encounter a very similar dilemma. The particle could go through the first slit, or the particle could go through the second slit, but the two are mutually exclusive, ...

If the wave function has physical reality, then they are not mutually exclusive. And there are other viable interpretations too.

 

[DevilsAvocado]

I don't think this result is extremely important, given that they say (my bolding):
"There is one catch, however. To ensure that more than 67% of the pairs were detected, the experiment was done in the lab with Alice, Bob and the source near to each other. As a result, this particular experiment did not simultaneously rule out the other two loopholes. According to Giustina, closing all three in a Canary Islands experiment would be extremely difficult."

This was unexpected. DM you are highly skilled and very smart (=opposite to me) and yet you come to this conclusion? What’s the premise? Thousands of advanced experiments have verified the predictions of QM and NOT ONE has shown the contrary, and all loopholes have been closed individually.

What’s the hypothesis?? Photons are (without our knowledge) “intelligent science terrorist”?? Gathering before every experiment and collectively agrees on “Today’s Plot”??

- Hey guys! Today it’s that dude Zeilinger running the silly light cone stuff! RUN LOOPHOLE #3!

Isn’t that more mind-boggling than Entanglement, FTL, Tachyons or whatever??

(Even the Pilot Wave sounds like Sunday-school compared to the “Photon Terrorist Hypothesis”! )

 

[DevilsAvocado]

What counts for evidence that there is more than Locality and Realism? Clauser shows that "History is Multivalued". Doesn't he?

I think he showed that history and science is complicated, and that not everybody understands what he’s talking about.

AFAIK, when DrC talks about 'locality' and 'freedom of choice', it’s in the concept of so called loopholes. When Clauser talks about 'locality' vs. 'realism' it’s in the concept of EPR-Bell and the options that are left to hang on to, and the potential problems with non-locality/causality.

I agree with DrC; 'freedom of choice' is a ridicules loophole and is basically the end of science (and life as we know it) if it were to be true. Every experiment about to be performed is determined in every microscopic detail. This means we could end all discussions right here – they would be pointless.

The Loophole discussions show that there are arguments that will support Locality Stubbornly!

With all due respect, IMHO it only shows that stubbornness is far more common than complete and rigorous knowledge.

 

[harrylin]

This was unexpected. DM you are highly skilled and very smart (=opposite to me) and yet you come to this conclusion? What’s the premise? Thousands of advanced experiments have verified the predictions of QM and NOT ONE has shown the contrary, and all loopholes have been closed individually.

What’s the hypothesis?? Photons are (without our knowledge) “intelligent science terrorist”?? Gathering before every experiment and collectively agrees on “Today’s Plot”??

- Hey guys! Today it’s that dude Zeilinger running the silly light cone stuff! RUN LOOPHOLE #3!

Isn’t that more mind-boggling than Entanglement, FTL, Tachyons or whatever??

(Even the Pilot Wave sounds like Sunday-school compared to the “Photon Terrorist Hypothesis”! )

An obvious hypothesis would be that we don't understand what is going on. From that follows that all common hypotheses could be completely off track, leading to false dilemmas of the kind that DrChinese poinnted out in post 13.

 

[Demystifier]

This was unexpected. DM you are highly skilled and very smart (=opposite to me) and yet you come to this conclusion? What’s the premise? Thousands of advanced experiments have verified the predictions of QM and NOT ONE has shown the contrary, and all loopholes have been closed individually.

What’s the hypothesis?? Photons are (without our knowledge) “intelligent science terrorist”?? Gathering before every experiment and collectively agrees on “Today’s Plot”??

I am not saying that all the existing evidence for quantum non-locality is not convincing. It certainly is. What I am saying is that, given all the already existing evidence (including a closed fair sampling loophole with charged particles), this single particular paper does not increase the quality of the overall evidence dramatically. When (and I am not saying "if", but "when") one day experimentalists do find a way to close all 3 loopholes simultaneously, then it will be a much more dramatic increase of the evidence quality.

 

[DevilsAvocado]

I am not saying that all the existing evidence for quantum non-locality is not convincing. It certainly is.

Thanks DM, apologies for my misinterpretation.

 

[DevilsAvocado]

An obvious hypothesis would be that we don't understand what is going on. From that follows that all common hypotheses could be completely off track, leading to false dilemmas of the kind that DrChinese poinnted out in post 13.

Could be, however there’s a ‘small’ problem within that logic. Mathematically we do understand exactly what’s going on, and not only that – it’s 100% compatible with the predictions of QM. And as we all know; QM is a “neat little theory” with precision equal to measuring the distance between L.A. and NYC with the accuracy of a human hair.

So, if someone is claiming that Entanglement/EPR-Bell is wrong because “we don't understand what is going on” – this person is also claiming that QM is wrong, which naturally would be a quite ‘meaty claim’.

I have no scientific backing – but my gut feeling is that the “Photon Terrorist Hypothesis” won’t be enough here, you’d have to rewrite the entire map between L.A. and NYC!

 

[harrylin]

Could be, however there’s a ‘small’ problem within that logic. Mathematically we do understand exactly what’s going on, and not only that – it’s 100% compatible with the predictions of QM. And as we all know; QM is a “neat little theory” with precision equal to measuring the distance between L.A. and NYC with the accuracy of a human hair.

So, if someone is claiming that Entanglement/EPR-Bell is wrong because “we don't understand what is going on” – this person is also claiming that QM is wrong, which naturally would be a quite ‘meaty claim’. [..]

No, not at all. Very different interpretations are possible (and already many abound) concerning the exact same predictions of what will be observed.

 

[DevilsAvocado]

No, not at all. Very different interpretations are possible (and already many abound) concerning the exact same predictions of what will be observed.

Philosophical interpretations do not prove anything, and as you say they must provide “the exact same predictions”.

There’s nothing stopping me from presenting a brand new philosophical interpretation - “The Photon Terrorist Interpretation”. It’s 100% QM compatible, but with the small caveat that according to my intellectual masterpiece – Little Green Men on Mars are playing a practical joke on us by controlling the photons remotely, and hence make them act like ‘intelligent science terrorist’, creating a lot of strange puzzles here on Earth, that we really shouldn’t care that much about...

[Of course I will never do this! (Because NASA is a big threat!)]

Unless you can’t prove (experimentally) that gazillion split universes or the pilot wave exists, it’s just a ‘philosophical twist’ on things – not empirical science/physics.

IMHO, Matt Leifer’s comment on Sean Carroll’s blog is absolutely brilliant, dealing with this issue:

http://www.preposterousuniverse.com/blog/2013/01/17/the-most-embarrassing-graph-in-modern-physics/

[...] The moral of this is that, if you believe what I am saying, then you shouldn’t stop at coming up with ideas of how to think about quantum theory. Once you have had those ideas you should see what changes to quantum theory are natural from that point of view and what predictions you can make. This is the only way we can hope to separate interpretations.
[...]
Adding things to the theory ad hoc that are not really independent of the rest of the physics could be the source of the problem. Therefore, many-worlds advocates should look at this again and figure out what current ideas about observables and probabilities in many-worlds theory tell us about what should happen in these modified theories.

 

[harrylin]

Philosophical interpretations do not prove anything, and as you say they must provide “the exact same predictions”. [..]

"Entanglement" as discussed in that paper ("correlations stronger than this limit, [Bell] reasoned, could only occur if the photons were entangled as defined by quantum mechanics") is what you call a "philosophical interpretation"; that's the opposite of "shut up and calculate".

And yes, I also like Matt Leifer’s comment.

 

[DevilsAvocado]

"Entanglement" as discussed in that paper ("correlations stronger than this limit, [Bell] reasoned, could only occur if the photons were entangled as defined by quantum mechanics") is what you call a "philosophical interpretation"; that's the opposite of "shut up and calculate".

Maybe it’s me - lost in translation – because I don’t understand...

Are you saying that I’m claiming that Bell's inequality is a philosophical interpretation? If so, I’m absolutely not.

Or are you claiming that Bell's inequality is a philosophical interpretation? If so, I’m afraid you’re incorrect – it’s 100% math that easily can be confirmed.

Axiom1: When entangled photons are measured along perfectly parallel alignments, they are always 100% correlated. And this is 100% compatible with QM theory and experiments.

Debunk1: This is a hoax! Exactly the same thing happens to red & blue marbles in a black box, when you pick up the red you know the one left in the box must be blue!

Axiom2: If we use Bell’s brilliant idea – to get out of the black box of marbles – and allow the marbles to obtain any color in the rainbow spectrum on verification, we can mathematically set an upper limit for the “color correlations” between the two marbles; if the correlation ought to be explained within the classical domain. And the extremely simple mathematical result we obtain proves that from a classical point of view 1 + 1 (must naturally) = 2. However, this is not what QM predicts, where 1 + 1 = 3, which is also empirical confirmed in every performed experiment this far. (i.e. philosophers are not even allowed in the dining room ;)

Debunk2: This is also a hoax! All loopholes are not closed simultaneously!

Debunk-Debunk2: Well, there are no loopholes in QM theory and all individually closed loopholes will one day be closed simultaneously. End of story.

Clever twist: This can all be explained in an almost classical manner if we accept that there are “things” out there, which no one has ever seen or measured, that will turn the experimental data upside-down! Let’s call it interpretations! (i.e. philosophers heaven ;)

 

[zonde]

I don't think this result is extremely important

Surely it is.
It's not too difficult to believe that different particles can behave differently in different experiments. It's much harder to believe that the same particle behaves differently in different experiments.

So hopefully more effort will be used to check results/repeat/improve this experiment than to come up with loophole free experiment.

 

[DevilsAvocado]

Philosophers are not even allowed in the dining room.

 

[Demystifier]

Surely it is.
It's not too difficult to believe that different particles can behave differently in different experiments. It's much harder to believe that the same particle behaves differently in different experiments.

OK, I can accept that argument.

 

[billschnieder]

It's much harder to believe that the same particle behaves differently in different experiments.

It is? You know somebody who successfully measured the same particle in different experiments?

 

[Demystifier]

It is? You know somebody who successfully measured the same particle in different experiments?

Stern and Gerlach?

 

[billschnieder]

Really? I googled "Stern Gerlach particle recovery system" and came up empty. Won't they need one in order to measure the "same particle" in different experiments.

 

[lugita15]

Surely it is.
It's not too difficult to believe that different particles can behave differently in different experiments. It's much harder to believe that the same particle behaves differently in different experiments.

So hopefully more effort will be used to check results/repeat/improve this experiment than to come up with loophole free experiment.

zonde, if I recall correctly you have been relying on the fair sampling loophole for photons to escape Bell's theorem. So has this result convinced you to abandon local realism, or are you going to wait for more confirmation?

 

[harrylin]

Maybe it’s me - lost in translation – because I don’t understand...

Are you saying that I’m claiming that Bell's inequality is a philosophical interpretation? If so, I’m absolutely not.
[..]

The fact that we don't understand each other here is a strong indicator that the topic is highly philosophical. I was referring to the paper which addresses interpretations; the issue is not so much what will be measured, but "what really happens". That kind of thing is often called philosophy, as it is non-verifiable. We can only verify what is expected to be measured. Bell's inequality is not philosophical, but claims about physical reality are usually considered to be philosophical.

 

[lugita15]

The fact that we don't understand each other here is a strong indicator that the topic is highly philosophical. I was referring to the paper which addresses interpretations; the issue is not so much what will be measured, but "what really happens". That kind of thing is often called philosophy, as it is non-verifiable. We can only verify what is expected to be measured. Bell's inequality is not philosophical, but claims about physical reality are usually considered to be philosophical.

But the thing is, unlike many other things in science, Bell's theorem is intended to go beyond mere experimental observation, and into physical reality itself. When the EPR paradox came out, different people came up with different explanations of the reality behind EPR correlations.

But Bell's goal was to say, "If the experimental predictions of quantum mechanics are all correct, then certain beliefs about physical reality cannot possibly be correct." Now you may disagree about the extent to which Bell achieved his goals (in which case I'm curious how and why), but I hope you at least agree that that's what's at stake in arguments about Bell.

 

[harrylin]

But the thing is, unlike many other things in science, Bell's theorem is intended to go beyond mere experimental observation, and into physical reality itself. [..]

But Bell's goal was to say, "If the experimental predictions of quantum mechanics are all correct, then certain beliefs about physical reality cannot possibly be correct." Now you may disagree about the extent to which Bell achieved his goals (in which case I'm curious how and why), but I hope you at least agree that that's what's at stake in arguments about Bell.

Yes, exactly. He proved that the classical models that people were using and considering - even the concepts they had in common - were incompatible with QM. That is not yet philosophical.
But his theorem goes even beyond that, as it makes a claim about the nature of physical reality itself. IMHO that goes too far, as it suggests that we can draw conclusions about all possible solutions, even ones that we have not considered or can't imagine.

 

[audioloop]

But his theorem goes even beyond that, as it makes a claim about the nature of physical reality itself. IMHO that goes too far.

to avoid a sterile discussion, just call it counterfactual definiteness. CFD for short.
.

 

[lugita15]

Yes, exactly. He proved that the classical models that people were using and considering - even the concepts they had in common - were incompatible with QM. That is not yet philosophical.
But his theorem goes even beyond that, as it makes a claim about the nature of physical reality itself. IMHO that goes too far

harrylin, do you agree that if quantum mechanics is experimentally correct, then physical reality cannot possibly obey both counterfactual definiteness and locality (excluding superdeterminism)? If so, how is that not a statement with philosophical significance?

as it suggests that we can draw conclusions about all possible solutions, even ones that we have not considered or can't imagine

I'm not sure what you mean by "all possible solutions." Do you mean "all possible experimental situations" or "all predictions of quantum mechanics"?

 

[Nugatory]

But his theorem goes even beyond that, as it makes a claim about the nature of physical reality itself.

I don't see it that way.

Bell's theorem makes (and proves, to the extent that we can say that any mathematical theorem is proven) a claim about the predictions of a particular class of physical theories; that class is defined by a particular set of common assumptions.

We can ask whether some candidate theory does or does not make those assumptions; and the experimentalists can tell us whether or not these predictions are born out by experiment, and with what level of confidence.

We don't end up with a claim about physical reality until we've brought all three elements together.

 

[DevilsAvocado]

But his theorem goes even beyond that, as it makes a claim about the nature of physical reality itself. IMHO that goes too far, as it suggests that we can draw conclusions about all possible solutions, even ones that we have not considered or can't imagine.

I think lugita15 & Nugatory did a splendid job showing you where you fall short, and frankly I think you go too far in claiming what Bell said and not. In fact he was very precautious not to mention any specific theory and thus making his claim as general as possible. Check it out yourself:

J.S. Bell's Concept of Local Causality
http://arxiv.org/abs/0707.0401

 

[gespex]

Pardon my ignorance, but how did they know they measured 75% of the entangled photons? Even if they can show they measured 75% of the photons, they can't say for sure they'll measure 75% of the *entangled* photons without assuming fair sampling. And the only way to know which photons got entangled, you have to measure them both and count the joint detection rate, right?
So how did they show they measured 75% of the entangled photons?

 

[DevilsAvocado]

Even if they can show they measured 75% of the photons, they can't say for sure they'll measure 75% of the *entangled* photons without assuming fair sampling. And the only way to know which photons got entangled, you have to measure them both and count the joint detection rate, right?

If you could give an example of any physical experiment that can effectively count 100% and report only 75% efficiency, that would be quite nice...

http://arxiv.org/abs/1212.0533]We[/PLAIN] estimate the number of produced pairs to N = 24.2⋅10⁶ per applied setting, yielding a normalized violation of J/N = –0.00524 (± 0.00008).

 

[lugita15]

In fact he was very precautious not to mention any specific theory and thus making his claim as general as possible. Check it out yourself:

J.S. Bell's Concept of Local Causality
http://arxiv.org/abs/0707.0401

I actually happen to think that Travis Norsen (and I suppose Bell, insofar as Travis is quoting him), is making his claim too general. Travis thinks that Bell's theorem allows you to reject locality, while I think that it's only the combination of locality and counterfactual definiteness that can be rejected using Bell. See my post here, in a thread about his views of Bell's theorem. Here is his argument for why counterfactual definiteness is unnecessary for Bell's theorem.

 

[nightlight]

Pardon my ignorance, but how did they know they measured 75% of the entangled photons? Even if they can show they measured 75% of the photons, they can't say for sure they'll measure 75% of the *entangled* photons without assuming fair sampling. And the only way to know which photons got entangled, you have to measure them both and count the joint detection rate, right?
So how did they show they measured 75% of the entangled photons?

That was single channel experiment (from CH inequalities) i.e. they discard half the photons on each side to begin with, before the additional explicit losses (25%) are compounded on the remaining channel on each side. It's a "little bit" misleading to claim you can fix the detection problem of the 2-channel (CHCS inequalities) experiment by simply dropping half of the data upfront, without making additional assumptions about the ignored half. E.g. in a 2-channel experiment, the half of the events which are deliberately dropped in 1-channel experiment, can result in elimination of some events that 1-channel experiment accepts as valid counts (such as double detection events on one or both sides of apparatus which corresponds to 2 PDC pairs; hence the 1-channel experiment assumes that no such multi-pair events would have occurred had they measured the ignored channel).

See preprint (latest V34) for analysis of this and similar recent experiments.

Generally, it was already shown in a series of papers by Marshall, Santos and coworkers (see ref. [22] in the above above preprint) that PDC experiment cannot produce genuine violations of classicality i.e. there is a classical Stochastic ED (SED) model which replicates PDC counts. Basically, PDC pairs, being generated by Poissonian hence classical source, laser, are Poissonian themselves, hence reproducable by classical/positive & non-singular Glauber-Sudarshan distribution. The mentioned assumption in the 1-channel experiments postulates that there are no multiple PDC pairs contradicts the already known (theoretically and experimentally) statistical properties of the PDC sources. The source of their 'enhancement' (resulting in 'apparently' better rate of pair detection than in 2-channel experiment) are precisely these multi-photon events on the remaining channel.

 

[DevilsAvocado]

while I think that it's only the combination of locality and counterfactual definiteness that can be rejected using Bell. See my post here, in a thread about his views of Bell's theorem.

Okay, I posted the reply in that thread.

 

[harrylin]

harrylin, do you agree that if quantum mechanics is experimentally correct, then physical reality cannot possibly obey both counterfactual definiteness and locality (excluding superdeterminism)? If so, how is that not a statement with philosophical significance?

Different people give different meaning to those concepts, which is perhaps why they are often held to be philosophical concepts. That's what I meant, I disagree with the suggestion that the underlying topic is not philosophical.

I'm not sure what you mean by "all possible solutions." Do you mean "all possible experimental situations" or "all predictions of quantum mechanics"?

Neither. Bell made a claim about all possible hypothetical solutions. I don't think that such a sweeping claim is warranted, as it includes anything that he could not imagine.

 

[DevilsAvocado]

Bell made a claim about all possible hypothetical solutions.

Reference please [because this is just dead wrong].

 

[harrylin]

Reference please [because this is just dead wrong].

Sure - it's always good to re-read those texts. For example "Bertlman's socks and the nature of reality". His theorem is a sweeping claim of "cannot be explained" about a class of possible models, without limiting its application to known models:

"the following argument will not mention particles, nor indeed fields, nor any particular picture of what goes on at the microscopic level. [..] The difficulty is not created by any such picture or terminology. It is created by the predictions about the correlations in the visible outputs of certain conceivable experimental set-ups.
[..]
"certain particular correlations, realizable according to quantum mechanics, are locally inexplicable. They cannot be explained, that is to say, without action at a distance."

 

[DevilsAvocado]

Bell made a claim about all possible hypothetical solutions.

Sure - it's always good to re-read those texts. For example "Bertlman's socks and the nature of reality". His theorem is a sweeping claim of "cannot be explained" about a class of possible models, without limiting its application to known models:

"the following argument will not mention particles, nor indeed fields, nor any particular picture of what goes on at the microscopic level. [..] The difficulty is not created by any such picture or terminology. It is created by the predictions about the correlations in the visible outputs of certain conceivable experimental set-ups.
[..]
"certain particular correlations, realizable according to quantum mechanics, are locally inexplicable. They cannot be explained, that is to say, without action at a distance."

[my bolding]

If you don’t see your problem here my friend, I’m afraid I can’t help you.

Healthy scientists normally wish their claims to be as generally valid as possible. Very few publish theorems that start: - This idea only works on Mondays, between 4 & 5 p.m., and only if you look the other way. Otherwise it’s perfectly rock-solid!

The only thing you’ve got is the last sentence “cannot be explained, that is to say, without action at a distance”. It can be explained by non-separability, but this doesn’t change the mathematical theorem one single bit.

Words are words and mathematics is mathematics – the old Local Realism has gone fishing in the Norwegian fjords together with the Blue Parrot. Period.

 

[lugita15]

Different people give different meaning to those concepts, which is perhaps why they are often held to be philosophical concepts. That's what I meant, I disagree with the suggestion that the underlying topic is not philosophical.

Well, let me tell you exactly what I mean by these concepts:
1. Locality: An event can only influence things in its future light cone.
2. Counterfactual definiteness: If you make a given measurement, it is always meaningful to ask "What result would you have gotten if you had made this measurement instead?", and there exist a definite (although possibly unknowable) answer to this question.
3. No-Conspiracy Condition: The answer to the question "What result would you get if you make this measurement" is independent of what measurement you actually choose to make.

According to Bell's theorem, if Quantum Mechanics is always right in its experimental predictions, then physical reality cannot possibly obey all 3 conditions. Do you agree or disagree with this statement? And do you agree that it makes a firm claim about physical reality, and not just empirical observations?

Neither. Bell made a claim about all possible hypothetical solutions. I don't think that such a sweeping claim is warranted, as it includes anything that he could not imagine.

By "hypothetical solutions" do you mean hypothetical explanations of observed quantum mechanical phenomena? In that case, yes, Bell did indeed make a claim about all possible explanations, including ones that he presumably could not conceive of; see my statement above.

But there is nothing wrong with such a sweeping claim. Are you familiar with Cantor's proof that there are more real numbers than natural numbers? It starts with the assumption, suppose you had some method, however complicated or unimaginable, to make a one-to-one correspondence between the real numbers and the natural numbers. Then Cantor showed that there would exist a real number which did not map to any natural number, so that method would be unable to make to make such a 1-to-1 correspondence. How was Cantor able to reason about really clever potential methods of counting the real numbers, methods that he never even thought of or imagined? That's the power of proof by contradiction.

Bell's proof works in the same way. It says, assume that reality obeys certain properties. Then you can show that reality must also obey this other property, and this other property implies that quantum mechanics is not always experimentally correct.

 

[harrylin]

[my bolding]

If you don’t see your problem here my friend, I’m afraid I can’t help you. [..]

I agree with your bolding... so, the same to you my friend.

 

[harrylin]

The discussion here was increasingly becoming like the thread on scholarpedia, and I think that it's better to stop drifting away here; I may rejoin the discussion there, which is indeed more general. However, still a few clarifications:

Well, let me tell you exactly what I mean by these concepts:
1. Locality: An event can only influence things in its future light cone.
2. Counterfactual definiteness: If you make a given measurement, it is always meaningful to ask "What result would you have gotten if you had made this measurement instead?", and there exist a definite (although possibly unknowable) answer to this question.
3. No-Conspiracy Condition: The answer to the question "What result would you get if you make this measurement" is independent of what measurement you actually choose to make.

According to Bell's theorem, if Quantum Mechanics is always right in its experimental predictions, then physical reality cannot possibly obey all 3 conditions. Do you agree or disagree with this statement?

Localty implies even more than you indicate, but that's not important IMHO. However the requirement called counterfactual definiteness appears to be stronger than how you present it here; this was discussed in an earlier thread and it appears that devilsavocado now brings it up again in the thread on scholarpedia ( https://www.physicsforums.com/showthread.php?t=592086&page=28 ). I have problems with understanding all the implications of that requirement, and thus I don't know what to conclude form Bell's theorem.

And do you agree that it makes a firm claim about physical reality, and not just empirical observations?

That was the issue that I brought up!

By "hypothetical solutions" do you mean hypothetical explanations of observed quantum mechanical phenomena? In that case, yes, Bell did indeed make a claim about all possible explanations, including ones that he presumably could not conceive of; see my statement above.

OK, at least we agree on that (contrary to Devilsavocado).

But there is nothing wrong with such a sweeping claim. [..] Cantor showed that there would exist a real number which did not map to any natural number, so that method would be unable [..] to make such a 1-to-1 correspondence. How was Cantor able to reason about really clever potential methods of counting the real numbers, methods that he never even thought of or imagined? That's the power of proof by contradiction.

Bell's proof works in the same way. It says, assume that reality obeys certain properties. Then you can show that reality must also obey this other property, and this other property implies that quantum mechanics is not always experimentally correct.

There is a big difference: Cantor did not try to prove a negative, while Bell did. Bell's proof appears indeed to include certain assumptions about models of reality, despite the claim that his argument did not "mention particles, nor indeed fields, nor any particular picture of what goes on at the microscopic level". But in practice, no physical assumption about reality can be made without any models of reality.

 

[lugita15]

Localty implies even more than you indicate, but that's not important IMHO.

What is the more general conception of locality you have in mind? Regardless, it's possible to give a proof of Bell's theorem with this meager definition of locality: events can only influence events within their future light cone.

However the requirement called counterfactual definiteness appears to be stronger than how you present it here; this was discussed in an earlier thread and it appears that devilsavocado now brings it up again in the thread on scholarpedia ( https://www.physicsforums.com/showthread.php?t=592086&page=28 ).

I have problems with understanding all the implications of that requirement, and thus I don't know what to conclude form Bell's theorem.

Yes, I just replied to DevilAvocado. To sum up, in principle the term "counterfactual definiteness" COULD refer to something more general, but for the purposes of Bell's theorem all we need is the meaningfullness of asking what a measurement that you didn't make would yield if you had made it.

OK, at least we agree on that (contrary to Devilsavocado).

I think DevilsAvocado may have been having a semantic disagreement with you. He may have been saying that you were incorrect to call it a "claim" as opposed to a proven theorem.

There is a big difference: Cantor did not try to prove a negative, while Bell did.

Cantor did try to prove a negative. He said that no attempt to make a one-to-one correspondence between the natural numbers and the real numbers can possibly work. Similarly, Bell said that no attempt to make a local realistic (non-superdeterministic) explanation of the experimental predictions of quantum mechanics can possibly work.

Bell's proof appears indeed to include certain assumptions about models of reality, despite the claim that his argument did not "mention particles, nor indeed fields, nor any particular picture of what goes on at the microscopic level". But in practice, no physical assumption about reality can be made without any models of reality.

Can you elaborate on why you think this?

Anyway, let me just ask you point blank: do you agree that if Quantum Mechanics is always right in its experimental predictions, then physical reality cannot possibly obey all 3 conditions I specified in post 47, using the definitions I provided for them? If you disagree, I can try to write you a proof.