Photons from separated sources can be entangled - after they were detected

[SpectraCat]

No, we can observe them so they are macroscopic. Microscopic stuff is something that we cannot directly observe (e.g., Bohmian trajectories, objective wave functions, objective collapse, absence of any objective microscopic reality, ...) so at the moment we can only speculate about it.

Ok, then I don't understand your comment about my previous post where you said that I needed to distinguish macroscopic with microscopic descriptions. All I mentioned in the post you quoted were Bell's Inequality violations and decoherence. But it doesn't really matter ... your position is clear to me now.

Incidentally, I still disagree (assuming I understand the situation correctly, which is something I am not completely sure about) ... I am working on a post to explain my (mis?) understanding in more detail

 

[DrChinese]

Incidentally, I still disagree (assuming I understand the situation correctly, which is something I am not completely sure about) ... I am working on a post to explain my (mis?) understanding in more detail

Really? I'm sitting in the bar waiting for my beers.

 

[SpectraCat]

Ok, so I have done some more reading and a lot more thinking about this, and I still can't understand how there can be any quantum teleportation if the entanglement of photons B & C occurs after A & D have been measured. This is *not* what was reported experimentally in this http://128.84.158.114/abs/quant-ph/0201134" (PRL 88, [2002] art. 017903) that we were discussing earlier. In that case, photons 1 & 2 (equivalent to B & C in Dr. Chinese's example) enter the fiber beam splitter and are entangled before 0 & 3 are measured. The only thing that is delayed is the measurement of the Bell state that 1 & 2 have been projected into, which does not reflect on the *fact* of their entanglement, only the measurement of the state. Dr. Chinese has claimed that this does not matter, and that the same teleportation would be observed if A & D are measured before B & C are entangled. I cannot see how this can be correct, and I have worked out some of my arguments mathematically below. Please let me know where my mistake lies, if there is one.

Paraphrasing equations 2 and 3 from the paper cited above, the total wavefunction is initially composed of two independent states, and can be written as:

\left|\Psi_{tot}\right\rangle = \left|\Psi^{-}_{AB}\right\rangle \otimes \left|\Psi^{-}_{CD}\right\rangle, where \left|\Psi^{-}_{xy}\right\rangle refers to the Bell state,
\left|\Psi^{-}_{xy}\right\rangle = \frac{1}{\sqrt{2}}\left[\left|H\right\rangle_{x}\left|V\right\rangle_{y} - \left|V\right\rangle_{x}\left|H\right\rangle_{y}\right],
and H and V refer to orthogonal polarization states. \Psi_{tot} can then be re-expressed in the basis of Bell states of the A/D and B/C pairs:

\left|\Psi_{tot}\right\rangle = \frac{1}{2}\left[\left|\Psi^{+}_{AD}\right\rangle \otimes \left|\Psi^{+}_{BC}\right\rangle - \left|\Psi^{-}_{AD}\right\rangle \otimes \left|\Psi^{-}_{BC}\right\rangle - \left|\Phi^{+}_{AD}\right\rangle \otimes \left|\Phi^{+}_{BC}\right\rangle + \left|\Phi^{-}_{AD}\right\rangle \otimes \left|\Phi^{-}_{BC}\right\rangle\right]

Therefore, at the moment when Charlie entangles B & C in his fiber beam splitter, the system is cast into one of the four states above, and this means that A & D must also be entangled. At some later point, the particular Bell state of B & C is measured at the detectors, and at that moment, Charlie knows which Bell state A & D are in as well.

This is all fine, but it only works if the first equation I wrote above is valid when B & C become entangled. This is not true if the measurements on A and D have already occurred. Immediately after those measurements have occurred, the total state of the system is known, that is, it has been resolved into some element of the set of separable states:

\left\{\left[\left|H\right\rangle_{A}\otimes\left|V\right\rangle_{B}\otimes\left|H\right\rangle_{C}\otimes\left|V\right\rangle_{D}\right],\:\:<br /> \left[\left|V\right\rangle_{A}\otimes\left|H\right\rangle_{B}\otimes\left|H\right\rangle_{C}\otimes\left|V\right\rangle_{D}\right],\:\:<br /> \left[\left|H\right\rangle_{A}\otimes\left|V\right\rangle_{B}\otimes\left|V\right\rangle_{C}\otimes\left|H\right\rangle_{D}\right],\:\:<br /> \left[\left|V\right\rangle_{A}\otimes\left|H\right\rangle_{B}\otimes\left|V\right\rangle_{C}\otimes\left|H\right\rangle_{D}\right]\right\}

(Note: I used the tensor product notation above to emphasize the separability, but it is just the 4 combinations: HVHV, VHHV, HVVH, VHVH)

So there is now no way to get from just one of these states to the case where there is entanglement between A & D. Note that B & C aren't entangled in this case either ... (otherwise it would be possible to generate entangled pairs from linearly polarized photons simply using beamsplitters).

Anyway, hopefully this makes my analysis and arguments clear. Have I made a deduction or math error somewhere? Note that at no point do I involve order of detection in my analysis, I only refer to the state of the system when the B & C photons enter the beamsplitter.

Finally, it is worth noting that in the paper I cited, the authors did not make the same claim that Dr. Chinese made in his post here. They claim that the space-time separation of the detection events doesn't matter, which is consistent with what they tested in their experiment.

 

[DrChinese]

... Finally, it is worth noting that in the paper I cited, the authors did not make the same claim that Dr. Chinese made in his post here. They claim that the space-time separation of the detection events doesn't matter, which is consistent with what they tested in their experiment.

Gee, I am not sure how much clearer they could have put it. And they come as close to the title of this thread as can be: "Photons from separated sources can be entangled - after they were detected!" I certainly get the feeling that you are going to great lengths to avoid buying a Cowboy fan a beer.

So here are the quotes, please reference again the attached diagram in Post 84. Those figures are from the Zeilinger article.

Article body:

"A seemingly paradoxical situation arises — as suggested by Peres [4] — when Alice’s Bellstate analysis is delayed long after Bob’s measurements. This seems paradoxical, because Alice’s measurement projects photons 0 and 3 into an entangled state after they have been measured. Nevertheless, quantum mechanics predicts the same correlations. Remarkably, Alice is even free to choose the kind of measurement she wants to perform on photons 1 and 2. Instead of a Bell-state measurement she could also measure the polarizations of these photons individually. Thus depending on Alice’s later measurement, Bob’s earlier results either indicate that photons 0 and 3 were entangled or photons 0 and 1 and photons 2 and 3. This means that the physical interpretation of his results depends on Alice’s later decision.

"Such a delayed-choice experiment was performed by including two 10 m optical fiber delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore, this result indicate [sic] that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."Figure 1: Shows diagram of setup.
"One photon from each pair is sent to Alice who subjects them to a Bell-state measurement, projecting them randomly into one of four possible entangled states. ... This procedure
projects photons 0 and 3 into a corresponding entangled state. [Bob] hands his results also to Victor, who sorts them into subsets according to Alice’s results, and checks each subset for a violation of Bell’s inequality. This will show whether photons 0 and 3 became entangled although they never interacted in the past. Interestingly, the quantum prediction for the observations does not depend on the relative space-time arrangement Alice’s and Bob’s detection events. "

Figure 3: Shows Fidelity, and Fidelity with Delayed Choice. [Note the words "delayed choice"]
"The square dots represent the fidelity for the case that Alice’s and Bob’s events are space-like separated, thus no classical information transfer between Alice and Bob can influence the results. The circular dot is the fidelity for the case, that Alice’s detections are delayed by 50 ns with respect to Bob’s detections. This means, that Alice’s measurement projects photon 0 and 3 in an entangled state, at a time after they have already been registered."

So can you point to any sentence above which makes you think that a) delayed choice version was not observed; b) entanglement did not occur; or c) the order of Alice and Bob's actions makes ANY difference to the outcome? Because it certainly seems clear to me. I don't even see in your analysis where the outcomes are different based on ordering.

 

[DrChinese]

...This is all fine, but it only works if the first equation I wrote above is valid when B & C become entangled. This is not true if the measurements on A and D have already occurred. Immediately after those measurements have occurred, the total state of the system is known, that is, it has been resolved into some element of the set of separable states:

\left\{\left[\left|H\right\rangle_{A}\otimes\left|V\right\rangle_{B}\otimes\left|H\right\rangle_{C}\otimes\left|V\right\rangle_{D}\right],\:\:<br /> \left[\left|V\right\rangle_{A}\otimes\left|H\right\rangle_{B}\otimes\left|H\right\rangle_{C}\otimes\left|V\right\rangle_{D}\right],\:\:<br /> \left[\left|H\right\rangle_{A}\otimes\left|V\right\rangle_{B}\otimes\left|V\right\rangle_{C}\otimes\left|H\right\rangle_{D}\right],\:\:<br /> \left[\left|V\right\rangle_{A}\otimes\left|H\right\rangle_{B}\otimes\left|V\right\rangle_{C}\otimes\left|H\right\rangle_{D}\right]\right\}

(Note: I used the tensor product notation above to emphasize the separability, but it is just the 4 combinations: HVHV, VHHV, HVVH, VHVH)

So there is now no way to get from just one of these states to the case where there is entanglement between A & D. Note that B & C aren't entangled in this case either ... (otherwise it would be possible to generate entangled pairs from linearly polarized photons simply using beamsplitters).

This analysis is just not true! The observation of A & D first changes nothing. You can see this is wrong quite easily. Just split the A & D observations into 2 separate events, which by your reasoning they now are. And let's consider ONLY the A & B case. According to you, A & B are now in a product state. But that is flat out incorrect, they are in the entangled state and produce statistics to match (assuming you measure B's polarization)!

Ditto with C & D. I hope you can see that the observation of A & D does NOT produce the state you describe above UNTIL, and UNLESS, and AFTER the polarizations of B & C are also measured. Which in the Zeilinger experiment they are NOT, because a Bell State Measurement is performed instead. Due to the BSM, regardless of when it occurs, A & D are projects into an entangled state and this is what is shown to violate a Bell Inequality in their Figure 3.

In reality, the context of the A & B pair measurement and the context of the C & D pair measurement is critical. CONTEXT is always critical, and must be considered carefully. Although we talk about collapse as if it is instantaneous, it really transcends that description. It cannot strictly be said to occur upon first measurement. Otherwise, the observation of the second member of an entangled pair would not lead to the violation of a Bell Inequality (because then they would be separated once the entanglement is collapsed).

 

[SpectraCat]

Gee, I am not sure how much clearer they could have put it. And they come as close to the title of this thread as can be: "Photons from separated sources can be entangled - after they were detected!" I certainly get the feeling that you are going to great lengths to avoid buying a Cowboy fan a beer.

So here are the quotes, please reference again the attached diagram in Post 84. Those figures are from the Zeilinger article.

Article body:

"A seemingly paradoxical situation arises — as suggested by Peres [4] — when Alice’s Bellstate analysis is delayed long after Bob’s measurements. This seems paradoxical, because Alice’s measurement projects photons 0 and 3 into an entangled state after they have been measured. Nevertheless, quantum mechanics predicts the same correlations. Remarkably, Alice is even free to choose the kind of measurement she wants to perform on photons 1 and 2. Instead of a Bell-state measurement she could also measure the polarizations of these photons individually. Thus depending on Alice’s later measurement, Bob’s earlier results either indicate that photons 0 and 3 were entangled or photons 0 and 1 and photons 2 and 3. This means that the physical interpretation of his results depends on Alice’s later decision.

"Such a delayed-choice experiment was performed by including two 10 m optical fiber delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore, this result indicate [sic] that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."


Figure 1: Shows diagram of setup.
"One photon from each pair is sent to Alice who subjects them to a Bell-state measurement, projecting them randomly into one of four possible entangled states. ... This procedure
projects photons 0 and 3 into a corresponding entangled state. [Bob] hands his results also to Victor, who sorts them into subsets according to Alice’s results, and checks each subset for a violation of Bell’s inequality. This will show whether photons 0 and 3 became entangled although they never interacted in the past. Interestingly, the quantum prediction for the observations does not depend on the relative space-time arrangement Alice’s and Bob’s detection events. "

Figure 3: Shows Fidelity, and Fidelity with Delayed Choice. [Note the words "delayed choice"]
"The square dots represent the fidelity for the case that Alice’s and Bob’s events are space-like separated, thus no classical information transfer between Alice and Bob can influence the results. The circular dot is the fidelity for the case, that Alice’s detections are delayed by 50 ns with respect to Bob’s detections. This means, that Alice’s measurement projects photon 0 and 3 in an entangled state, at a time after they have already been registered."

So can you point to any sentence above which makes you think that a) delayed choice version was not observed; b) entanglement did not occur; or c) the order of Alice and Bob's actions makes ANY difference to the outcome? Because it certainly seems clear to me. I don't even see in your analysis where the outcomes are different based on ordering.

Look, I am not trying to avoid anything ... but you missed the point of my argument. I am familiar with all of the quotations from the paper, and I can sketch the figures from memory, that is not the issue. What I am saying is that the entanglement and detection events are two separate parts of the BSM. To put it another way, will there be any teleportation if the fibre beam splitter in figure 2 is removed? I don't think so, because it is only by interfering inside that beam splitter that the photons 1 & 2 (and hence 0 & 3) become entangled. My entire analysis is based on that point. To put it yet another way, I am saying that I agree that detection order doesn't matter provided that the entanglement of pairs 0 & 1 and 2 & 3 still exists when 1 & 2 enter the beam-splitter. However if 0 and 3 have already been projected into definite polarization states by interacting with their detectors, then by definition, they are no longer entangled with 1 & 2, so there is nothing to teleport.

Do you somehow disagree that when one member of an entangled pair is measured, they cease to be entangled? If you do, please explain why. If you do not, please explain how the two photons with well-defined polarization states can become entangled in the specific example under consideration, that is, where 0 & 3 have been measured *before* 1 & 2 reach the fiber beam splitter. Where is the flaw in my mathematical analysis from my previous post? Please be specific. If I am wrong here, I want to know why.

Also, please don't rely on quotations from the paper, because their measurements do not address this specific case. I have absolutely no problem with the results or conclusions of that paper. My only issue is with your original example from this thread. You have not really provided any solid evidence for why you think the results of the Zeilinger paper can be extended to your example. Your vague argument about being able to "reconstruct the two-photon initial state" was far from clear, and was not supported by the reference you posted. Can you provide a more detailed explanation of what you were talking about?

 

[DrChinese]

What I am saying is that the entanglement and detection events are two separate parts of the BSM. To put it another way, will there be any teleportation if the fibre beam splitter in figure 2 is removed? I don't think so, because it is only by interfering inside that beam splitter that the photons 1 & 2 (and hence 0 & 3) become entangled. My entire analysis is based on that point. To put it yet another way, I am saying that I agree that detection order doesn't matter provided that the entanglement of pairs 0 & 1 and 2 & 3 still exists when 1 & 2 enter the beam-splitter. However if 0 and 3 have already been projected into definite polarization states by interacting with their detectors, then by definition, they are no longer entangled with 1 & 2, so there is nothing to teleport.

Do you somehow disagree that when one member of an entangled pair is measured, they cease to be entangled? If you do, please explain why.

The entanglement ends when both are measured, not before. That is a statistical fact, and I mention that in my post 105 (so you can see that for the explanation instead of me repeating).

Now, you say that there is some difference between the entanglement and the detection portions of the BSM. Could you please cite a reference for that? Because the only thing that matters is the final configuration, and everything in the Zeilinger article supports that view instead of yours. It NEVER matters what happens at a beam splitter until the final irreversible detection occurs. Certainly you must have seen any number of experiments that prove this. There are the series regarding Wheeler's Delayed Choiice, for example, of which I can provide as many papers as you like with some variation on the Wheeler quotes:

"We have no right to say what the photon is doing in all its long course from point of entry to point of detection."

"There is no experiment without a measurement result."

 

[DrChinese]

1. You have not really provided any solid evidence for why you think the results of the Zeilinger paper can be extended to your example.

Your vague argument about being able to "reconstruct the two-photon initial state" was far from clear, and was not supported by the reference you posted. Can you provide a more detailed explanation of what you were talking about?

1. They are identical, that is why. And Zeilinger says exactly what I stated, despite your assertion that they did NOT do a delayed choice as they claimed. This is a top notch reference, not really sure where you are going on this one. Do you think that any reader of the referenced paper will accept your interpretation over the interpretation of the authors?

2. The reference for this is as follows:

http://www.pas.rochester.edu/~AdvLab/Eberly_Bell_Inequalities_AJP.pdf

This in turn relies on French and Taylor, An Introduction to Quantum Physics, Section 7-2, Figures 7-1 and 7-3. See attached. I hope this settles the question.

"The experiments discussed in this chapter involve a piece of equipment we call an analyzer loop. This is a two-part device of which the first part is just an analyzer as defined in Chapter 6. The second part of the analyzer loop is a "reversed" analyzer of the same type, which recombines the beams separated by the first analyzer in such a way as to reconstruct the original beam in every detail (see Figure 7-1)."

You CAN reconstruct the input to a BS, in principle, and if it was entangled it will return to that state (if you can say that it ever was NOT entangled in the first place).

 

[yoda jedi]

I have been under the impression that the line between 'micro' and 'macro' in QM is (forgive me) fuzzy. Isn't the notion of where and when macroscopic reality emerges from quantum behaviour one of the bigger unsolved questions of any interpretation of QM?

good insight


http://www.springerlink.com/content/p57117239x631547/fulltext.pdf


....it has been suggested that decoherence, due to the interaction of the system with its environment, could provide the desired mechanism: The environment, constantly interacting with the body, could somehow act as a measuring device of
the macroscopic variables of the body (say, its center of mass) producing in this way a narrow wave function in the macroscopic directions of its configuration space But the composite system formed by the system of interest and its environment is itself a closed system, and Schrödinger’s evolution of this enlarged system tends to produce spreading of the total wave function over the total configuration space. Thus, decoherence alone is not sufficient to explain the emergence of the classical world from standard quantum mechanics.....

 

[Frame Dragger]

good insight


http://www.springerlink.com/content/p57117239x631547/fulltext.pdf


....it has been suggested that decoherence, due to the interaction of the system with its environment, could provide the desired mechanism: The environment, constantly interacting with the body, could somehow act as a measuring device of
the macroscopic variables of the body (say, its center of mass) producing in this way a narrow wave function in the macroscopic directions of its configuration space But the composite system formed by the system of interest and its environment is itself a closed system, and Schrödinger’s evolution of this enlarged system tends to produce spreading of the total wave function over the total configuration space. Thus, decoherence alone is not sufficient to explain the emergence of the classical world from standard quantum mechanics.....

Thank you! SQM is wonderful, but it's not an answer, just a partial roadmap. Nothing makes it clearer than the constant retreat from the formal view that macrocopic is macro, and microscropic is micro. PERIOD. No one really bothers to give answers as convincing as a the fundamentals of QM as to when and where this emergence occurs. I wonder if it ever does. I throw out as pure speculation, open to ridicule, that there is no emergence; that we at some point as humans and more widely as life become unable to make the distinction in a manne that is descriptive or predictive (aka valid).

We keep measuring quantum systems with fundamentally classical measuring devices, (for those TCI and related folks) and that is a crippling limitation I don't know that we can overcome.

EDIT: I have it! You know that ridiculous add for text messaging answers to 'any' question (KGB... very tasteful) and ask them all about this. I understand the answer is guaranteed. We could merge this thread with the general topics 'what is the hardest question to ask a qunantum physicist" and make a new thread, "The nastiest physics questions we believe a text answers service can't asnwer with google. lol. lol. haha.". It's a thought.

 

[SpectraCat]

The entanglement ends when both are measured, not before. That is a statistical fact, and I mention that in my post 105 (so you can see that for the explanation instead of me repeating).

Wow ... I really wasn't expecting you to say that. Is it possible that you are confusing entanglement and coincidence measurements, which are required to test the results against a Bell inequality?

Everything I have ever read on this subject indicates that an entangled state only persists until the first measurement on either of the entangled particles. At that point, the state of the other particle is immediately known, irrespective of space-time separation. As far as I can tell, this conclusion is inextricably linked to the concept of measurement in Q.M.

This seems important, so let's consider the simple 2-photon case of a \Psi^{-} Bell's state. This state is the antisymmetric superposition of the HV and VH polarizations for the photons. As is customary, let Alice measure the first particle ... whatever result she gets, she immediately knows what the other particle's polarization must be. Thus the states of both particles are completely resolved by her measurement, which means that the entanglement is destroyed, whether or not Bob ever conducts his measurement. AFAIK, this is the standard interpretation of measurement on entangled states ... I mean, it is the basis of the photonic version of the EPR paradox after all.

Now, if one doesn't believe this, and is trying to test whether or not it is true, then in that case Bob's measurement is essential so that a Bell's test can be performed. But from the point of view of QM, there was never any doubt what Bob's result would be.

Do you really think that what I have written above is incorrect?

"We have no right to say what the photon is doing in all its long course from point of entry to point of detection."

"There is no experiment without a measurement result."

Yep!

1. They are identical, that is why. And Zeilinger says exactly what I stated, despite your assertion that they did NOT do a delayed choice as they claimed. This is a top notch reference, not really sure where you are going on this one. Do you think that any reader of the referenced paper will accept your interpretation over the interpretation of the authors?

Now, here I really have to object to your characterization of my position. I have always consistently stated that I have no problem with any aspect of the Zeilinger paper, and I have never tried to re-interpret any of its claims. My only issue is with *your* extension of the results of that paper, as I have extensively described. You say it is identical, but you have not convinced me yet, and I have done my utmost to explain why I find your arguments flawed or unconvincing. Furthermore, I have tried to make it clear that I am open to the possibility that my understanding here is incorrect, but I am not just going to change my point of view without either experimental evidence or a worked mathematical proof, neither of which has surfaced so far. Finally, I have never tried to push my "interpretation" over anyone else's, I have just debated the matter openly on the scientific merits of each position.

2. The reference for this is as follows:

http://www.pas.rochester.edu/~AdvLab/Eberly_Bell_Inequalities_AJP.pdf

This in turn relies on French and Taylor, An Introduction to Quantum Physics, Section 7-2, Figures 7-1 and 7-3. See attached. I hope this settles the question.

"The experiments discussed in this chapter involve a piece of equipment we call an analyzer loop. This is a two-part device of which the first part is just an analyzer as defined in Chapter 6. The second part of the analyzer loop is a "reversed" analyzer of the same type, which recombines the beams separated by the first analyzer in such a way as to reconstruct the original beam in every detail (see Figure 7-1)."

You CAN reconstruct the input to a BS, in principle, and if it was entangled it will return to that state (if you can say that it ever was NOT entangled in the first place).

No, that is the same reference you posted before, and as was pointed out to you by myself and others, it does not pertain to the situation at hand. That source takes one input to two outputs, and back again. It does not follow at all that it should be extensible to the case we are dealing with here where there are two inputs and two outputs, and this point is not addressed in the paper. Can you please explain in some detail why you think that should be possible?

 

[DrChinese]

1. Wow ... I really wasn't expecting you to say that. Is it possible that you are confusing entanglement and coincidence measurements, which are required to test the results against a Bell inequality?

Everything I have ever read on this subject indicates that an entangled state only persists until the first measurement on either of the entangled particles. At that point, the state of the other particle is immediately known, irrespective of space-time separation. As far as I can tell, this conclusion is inextricably linked to the concept of measurement in Q.M.

This seems important, so let's consider the simple 2-photon case of a \Psi^{-} Bell's state. This state is the antisymmetric superposition of the HV and VH polarizations for the photons. As is customary, let Alice measure the first particle ... whatever result she gets, she immediately knows what the other particle's polarization must be. Thus the states of both particles are completely resolved by her measurement, which means that the entanglement is destroyed, whether or not Bob ever conducts his measurement. AFAIK, this is the standard interpretation of measurement on entangled states ... I mean, it is the basis of the photonic version of the EPR paradox after all.

Now, if one doesn't believe this, and is trying to test whether or not it is true, then in that case Bob's measurement is essential so that a Bell's test can be performed. But from the point of view of QM, there was never any doubt what Bob's result would be.

Do you really think that what I have written above is incorrect?

2. Now, here I really have to object to your characterization of my position. I have always consistently stated that I have no problem with any aspect of the Zeilinger paper, and I have never tried to re-interpret any of its claims. My only issue is with *your* extension of the results of that paper, as I have extensively described. You say it is identical, but you have not convinced me yet, and I have done my utmost to explain why I find your arguments flawed or unconvincing. Furthermore, I have tried to make it clear that I am open to the possibility that my understanding here is incorrect, but I am not just going to change my point of view without either experimental evidence or a worked mathematical proof, neither of which has surfaced so far. Finally, I have never tried to push my "interpretation" over anyone else's, I have just debated the matter openly on the scientific merits of each position.

3. No, that is the same reference you posted before, and as was pointed out to you by myself and others, it does not pertain to the situation at hand. That source takes one input to two outputs, and back again. It does not follow at all that it should be extensible to the case we are dealing with here where there are two inputs and two outputs, and this point is not addressed in the paper. Can you please explain in some detail why you think that should be possible?

1. IF... entanglement ended upon first measurement, then why would there be a THETA relationship? Yours is the same as a Hidden Variable (HV) interpretation. But we know from a simple and elegant example like Mermin's 0-120-240 setup that there is a bias towards the second direction measured. As a specific example: Assume Alice measured clearly before Bob, Type I entanglement to make it easy and so Alice=Bob (correlated case). Alice = 0 degrees. Bob is measured at either 120 or 240 degrees, to be decided "later" than Alice. Follow?

Now... according to your idea, the particle is oriented at 0 degrees, because Alice is measured first. So IT DOESN'T MATTER whether Bob is oriented at 120 or 240, the probablity of coincidence is equal. And that probability cannot be less than .3333 according to standard probability theory. That is what Mermin taught us. You can work it out yourself (or I can give you a link to my page on it ).

But... the actual probability is .25. That is QM. That is the experimental value. Not .3333, as you predict. You see, entanglement is entanglement. It is not the uncovering of a pre-existing value. If it were, you would be correct. Instead, you MUST MUST MUST consider the entire context, as I keep saying. The context is THETA, the difference between Alice and Bob. You don't know that until the SECOND measurement (i.e. you know what Alice and Bob decided for their observations). Otherwise EPR would have been correct and reality would NOT depend on the nature of observation. But, Bell's Theorem shows that to be incorrect - reality DOES depend on the observer.

So yes, I disagree.2. OK, fine. You think my position is not identical and you agree entirely with the referenced paper. Then simply repeat the words of Zeilinger et al: *The order of measurements does not matter.* Once we have that, I think we are 100% back to my position. Then we can debate the details. The order doesn't matter. You CAN detect entanglement BEFORE it is created! Precisely because order doesn't matter.3. Yes, this is a different case. If you take an input, split into 2, then reassemble to 1, then have the original...

Do you really want to say that the entanglement ends at the Beam Splitter? Because it should be pretty clear that the entanglement can be restored. So if it can be restored, then the FINAL CONTEXT must be important. If it weren't, then the collapse would be irreversible. But... it isn't! It is reversible! That is because you want to look at the beginning and the ending setup more than simply trying to say "the beam splitter collapses irreversibly". It doesn't and the statistics show it by violating a Bell Inequality.

The reference is intended strictly to demonstrate that collapse CAN be erased. I think there are plenty of examples to indicate this already, the point of mentioning this one is simply to show - in case of any doubt - that a beam splitter can be reversed specifically. Since that is the technique used for the BSM, you cannot be sure that the beam splitter was not reversed UNTIL and UNLESS the photons are finally detected. So it is the final context that matters, and the intermediate steps are not to be considered as final (since erasure is always a possiblity). Which was the point I thought I had made, and my sincere apology if I was not clear on that point.

 

[Demystifier]

good insight


http://www.springerlink.com/content/p57117239x631547/fulltext.pdf


....it has been suggested that decoherence, due to the interaction of the system with its environment, could provide the desired mechanism: The environment, constantly interacting with the body, could somehow act as a measuring device of
the macroscopic variables of the body (say, its center of mass) producing in this way a narrow wave function in the macroscopic directions of its configuration space But the composite system formed by the system of interest and its environment is itself a closed system, and Schrödinger’s evolution of this enlarged system tends to produce spreading of the total wave function over the total configuration space. Thus, decoherence alone is not sufficient to explain the emergence of the classical world from standard quantum mechanics.....

I agree that decoherence alone is not sufficient (even though it does represent a crucial ingredient). Nevertheless, I think this
http://xxx.lanl.gov/abs/quant-ph/0112005
IS sufficient. The question, of course, is whether all this is also necessary.

 

[DrChinese]

Wow ... I really wasn't expecting you to say that. Is it possible that you are confusing entanglement and coincidence measurements, which are required to test the results against a Bell inequality?

Everything I have ever read on this subject indicates that an entangled state only persists until the first measurement on either of the entangled particles. At that point, the state of the other particle is immediately known, irrespective of space-time separation. As far as I can tell, this conclusion is inextricably linked to the concept of measurement in Q.M.

...

Just to take this a step further: It is true that entangled Alice acts "as if" her entanglement has ended once her polarization is known. So you are partially correct there. However, mutually entangled Bob does not stop "acting" entangled UNTIL and UNLESS his polarization is learned. So you can see that the entnaglement collapse isn't something that happens at a single point in spacetiime (as I mentioned earlier).

Now, why is it commonly said - as you mention - that the entanglement collapses non-locally upon first measurement? It is because most setups are intended to show the non-local side of things, and so to do that you measure Alice and Bob simultaneously to accomplish this. I.e. as close to simultaneously as possible, so that the required transmission speed of a signal would need to be far greater than c.

But the Zeilinger et al experiment is intended to show the non-causal side of the equation. So it is qualitatively different. That non-causal side packs quite a wallop, doesn't it! All delayed choice experiments - surely these are well documented by now, and this is simply another variation - exploit the non-temporal nature of collapse.

 

[Frame Dragger]

Just to take this a step further: It is true that entangled Alice acts "as if" her entanglement has ended once her polarization is known. So you are partially correct there. However, mutually entangled Bob does not stop "acting" entangled UNTIL and UNLESS his polarization is learned. So you can see that the entnaglement collapse isn't something that happens at a single point in spacetiime (as I mentioned earlier).

Now, why is it commonly said - as you mention - that the entanglement collapses non-locally upon first measurement? It is because most setups are intended to show the non-local side of things, and so to do that you measure Alice and Bob simultaneously to accomplish this. I.e. as close to simultaneously as possible, so that the required transmission speed of a signal would need to be far greater than c.

But the Zeilinger et al experiment is intended to show the non-causal side of the equation. So it is qualitatively different. That non-causal side packs quite a wallop, doesn't it! All delayed choice experiments - surely these are well documented by now, and this is simply another variation - exploit the non-temporal nature of collapse.

*coughs and nudges Dr. Chinese* 'apparant' atemporal nature of the collapse. I suspect this presages a strong boot to the fanny of our notions of 'time', rather than a mysterious natural event that occurs at no time or any time.

 

[SpectraCat]

1. IF... entanglement ended upon first measurement, then why would there be a THETA relationship?

Because of Malus's Law? (see below)

As a specific example: Assume Alice measured clearly before Bob, Type I entanglement to make it easy and so Alice=Bob (correlated case). Alice = 0 degrees. Bob is measured at either 120 or 240 degrees, to be decided "later" than Alice. Follow?

Ok, as long as "0 degrees" corresponds to one of the directions of polarization (H or V).

Now... according to your idea, the particle is oriented at 0 degrees, because Alice is measured first.

Yes, rephrasing for your example, if Alice measures transmission at 0 degrees polarization, Bob's photon is also polarized at 0 degrees. If he set his polarizer at 0 degrees as well, he would find the photon to be transmitted 100% of the time. But you have specified that he has set it at 120 or 240 degrees .. that's fine.

So IT DOESN'T MATTER whether Bob is oriented at 120 or 240, the probablity of coincidence is equal.

Correct, because according to Malus's law, the probability of transmission through rotated polarizers is given by, cos^{2}\left(\theta_{Alice}-\theta_{Bob}\right), and for both cases considered here, that works out to 1/4 [cos(120)=cos(240)=-1/2].

And that probability cannot be less than .3333 according to standard probability theory.

Ok, here's where you lose me. I have no idea why one should apply "standard probability theory" to this case. I certainly have NEVER advocated doing so. The particles are entangled, and Alice's measurement COMPLETELY determines the the polarization of Bob's photon. It has 0 degrees polarization with 100% certainty, as I said above.

But... the actual probability is .25. That is QM. That is the experimental value. Not .3333, as you predict.

Again, I *don't* predict 0.333, I predict 0.25. I guess you would then say that I am somehow mis-interpreting my example, but I can't see how.

You see, entanglement is entanglement. It is not the uncovering of a pre-existing value. If it were, you would be correct. Instead, you MUST MUST MUST consider the entire context, as I keep saying. The context is THETA, the difference between Alice and Bob. You don't know that until the SECOND measurement (i.e. you know what Alice and Bob decided for their observations). Otherwise EPR would have been correct and reality would NOT depend on the nature of observation. But, Bell's Theorem shows that to be incorrect - reality DOES depend on the observer.

Yep .. I agree with all of that. I just fail to see how it refutes anything I have been saying. Alice and Bob are completely free to agree on any value of theta that they wish, and their results can only be interpreted if they share information about their results and polarizer settings. That is completely consistent with everything I have said so far. Nothing at all can be predicted until *one* of them makes a measurement, at which point everything about the system is known.

Consider the following modification. Alice's detector is at the end of a 1 m fiber, while Bob's fiber is a 1 light-minute long coil, but they are nest to each other in the lab. When Alice makes her measurement, she can tell Bob, "Hey dude, your photon's going to come out at 0 degrees .. set your polarizer there." There is nothing either of them can do to change the outcome. If Bob is ornery and sets his polarizer at another angle, he may or may not see the photon, with the probability as determined by Malus's Law, which can be confirmed by multiple measurements. This scenario is correct as far as I know, and it is completely consistent with the idea that Bob's measurement is superfluous once Alice has made hers, i.e. that the entanglement was destroyed by her measurement.

So yes, I disagree.

Ok, sorry if I am being dense or slow here, but can you please re-state your objection in light of my latest comments?

3. Yes, this is a different case. If you take an input, split into 2, then reassemble to 1, then have the original...

Do you really want to say that the entanglement ends at the Beam Splitter? Because it should be pretty clear that the entanglement can be restored. So if it can be restored, then the FINAL CONTEXT must be important. If it weren't, then the collapse would be irreversible. But... it isn't! It is reversible! That is because you want to look at the beginning and the ending setup more than simply trying to say "the beam splitter collapses irreversibly". It doesn't and the statistics show it by violating a Bell Inequality.

The reference is intended strictly to demonstrate that collapse CAN be erased. I think there are plenty of examples to indicate this already, the point of mentioning this one is simply to show - in case of any doubt - that a beam splitter can be reversed specifically. Since that is the technique used for the BSM, you cannot be sure that the beam splitter was not reversed UNTIL and UNLESS the photons are finally detected. So it is the final context that matters, and the intermediate steps are not to be considered as final (since erasure is always a possiblity). Which was the point I thought I had made, and my sincere apology if I was not clear on that point.

*Sigh* but still I don't see how that the example you gave guarantees that the interference between two entangled input photons at the beamsplitter can be reversed, which is what is required in our case. It seems like that *would* require a hidden-variables theory to explain, since the polarizations of the entangled photons when they enter the BS cannot be known.

 

[yoda jedi]

Thank you! SQM is wonderful, but it's not an answer, just a partial roadmap. Nothing makes it clearer than the constant retreat from the formal view that macrocopic is macro, and microscropic is micro. PERIOD. No one really bothers to give answers as convincing as a the fundamentals of QM as to when and where this emergence occurs. I wonder if it ever does. I throw out as pure speculation, open to ridicule, that there is no emergence; that we at some point as humans and more widely as life become unable to make the distinction in a manne that is descriptive or predictive (aka valid).

We keep measuring quantum systems with fundamentally classical measuring devices, (for those TCI and related folks) and that is a crippling limitation I don't know that we can overcome.

EDIT: I have it! You know that ridiculous add for text messaging answers to 'any' question (KGB... very tasteful) and ask them all about this. I understand the answer is guaranteed. We could merge this thread with the general topics 'what is the hardest question to ask a qunantum physicist" and make a new thread, "The nastiest physics questions we believe a text answers service can't asnwer with google. lol. lol. haha.". It's a thought.

right.
and explicative.

I agree that decoherence alone is not sufficient (even though it does represent a crucial ingredient). Nevertheless, I think this
http://xxx.lanl.gov/abs/quant-ph/0112005
IS sufficient. The question, of course, is whether all this is also necessary.

indeed, recently, a lot of work on that area.


...concerning both Bohiam-like and GRW-like approaches to the relativistic macro-objectification process...

 

[Frame Dragger]

right.
and explicative.





indeed, recently, a lot of work on that area.


...concerning both Bohiam-like and GRW-like approaches to the relativistic macro-objectification process...

I'll say it again, this is a good time in the history of human life to be alive. Forget the medicine, nutirtion etc... just for the intellectual stimulation alone. Thank you btw YJ.

EDIT: SpectraCat & Dr. Chinese: If you ever feel like taking this to a PM, please don't. I for one am truly enjoying this, and learning from your different approaches to proof/disproof in this case. This is a bit like watching a couple of excellent teachers argue in front of a class; always a good experience for all involved when it's civil (and this clearly is).

 

[DrChinese]

...

Yep .. I agree with all of that. I just fail to see how it refutes anything I have been saying. Alice and Bob are completely free to agree on any value of theta that they wish, and their results can only be interpreted if they share information about their results and polarizer settings. That is completely consistent with everything I have said so far. Nothing at all can be predicted until *one* of them makes a measurement, at which point everything about the system is known.

...

OK, maybe we agree on some of these things and I didn't realize it.

I don't think we disagree on the statistical predictions. So that is good. I have a few quibbles but I don't think we are far enough apart on some of them to keep hammering.

The issue that is open between us is the timing of collapse. Does it occur upon first measurement? Or does it relate to the context of the entire setup. I say - absolutely - it is the entire context that is important. On the other hand, I readily admit that for *some* contexts - such as basic entanglement followed by polarization measurements - it can be interpreted as occurring "as if" based on "first measured". So I saying that is a special case which is a bit deceptive. You could just as easily interpret collapse as occurring on the SECOND measurement as occurring on the first in that case.

...Since order does not matter.

 

[Demystifier]

The issue that is open between us is the timing of collapse. Does it occur upon first measurement? Or does it relate to the context of the entire setup.

Let us see what the theory of decoherence says on this question, augmented by either many-world or Bohmian interpretation. According to such a view, we can say that:
1. There is no true collapse at all.
2. There is an effective collapse which occurs locally when interaction with the environment takes place.
3. When there are many particles, there are many effective local collapses, each associated with another particle.
4. Since dynamics is deterministic, these different effective collapses are not independent. Instead, if you know the result of one effective collapse, you can also predict something about the results of other effective collapses.
5. It doesn't matter which effective collapse occurs first.

 

[Frame Dragger]

Let us see what the theory of decoherence says on this question, augmented by either many-world or Bohmian interpretation. According to such a view, we can say that:
1. There is no true collapse at all.
2. There is an effective collapse which occurs locally when interaction with the environment takes place.
3. When there are many particles, there are many effective local collapses, each associated with another particle.
4. Since dynamics is deterministic, these different effective collapses are not independent. Instead, if you know the result of one effective collapse, you can also predict something about the results of other effective collapses.
5. It doesn't matter which effective collapse occurs first.

Well, that is the upside of believing in a Pilot Wave and particles coming in ensembles. I don't see how that is any easier to swallow that SQM.

 

[Demystifier]

Well, that is the upside of believing in a Pilot Wave and particles coming in ensembles. I don't see how that is any easier to swallow that SQM.

Why exactly do you find it difficult to swallow?

 

[SpectraCat]

OK, maybe we agree on some of these things and I didn't realize it.

I don't think we disagree on the statistical predictions. So that is good. I have a few quibbles but I don't think we are far enough apart on some of them to keep hammering.

The issue that is open between us is the timing of collapse. Does it occur upon first measurement? Or does it relate to the context of the entire setup. I say - absolutely - it is the entire context that is important. On the other hand, I readily admit that for *some* contexts - such as basic entanglement followed by polarization measurements - it can be interpreted as occurring "as if" based on "first measured". So I saying that is a special case which is a bit deceptive. You could just as easily interpret collapse as occurring on the SECOND measurement as occurring on the first in that case.

...Since order does not matter.

Hmmm ... I will write up a more detailed response to this when I have more time, but for now I just want to say that I can't see how your above description could hold based on our earlier discussion.

Given the staring conditions (A & B, and C & D entangled) and the experimental setup, I agree that A will be entangled with either B or D whenever it is detected (ignoring possible external perturbations). It all depends on whether or not Charlie has acted before A is measured by Alice. If he has, then A will be entangled with D, if he has not, then A will be entangled with B. That is my view, and by "acted" here, I mean, that Charlie has allowed B and C to interfere at his fiber beamsplitter.

Two points that I will expand on in a future post:

1) I am still trying to find a reference to justify my separation of the interference and detection events in Charlie's BSM. However, I contend that it is the interference that is essential here, because the particles are detected in all configurations. Basically, if B & C are not entangled because A & D have been measured, then there is no entanglement after the interference event either.

2) Regarding the "delayed choice" aspect of this experiment, I am not sure that it is not appropriate to compare this with the "standard" delayed-choice double-slit experiment, such as the Scully paper. In the vein of the Wheeler gedanken, it seems interesting to ask the question in this case, what if Charlie inserted the fiber beamsplitter *after* B & C passed that point, but before they were detected. I guess by your contextual analysis, A & D would still be found to be entangled. Hmmm ... I will have to think about this some more ...

 

[Frame Dragger]

Why exactly do you find it difficult to swallow?

All of my objections to dBB have been raised in the 'end of local realism?' thread, as have nearly all of my objections to SQM. I am as yet, unconvinced by lengths each theory must go to, if they are to remain valid. SQM tries to stick to the math MOST of the time becuase the implications otherwise are too paradoxical, disturbing, or just plain weird.

dBB is the only of an ensemble (yes, a joke) of theories that WERE LHV theories. Now dBB is a Non-Local Hidden Variable Theory, and while SQM would require enormous refuation, a few experiments could kill dBB. Feet of clay are not a good place for a theory to begin. If HR is confirmed, dBB dies... if DCQE experiments lead to convincing evidence of atemporal collapse dBB is hurting. If the Higg's boson is found, dBB is going to have to find an alternate explanation for scalar fields that exists in tandem with the force carrier for that field.

What experiment is dBB (as a whole) going to run that in one shot does the same damage to SQM? QM as a whole can't just die, because dBB's predictions must be in line with QM for that theory to remain valid. What specific evidence of a pilot wave, or ensemble action... what NON post hoc explanation for an experiment carried out to prove a point in SQM will dBB offer?

Sell me on it. I don't find SQM terribly compelling, in no small part because SQM without an interpretation hurts my head. Still, it seems to be the best way to the approach the science.

EDIT: I should add, purely as a layperson, that the notion of a pilot wave seems like a desire to maintain the guiding hand of fate, if not god in matters. It's a fascinating idea that seems like a needless complication. QM is a partial theory, and knows it. dBB is trying to be complete, and it's clear we're FAR from that. Again... these are my completely 'gut' related concerns.

 

[Demystifier]

a few experiments could kill dBB.

I disagree.

If HR is confirmed, dBB dies...

What is HR?

if DCQE experiments lead to convincing evidence of atemporal collapse dBB is hurting.

Not true.

If the Higg's boson is found, dBB is going to have to find an alternate explanation for scalar fields that exists in tandem with the force carrier for that field.

Classical Higgs field can be described as a coherent state of Higgs particles, just as classical electromagnetic field can be described as a coherent state of photons.

 

[Frame Dragger]

I disagree.


What is HR?


Not true.


Classical Higgs field can be described as a coherent state of Higgs particles, just as classical electromagnetic field can be described as a coherent state of photons.

-I disagree with your disagreement!

-Hawking Radiation

-I think so

-That strikes me as a stretch for EM, and it just feels like more of one when you're trying to formulate the scalar 'mass'.

 

[Demystifier]

QM is a partial theory, and knows it.

Many would disagree.

dBB is trying to be complete, and it's clear we're FAR from that.

You are probably right on that. But how to find the complete theory without even trying? dBB is the best try so far. So it could be closer than SQM to the final goal. Or maybe it's on the wrong track, but again, how to know it without even trying?

 

[Demystifier]

-Hawking Radiation

Hawking radiation is not in contradiction with dBB.

 

[Demystifier]

-I think so

Do you have an argument?

-That strikes me as a stretch for EM, and it just feels like more of one when you're trying to formulate the scalar 'mass'.

I don't understand what is that supposed to mean.

 

[torquil]

According to QM, the path lengths of the various measuring devices can be set to any length in principle. And length implies time as well.

Why are you allowed to change the order of events? That is certainly not the case usually in quantum mechanics. I'm not sure this is correct, even if relativity has been mentioned in this thread.

Doesn't a simple application of nonrelativistic quantum theory disprove this, e.g. using the Schrödinger equation? Examining the state at a time before the BSM is made should easily tell you that photons A and D are uncorrelated?

EDIT: Sorry I didn't notice that there had been a long discussion. My viewpoint has probably already been mentioned.

Torquil

 

[DrChinese]

Why are you allowed to change the order of events? That is certainly not the case usually in quantum mechanics. I'm not sure this is correct, even if relativity has been mentioned in this thread. ... Examining the state at a time before the BSM is made should easily tell you that photons A and D are uncorrelated?

The results appear random when you look at A & D. You can't make any sense of them until you look at the B & C results - which can either be a BSM or ordinary polarization measurements (or anything else depending on a later choice).

 

[DrChinese]

Hmmm ... I will write up a more detailed response to this when I have more time, but for now I just want to say that I can't see how your above description could hold based on our earlier discussion.

Given the staring conditions (A & B, and C & D entangled) and the experimental setup, I agree that A will be entangled with either B or D whenever it is detected (ignoring possible external perturbations). It all depends on whether or not Charlie has acted before A is measured by Alice. If he has, then A will be entangled with D, if he has not, then A will be entangled with B. That is my view, and by "acted" here, I mean, that Charlie has allowed B and C to interfere at his fiber beamsplitter.

Two points that I will expand on in a future post:

1) I am still trying to find a reference to justify my separation of the interference and detection events in Charlie's BSM. However, I contend that it is the interference that is essential here, because the particles are detected in all configurations. Basically, if B & C are not entangled because A & D have been measured, then there is no entanglement after the interference event either.

2) Regarding the "delayed choice" aspect of this experiment, I am not sure that it is not appropriate to compare this with the "standard" delayed-choice double-slit experiment, such as the Scully paper. In the vein of the Wheeler gedanken, it seems interesting to ask the question in this case, what if Charlie inserted the fiber beamsplitter *after* B & C passed that point, but before they were detected. I guess by your contextual analysis, A & D would still be found to be entangled. Hmmm ... I will have to think about this some more ...

Picking up on our earlier discussion around whether the collapse occurs when the photon encounters the PBS or upon detection:

1. Would there be any way to tell the difference? I think we might agree that there is no way to detect this.

2. Is an observation reversible? Clearly if an observation is erased, then there effectively was no observation. If it is not reversed, then the observation occurred. We probably agree on this too.

3. If so, the only difference in our position is probably one of semantics. Because the decision to erase (or not) can be delayed until the point of detection, but not after. So to me, that signals the point of no return and therefore the point of observation. Whereas to you, the observation DID occur at the PBS as long as it wasn't erased.

 

[DrChinese]

A couple of comments. Making sure that everyone has seen this additional reference from Zeilinger's group, 2008:

http://arxiv.org/abs/0809.3991

"Entanglement swapping allows to establish entanglement between independent particles that never interacted nor share any common past. This feature makes it an integral constituent of quantum repeaters. Here, we demonstrate entanglement swapping with time-synchronized independent sources with a fidelity high enough to violate a Clauser-Horne-Shimony-Holt inequality by more than four standard deviations. The fact that both entangled pairs are created by fully independent, only electronically connected sources ensures that this technique is suitable for future long-distance quantum communication experiments as well as for novel tests on the foundations of quantum physics. "

 

[DrChinese]

Now, regarding the above: Zeilinger et al say that the experiment does "establish entanglement between independent particles that never interacted nor share any common past." This is actually a slight - but fair - distortion. The photons do not interact IF you define entangled photons as independent entities. In QM, however, entangled photons do share a state equation and therefore should not truly be considered independent in spacetime.

Therefore, with entanglement swapping, there is a meeting of entangled pairs.

 

[Demystifier]

In QM, however, entangled photons do share a state equation and therefore should not truly be considered independent in spacetime.

Therefore, with entanglement swapping, there is a meeting of entangled pairs.

I completely agree with this.

 

[Frame Dragger]

I completely agree with this.

For me that ultimately means that intuition and LR are just out of the window.

EDIT: I also agree.

 

[DrChinese]

Here is a great new experimental report on the subject. Entanglement from fully independent sources are entangled, in a cleaner and much more accurate fashion that some of the earlier references provided. Here S=2.54, and Bell's Inequality is violated by 27 standard deviations.

http://arxiv.org/abs/1005.1426

Experimental non-local generation of entanglement from independent sources

Authors: Xian-Min Jin, Jügen Röch, Juan Yin, Tao Yang (2010)

"We experimentally demonstrate a non-local generation of entanglement from two independent photonic sources in an ancilla-free process . Two bosons (photons) are entangled in polarization space by steering into a novel interferometer setup, in which they have never meet each other. The entangled photons are delivered to polarization analyzers in different sites, respectively, and a non-local interaction is observed. Entanglement is further verified by the way of the measured violation of a CHSH type Bell's inequality with S-values of 2.54 and 27 standard deviations. Our results will shine a new light into the understanding on how quantum mechanics works, have possible philosophic consequences on the one hand and provide an essential element for quantum information processing on the other hand. Potential applications of our results are briefly discussed. "

 

[billbray]

Dr. Chinese, you are the master of this stuff -so you would know - Michio Kaku held a laser on a beam splitter and said the photons at the end of the beams were superpositioned. I tried this trick in my lab with a light meter and measured about half the intensity of each outgoing beam when compared to the ingoing beam.

It doesn't seem to me that the resulting beams are superpositioned. Unless you have a better way to do the experiment with a laser, beam splitter, and light meter?

 

[Frame Dragger]

Dr. Chinese, you are the master of this stuff -so you would know - Michio Kaku held a laser on a beam splitter and said the photons at the end of the beams were superpositioned. I tried this trick in my lab with a light meter and measured about half the intensity of each outgoing beam when compared to the ingoing beam.

It doesn't seem to me that the resulting beams are superpositioned. Unless you have a better way to do the experiment with a laser, beam splitter, and light meter?

Great scientist, lousy shows that he's on, so the explanations tend towards the painfully simplistic or misleading.

 

[DrChinese]

Dr. Chinese, you are the master of this stuff -so you would know - Michio Kaku held a laser on a beam splitter and said the photons at the end of the beams were superpositioned. I tried this trick in my lab with a light meter and measured about half the intensity of each outgoing beam when compared to the ingoing beam.

It doesn't seem to me that the resulting beams are superpositioned. Unless you have a better way to do the experiment with a laser, beam splitter, and light meter?

Cool that you did this!

So we know the output beams should be 1/2 the intensity, so no surprise there.

Perhaps Kaku was trying to say that before they are detected, they are in a superposition. That would be accurate in my view.

 

[Lord Crc]

As someone who has only read pop-sci stuff about QM, I'm a bit slow on how to interpret the setup proposed in the first post. If anyone could help me out I'd be very grateful as I find these things very fascinating! My main confusion lies with what Alice and Bob will measure.

Let's say Alice sets her detector to theta degrees and say Bob also sets his to theta degrees. If photons A and D are entangled then they will measure the same 100% of the time (or 0% if the photons are anti-correlated). Now, if A and D are NOT entangled, then, since the pairs AB and CD are independent, they would measure the same 50% of the time (ie independent uniformly random distributions).

Assuming the above is correct, then I fail to see how Charlie's information is needed for Alice and Bob to figure out if the photons are entangled. That is, I understand they can't do so on a single pair basis, but assume that Charlie ensures that the pairs are always entangled. Then how is this different from the situation where Charlie doesn't exist and Alice and Bob performs the measurements on a single pair of entangled photons?

However if it results in the same situation as a single pair measurement, then it would seem that Charlie has no free will, assuming he doesn't change his mind too often :)

Due to this I assume that I missed a turn somewhere, invalidating my subsequent assumptions. Anyone care to shed some light? Thanks in advance.

 

[zonde]

Here is a great new experimental report on the subject. Entanglement from fully independent sources are entangled, in a cleaner and much more accurate fashion that some of the earlier references provided. Here S=2.54, and Bell's Inequality is violated by 27 standard deviations.

http://arxiv.org/abs/1005.1426

Experimental non-local generation of entanglement from independent sources

Authors: Xian-Min Jin, Jügen Röch, Juan Yin, Tao Yang (2010)

"We experimentally demonstrate a non-local generation of entanglement from two independent photonic sources in an ancilla-free process . Two bosons (photons) are entangled in polarization space by steering into a novel interferometer setup, in which they have never meet each other. The entangled photons are delivered to polarization analyzers in different sites, respectively, and a non-local interaction is observed. Entanglement is further verified by the way of the measured violation of a CHSH type Bell's inequality with S-values of 2.54 and 27 standard deviations. Our results will shine a new light into the understanding on how quantum mechanics works, have possible philosophic consequences on the one hand and provide an essential element for quantum information processing on the other hand. Potential applications of our results are briefly discussed. "

I would say that experiment from Zeilinger's group is much better then this one. Here this non-locality argument is quite dubious as all photons go through the same interferometer.

And if you compare them from perspective of practical application it's clear that experiment from Zeilinger's group can be used to develop quantum repeaters where this one can't. I think it's quite reasonable way how to make distinction between true quantum "non-locality" and feigned quantum "non-locality".

EDIT: Thought that in last sentence I should have used "true/feigned non-local generation of entanglement" instead of "true/feigned quantum non-locality".

 

[DrChinese]

As someone who has only read pop-sci stuff about QM, I'm a bit slow on how to interpret the setup proposed in the first post. If anyone could help me out I'd be very grateful as I find these things very fascinating! My main confusion lies with what Alice and Bob will measure.

Let's say Alice sets her detector to theta degrees and say Bob also sets his to theta degrees. If photons A and D are entangled then they will measure the same 100% of the time (or 0% if the photons are anti-correlated). Now, if A and D are NOT entangled, then, since the pairs AB and CD are independent, they would measure the same 50% of the time (ie independent uniformly random distributions).

Assuming the above is correct, then I fail to see how Charlie's information is needed for Alice and Bob to figure out if the photons are entangled. That is, I understand they can't do so on a single pair basis, but assume that Charlie ensures that the pairs are always entangled. Then how is this different from the situation where Charlie doesn't exist and Alice and Bob performs the measurements on a single pair of entangled photons?

However if it results in the same situation as a single pair measurement, then it would seem that Charlie has no free will, assuming he doesn't change his mind too often :)

Due to this I assume that I missed a turn somewhere, invalidating my subsequent assumptions. Anyone care to shed some light? Thanks in advance.

I think you have summarized things pretty well. The issue is that clearly, sometimes A & D ARE in fact entangled. And that depends on what happens with B & C. There is no questioning the effect really. It is just: how do you interpret it? (And no interpretation is even required!)

 

[zonde]

Assuming the above is correct, then I fail to see how Charlie's information is needed for Alice and Bob to figure out if the photons are entangled. That is, I understand they can't do so on a single pair basis, but assume that Charlie ensures that the pairs are always entangled. Then how is this different from the situation where Charlie doesn't exist and Alice and Bob performs the measurements on a single pair of entangled photons?

Two photons from two different pairs interact at Charlie's beam splitter. Now there are two possibilities either both photons appear at the same output of Charlie's beam splitter or they appear at different outputs. Entangled pairs at Alice and Bob are those where corresponding photons at Charlie are detected in different outputs. And Charlie have to communicate this information to Alice and Bob so that they could create appropriate subsample. If Alice and Bob do not check they pairs against Charlie's data they do not see any entanglement.

 

[Lord Crc]

Entangled pairs at Alice and Bob are those where corresponding photons at Charlie are detected in different outputs.

Ah there's my misunderstanding. I was under the impression Charlie could determine which pair to entangle, not that he merely observed which ones were which. That changes things a bit :)

Thanks to both of you. While the experiment seems a bit less weird now, it's never the less very fascinating to contemplate.