Another loophole-free test of Bell's theorem

[DrChinese]

This just showed up from a team led by Zeilinger, for those interested in loophole-free Bell tests:

http://arxiv.org/abs/1511.03190

A significant-loophole-free test of Bell's theorem with entangled photons

Marissa Giustina, Marijn A. M. Versteegh, Soeren Wengerowsky, Johannes Handsteiner, Armin Hochrainer, Kevin Phelan, Fabian Steinlechner, Johannes Kofler, Jan-Ake Larsson, Carlos Abellan, Waldimar Amaya, Valerio Pruneri, Morgan W. Mitchell, Joern Beyer, Thomas Gerrits, Adriana E. Lita, Lynden K. Shalm, Sae Woo Nam, Thomas Scheidl, Rupert Ursin, Bernhard Wittmann, Anton Zeilinger

(Submitted on 10 Nov 2015)

"Local realism is the worldview in which physical properties of objects exist independently of measurement and where physical influences cannot travel faster than the speed of light. Bell's theorem states that this worldview is incompatible with the predictions of quantum mechanics, as is expressed in Bell's inequalities. Previous experiments convincingly supported the quantum predictions. Yet, every experiment performed to date required assumptions that provide loopholes for a local realist explanation. Here we report a Bell test that closes the most significant of these loopholes simultaneously. Using a well-optimized source of entangled photons, rapid setting generation, and highly efficient superconducting detectors, we observe a violation of a Bell inequality with high statistical significance."

 

[DrChinese]

And apparently today was as well another team's choice to post their results:

http://arxiv.org/abs/1511.03189

A strong loophole-free test of local realism

Lynden K. Shalm, Evan Meyer-Scott, Bradley G. Christensen, Peter Bierhorst, Michael A. Wayne, Martin J. Stevens, Thomas Gerrits, Scott Glancy, Deny R. Hamel, Michael S. Allman, Kevin J. Coakley, Shellee D. Dyer, Carson Hodge, Adriana E. Lita, Varun B. Verma, Camilla Lambrocco, Edward Tortorici, Alan L. Migdall, Yanbao Zhang, Daniel R. Kumor, William H. Farr, Francesco Marsili, Matthew D. Shaw, Jeffrey A. Stern, Carlos Abellán, Waldimar Amaya, Valerio Pruneri, Thomas Jennewein, Morgan W. Mitchell, Paul G. Kwiat, Joshua C. Bienfang, Richard P. Mirin, Emanuel Knill, Sae Woo Nam

(Submitted on 10 Nov 2015)

"We present a loophole-free violation of local realism using entangled photon pairs. We ensure that all relevant events in our Bell test are spacelike separated by placing the parties far enough apart and by using fast random number generators and high-speed polarization measurements. A high-quality polarization-entangled source of photons, combined with high-efficiency, low-noise, single-photon detectors, allows us to make measurements without requiring any fair-sampling assumptions. Using a hypothesis test, we compute p-values as small as 5.9×10−9 for our Bell violation while maintaining the spacelike separation of our events. We estimate the degree to which a local realistic system could predict our measurement choices. Accounting for this predictability, our smallest adjusted p-value is 2.3×10−7. We therefore reject the hypothesis that local realism governs our experiment."

 

[DrChinese]

There is some overlap in the teams for the above papers. We have already had some threads talking about similar results published by another team, which also features some cross-members.

http://arxiv.org/abs/1508.05949

 

[ZapperZ]

So the caveat here is the definition of "loophole free". It appears that they are using the criteria of minimal set of assumption set out by Larsson (Ref. 16) to define what is meant by "loophole free". Is this stronger than the most recent claim made by Hensen et al.? It doesn't appear to be that way to me.

Zz.

 

[DrChinese]

My post 3 is a reference to the Hensen et al paper, which was the first of the 3. Morgan W. Mitchell was a member of all 3 teams. Larsson (who you mentioned) is a member of the Zeilinger team.

So I think they are all using similar criteria. The main loopholes addressed are the locality, fair sampling and freedom of choice loopholes. (Freedom of choice meaning that the measurement settings are selected independently at times and in a manner at which they cannot affect the outcomes in a local realistic world.)

 

[Mentz114]

This just showed up from a team led by Zeilinger, for those interested in loophole-free Bell tests:

http://arxiv.org/abs/1511.03190

A significant-loophole-free test of Bell's theorem with entangled photons

They find the probability of getting their value for the J statistic from local realism is of the order 10^-31. Effectively zero.

Amazing.

It's good this so well understood otherwise a lot of heads would be getting scratched (irony).

 

[Heinera]

All of the experiments linked to in this thread simultaneously close the detection loophole, coincidence loophole, and the locality loophole. The remaining "loophole", superdeterminism (or non-freedom of choice), can't be closed by experiment anyway. It's a purely philosophical objection to Bell's theorem.

All in all, it's quite a year for Bell experiments.

 

[akhmeteli]

Looks like all of these three groups explicitly assume that measurement outcomes are fixed at some point. This assumption seems to be in contradiction with unitary evolution of quantum theory. As far as I understand, if unitary evolution is correct, measurement outcome, strictly speaking, can never be final. Does that mean they eliminate local realism assuming quantum theory is wrong?

 

[nightlight]

Interestingly, these two new claims (apparently choreographed to appear the same day) came out a day before the fatal signalling loophole was identified in the raw data of the most recent "loophole free" test from August 2015. The new claims haven't provided the raw data yet, hence it would be wise to hold off on the champagne.

 

[DrChinese]

... the fatal signalling loophole was identified in the raw data of the most recent "loophole free" test...

I would not call this analysis convincing enough to be fatal. I am not sure it means anything - I suspect a firm rebuttal from the authors is likely quite soon. Unfortunately, the Hensen paper has a relatively small data set as it takes a longer time to accumulate each heralded event.

But true enough, all 3 of these experiments are new and therefore subject to peer scrutiny.

 

[Heinera]

There is no "signaling loophole" in Bell tests. If someone tries to explain the results by invoking FTL signaling, then obviously the underlying model is not local, so local realistic models have anyway been ruled out. In this particular paper the author notes that the randomized settings of the angles should be independent of whether the entanglement swap was successful or not (aka heralded; these events are recorded with space-like separation), so the number of successful swaps should be approximately equal for all of the four settings combinations. In the Hensen et.al. experiment, they are rather unequal. I did a chi-square test of this with a null hypothesis of a uniform distribution, and got a p-value of 0.046. But no one in their right mind would reject the correctness of special relativity based on a p-value of 4.6 % in a single experiment.

 

[Mentz114]

There is no "signaling loophole" in Bell tests. If someone tries to explain the results by invoking FTL signaling, then obviously the underlying model is not local, so local realistic models have anyway been ruled out.

Seems logical.

In this particular paper the author notes that the randomized settings of the angles should be independent of whether the entanglement swap was successful or not (these events are recorded with space-like separation), so the number of successful swaps should be approximately equal for all of the four setting combinations.

Surely this must be true unconditionally, as a sign that the experimental setup is behaving as expected.

In the Hensen et.al. experiment, they are rather unequal. I did a chi-square test of this with a null hypothesis of a uniform distribution, and got a p-value of 0.046. But no one in their right mind would reject the correctness of special relativity based on a p-value of 4.6 % in a single experiment.

That is a big deviation from what one would expect. Either we have a rare-event run ( odds about 20:1 against) or the setup is flawed. It must reduce confidence in the hypothesis that the inequality is violated ( also about 20:1 with the 'uncorrected' data).

More data is needed. A lot more.

 

[Heinera]

That is a big deviation from what one would expect. Either we have a rare-event run ( odds about 20:1 against) or the setup is flawed. It must reduce confidence in the hypothesis that the inequality is violated ( also about 20:1 with the 'uncorrected' data).

More data is needed. A lot more.

Yes. But given what we know about the experimental setup, it is hard to come up with a plausible flaw that could generate this kind of dependence. Which makes the "rare-event run" hypothesis that more edible.

(The author of the paper mentioned earlier in the thread speculates about FTL signaling as an explanation. This is of course even less likely than all other alternatives, since in that case, the anomaly could actually be used to facilitate FTL communication).

 

[nikman]

The remaining "loophole", superdeterminism (or non-freedom of choice), can't be closed by experiment anyway. It's a purely philosophical objection to Bell's theorem.

Don't overlook Kaiser et al's cosmic photons proposal. I believe Zeilinger (among others) has also been thinking along those lines. So anyway are we entering an era when metaphysical issues will be settled by experiment? It's already hard enough for philosophy postdocs to find careers.

 

[zonde]

And apparently today was as well another team's choice to post their results:

http://arxiv.org/abs/1511.03189

A strong loophole-free test of local realism

I like how seriously these experimenters approached the question of RNG. They XORed outputs from two different QRNG and pseudorandom source (!).
For pseudorandom number source they used different data ... including movie Back to the Future

 

[f95toli]

So the caveat here is the definition of "loophole free". It appears that they are using the criteria of minimal set of assumption set out by Larsson (Ref. 16) to define what is meant by "loophole free".

Hensen has always been very careful about stating exactly what they measure and what they can/can not rule out. He gave a talk at a conference I attended recently (Quantum Information Processing and Communication in Leeds, UK) and he spent about a 1/3 of his talk talking about potential loopholes. However, most of these are philosophical and/or would violate SR; as far as I can remember there were no "serious" loopholes in their experiment.

 

[gill1109]

Looks like all of these three groups explicitly assume that measurement outcomes are fixed at some point. This assumption seems to be in contradiction with unitary evolution of quantum theory. As far as I understand, if unitary evolution is correct, measurement outcome, strictly speaking, can never be final. Does that mean they eliminate local realism assuming quantum theory is wrong?

That's right. They all assume that measurement outcomes are fixed at some quite definite point. They also assume that randomly determined measurement settings were also fixed at a slightly earlier definite point.

If you believe in the many world theory then (it seems to me) you do not believe in any ordinary reality at all. There are no "measurement outcomes". Not ever. They are somehow illusory. Many world theorists do not have any problem with non-locality because there is no classical world where measurements actually have outcomes.

In fact the pages of the journals where the results are published do not exist either because they remain forever in quantum superposition with the pages which would have been published if the measurement outcomes had been different.

 

[akhmeteli]

That's right. They all assume that measurement outcomes are fixed at some quite definite point. They also assume that randomly determined measurement settings were also fixed at a slightly earlier definite point.

If you believe in the many world theory then (it seems to me) you do not believe in any ordinary reality at all. There are no "measurement outcomes". Not ever. They are somehow illusory. Many world theorists do not have any problem with non-locality because there is no classical world where measurements actually have outcomes.

As one can see from my posts here or from my articles, I have nothing to do with many worlds. And I believe there is reality out there (at least I don't think there is a valid reason to doubt it:-) ). Ordinary reality? I guess it depends on how you define "ordinary":-)

In fact the pages of the journals where the results are published do not exist either because they remain forever in quantum superposition with the pages which would have been published if the measurement outcomes had been different.

Obviously, you are sarcastic here:-) Obviously, Schroedinger was sarcastic about the Shroedinger's cat:-) However, I listened to lectures of Haroche and Wineland once (It was at APS March meeting last year). Both of them mentioned the Schroedinger's cat quite seriously, and one of them said (it was Haroche, if I am not mistaken): "There is no fundamental decoherence." So which of the following do you dispute?

1). Unitary evolution, strictly speaking, is not compatible with definite outcomes of measurements.
2). Unitary evolution is always correct (meaning it will never be in disagreement with experimental results).

If you dispute 1), I'd like to hear your arguments. If you dispute 2), then, strictly speaking, you don't think quantum theory is correct. In this case, the assumptions of the Bell's theorem are not correct, so what's there to discuss?

So you may ask me, if measurement outcomes are never final, do I discard all experimental physics? No, I don't. Do I contradict myself then? I am not sure. For example, I very much like the approach of http://arxiv.org/abs/1107.2138 (published in Phys. Rep.). I interpret their results as follows (maybe they will disagree though:-)): while measurement outcomes are never final, the time required for the outcomes to change can be comparable to the Poincare's recurrence time. But I am not sure if the recurrence time is that big in the experiments that we discuss. Anyway, I believe the experimentalists' assumption of fixed outcomes is wrong if quantum theory is right. Then what did they demonstrate? That local realism is wrong assuming quantum theory is wrong?

 

[akhmeteli]

Mind you, I mean no disrespect. The experimentalists have pushed the envelope, so now we know more than we knew last year. However, we should be careful with the conclusions.

 

[Smattering]

If you believe in the many world theory then (it seems to me) you do not believe in any ordinary reality at all.

They do believe in the objective reality of the universal wavefunction, don't they?

There are no "measurement outcomes". Not ever. They are somehow illusory.

Yes, there are measurement outcomes. Each measurement is supposed to "split" the universe, and the outcome of the measurement correpsonds to one of the succeeding universes that result from the split.

Many world theorists do not have any problem with non-locality because there is no classical world where measurements actually have outcomes.

Can you please elaborate a bit more on this?

As far as I can see, non-locality does not seem any more natural with repsect to the MWI than it does with respect to the Copenhagen interpretation.

 

[atyy]

1). Unitary evolution, strictly speaking, is not compatible with definite outcomes of measurements.
2). Unitary evolution is always correct (meaning it will never be in disagreement with experimental results).

If you dispute 1), I'd like to hear your arguments. If you dispute 2), then, strictly speaking, you don't think quantum theory is correct. In this case, the assumptions of the Bell's theorem are not correct, so what's there to discuss?

The usual Bell's theorem assumes (2). The usual terminology is that quantum mechanics itself assumes (2).

Your terminology is fine for something like MWI, but that is not the "orthodox" interpretation.

 

[akhmeteli]

The usual Bell's theorem assumes (2). The usual terminology is that quantum mechanics itself assumes (2).

I agree with both of these statements. And I also agree with (2) (at least I don't see any reason to disagree with it:-)). However, I also agree with (1), whereas both the Bell theorem and standard quantum mechanics assume the projection postulate (or something similar), which is in contradiction with (1) - this is the notorious measurement problem. By the way, do you agree with (1)? If not, then what are your arguments?

Your terminology is fine for something like MWI, but that is not the "orthodox" interpretation.

Again, I don't accept MWI. And I don't understand which of my words imply the opposite. That does not mean I accept the orthodox interpretation.

 

[stevendaryl]

1). Unitary evolution, strictly speaking, is not compatible with definite outcomes of measurements.
2). Unitary evolution is always correct (meaning it will never be in disagreement with experimental results).

Philosophically, the two are incompatible, but strangely don't seem to lead to any inconsistencies in practice.

For practical purposes, the Von Neumann recipe seems to work very well:

1. Between observations, the wave function evolves unitarily.
2. After a measurement, the wave function collapses into the eigenstate of the observable corresponding to the measured eigenvalue.

The second step appears to violate unitary evolution. However, I think most people believe that you could, in principle, if not in practice, consider the evolution of "System + Measuring Device" to be unitary, as well, and push off the "collapse" until a later time. If you can always push off the collapse until later (and you can, if you're willing to consider the quantum mechanics of arbitrarily large systems), then there is no inconsistency between assuming unitary evolution (now) and definite outcomes (at some point).

 

[stevendaryl]

The usual Bell's theorem assumes (2).

I don't understand that comment. Bell's theorem doesn't assume anything about quantum mechanics; it describes a class of physical theories, and then shows that each theory in that class satisfies a certain inequality. That our world violates that inequality is not an assumption--it's demonstrable by experiment.

 

[stevendaryl]

Again, I don't accept MWI. And I don't understand which of my words imply the opposite. That does not mean I accept the orthodox interpretation.

To me, the heart of MWI is the possibility that macroscopic objects can be in superpositions. But that possibility follows from the assumption that macroscopic objects are described by the same laws of physics that describe microscopic objects.

A lot of the controversy over MWI is about whether it is possible to derive probabilities from pure unitary evolution, or even whether probabilities make any sense for a deterministic theory. Those are tough problems, but it seems to me that they are tough problems for any interpretation of QM that allows for macroscopic superpositions, and DISallowing macroscopic superposition requires going beyond standard QM. So to me, the difficult issues with MWI are difficult issues with QM, period. They shouldn't be blamed on MWI. (Although you could blame MWI for failing to solve them, I suppose)

 

[akhmeteli]

I don't understand that comment. Bell's theorem doesn't assume anything about quantum mechanics; it describes a class of physical theories, and then shows that each theory in that class satisfies a certain inequality. That our world violates that inequality is not an assumption--it's demonstrable by experiment.

I am not sure I agree. I would say the Bell theorem typically includes two parts: the first one states that some inequalities are satisfied for local realistic theories, and this part, indeed, "doesn't assume anything about quantum mechanics", but the second part states that these inequalities are violated in quantum theory (not in experiments), and this part does make assumptions about quantum mechanics, for example, it uses the assumption that the total angular momentum of the two particles of the singlet is conserved, and this is a consequence of unitary evolution. So the Bell theorem states that local realistic theories cannot reproduce all the predictions of quantum theory. Experimental demonstrations of violations of the inequalities (or lack thereof) are not a part of the Bell theorem.

 

[stevendaryl]

I am not sure I agree. I would say the Bell theorem typically includes two parts: the first one states that some inequalities are satisfied for local realistic theories, and this part, indeed, "doesn't assume anything about quantum mechanics", but the second part states that these inequalities are violated in quantum theory (not in experiments)

Yes, but in a sense, that derivation is unnecessary, since experiments prove the violation. This "second part" that you're talking about is just a calculation that shows that the experimental results are in fact predicted by QM (or at least, is predicted by a particular "recipe" for using QM).

 

[akhmeteli]

To me, the heart of MWI is the possibility that macroscopic objects can be in superpositions. But that possibility follows from the assumption that macroscopic objects are described by the same laws of physics that describe microscopic objects.

I might agree with that, but this "heart" is not everything there is to MWI, otherwise why is it called "many world interpretation"? I don't see how "many worlds" are a direct consequence of macroscopic superpositions.

 

[stevendaryl]

I might agree with that, but this "heart" is not everything there is to MWI, otherwise why is it called "many world interpretation"? I don't see how "many worlds" are a direct consequence of macroscopic superpositions.

Well, it seems to me that it is. Suppose that I prepare an electron to have spin-up in the z-direction. Then I plan to measure the electron's spin in the x-direction. I resolve ahead of time: If it's spin-up, I will go to Paris for vacation. If it's spin-down, I will go to Hawaii. Then, after I've done my measurement, and gone on my vacation, it seems that someone from a distant planet describing the whole Earth using quantum mechanics would have to describe it as a superposition of an Earth where I am in Hawaii and an Earth where I am in Paris. That's many-worlds, it seems to me.

 

[Demystifier]

So which of the following do you dispute?

1). Unitary evolution, strictly speaking, is not compatible with definite outcomes of measurements.
2). Unitary evolution is always correct (meaning it will never be in disagreement with experimental results).

I dispute 1). For instance, unitary evolution is compatible with Bohmian mechanics (BM), and BM is compatible with definite outcomes of measurements.

 

[stevendaryl]

I dispute 1). For instance, unitary evolution is compatible with Bohmian mechanics (BM), and BM is compatible with definite outcomes of measurements.

Well, in the Bohmian view, the only measurement outcome that counts is a measurement of position of some collection of particles. So even though a "spin measurement" has a definite outcome, it can't be interpreted--as it is in mainstream QM--as the particle being a state of definite spin.

 

[jerromyjon]

So even though a "spin measurement" has a definite outcome, it can't be interpreted--as it is in mainstream QM--as the particle being a state of definite spin.

Bell only says that a value at a location of an object isn't enough to contribute exclusively to its state, there has to be non local value attributed, but does that mean one value interferes with all the universe's wavefunctions in all directions or can we isolate a chuck of space-time and call it all inclusive? Entanglement swapping seems to confirm that Bell's Inequality can be observed over mind-boggling distance of space and time. It contradicts our classic deterministic existence we have known since the see-saw.

To say it is up there with the other great experimentally verified theories of how the universe functions is impressive!

 

[akhmeteli]

I dispute 1). For instance, unitary evolution is compatible with Bohmian mechanics (BM), and BM is compatible with definite outcomes of measurements.

Not quite. It is my understanding that in BM there is still some overlap, however small, of different components of the wave functions related to different values of position. I guess we discussed this question before: https://www.physicsforums.com/threa...nt-or-not-to-standard-qm.307641/#post-2167542 , https://www.physicsforums.com/threa...-entanglement-and.369328/page-15#post-2602406 , https://www.physicsforums.com/threa...-entanglement-and.369328/page-16#post-2603650 .

 

[akhmeteli]

Well, it seems to me that it is. Suppose that I prepare an electron to have spin-up in the z-direction. Then I plan to measure the electron's spin in the x-direction. I resolve ahead of time: If it's spin-up, I will go to Paris for vacation. If it's spin-down, I will go to Hawaii. Then, after I've done my measurement, and gone on my vacation, it seems that someone from a distant planet describing the whole Earth using quantum mechanics would have to describe it as a superposition of an Earth where I am in Hawaii and an Earth where I am in Paris. That's many-worlds, it seems to me.

I don't understand why this is necessarily many-worlds. It is not obvious that this superposition cannot exist in one and only world - ours. If you believe this is impossible, and, say, you cannot be in Hawaii and in Paris at the same time in principle, then maybe you don't think unitary evolution is universal and maybe you don't think the Schroedinger's cat can exist in principle. You can take such a position, but that does not mean I have to take such a position.

 

[akhmeteli]

Yes, but in a sense, that derivation is unnecessary, since experiments prove the violation. This "second part" that you're talking about is just a calculation that shows that the experimental results are in fact predicted by QM (or at least, is predicted by a particular "recipe" for using QM).

I believe this derivation is indeed necessary. Let me remind you that the Bell theorem is 50+ years old, whereas there had been no loophole-free experimental demonstrations of violations of the inequalities at least until Q3 2015:-). And it is not obvious that the recent experiments offer such demonstration, e.g., for the reason we are discussing now. So without this derivation one could say: these inequalities must be satisfied in local realistic theories? So what? That does not contradict any experiments. At least it had not contradicted until recently.The strong and necessary point of the Bell theorem was that it demonstrated incompatibility of local realistic theories with predictions of quantum theory.