DCQE - how does/can the pattern change?

[SpectraCat]

…to me personally, isn't confusing because I tend to say that the photons were already in X or Y state upon emission. If your view is that they are not in either or are in both then I can see exactly what you mean and why it is confusing and such.

Well, that is demonstrated to be false .. the Aspect experiments first showed that there is no fixed polarization basis for the detection of entangled photons, and that result has since been confirmed, extended and verified in different contexts by other groups.

[EDIT: Actually, I am not sure that it was the Aspect experiment that first demonstrated that entangled photons do not have a unique polarization basis ... that may have already been known when he did his experiment. In any case, it is certainly true that it has been experimentally demonstrated. The 1981 Aspect experiment WAS the first experiment to demonstrate a Bell inequality violation, and thus show that local realism and QM are incompatible, as I mentioned below.]

The interpretation you gave above (i.e. that the photons have well-defined polarizations when they are generated is called local realism, and that has been shown experimentally to be incompatible with QM .. that entanglement is incompatible with local realism should be evident from the definition.

 

[Cthugha]

There isn't any debate , this has all been settled long ago. There are deluded people who are allowed to post again and again here on what is supposed to be a science forum and there are people who understand science (like me).

The analysis by Cthuga is bollocks, and has no relevance to the experiments.

Well you should read the threads again. When Cthugha first suggested the coincidence counters were to ensure classical (spatial) coherence between entangled pairs I thought he was being too ridiculous to argue with. You see you can't argue clearly with someone who has a wrong understanding of QM. And the fact that you think my arguments aren't made clearly is probably due to you not understanding QM either.

Hmm, as I am so obviously a crackpot, I should retract all my published papers quickly.

I suggest you reread the original threads again. At first, spatial coherence is not a quantity limited to classical physics. All I did was explaining the physics behind the experiment without using any special interpretation. The fact is that any experiment on entanglement relies on some conserved quantity which causes two subsystems to behave in line. Every of these subsystems when viewed on their own cannot be distinguished from classical systems having the same properties. This can be the polarization like in typical Bell test, energy or like in this case momentum or wavevector. I described what happens when you have two of these subsystems that develop according to such a conservation law. The non-classical part which requires using interpretation is always the question how this conservation law can be guaranteed to hold once one of the two subsystems is measured and the other one needs to know instantly what the measurement result has to be if it gets detected now, too. I did not answer this question as this is the question answered by the interpretation. I just answered the calculatable physics part containing the two subsystems.

By the way: I NEVER stated that the explanation is classical. Saying this is classical because spatial coherence plays a role is like the results from Bell tests are classical too because light gets absorbed at the detectors and absorption is a classical process, too. Common Bell tests use polarization sensitive detection mechanisms (polarizers), so you need polarization to explain them. Most DCQE experiments use a measurement setup which measures spatial coherence (the double slit), so you will need spatial coherence to explain them.

and don't falsely state that peer reviewed references were provided to support an argument that the DCQE can be explained by classical phase relationships, there were none. There may have been some links to irrelevant results from quantum optics and an obscure german phd thesis (which has since gone offline), but that doesn't hide the basic fact the the DCQE has NO classical explanation. And no amount of obfuscation will fix that.

If you don't think QM is correct then you will have a hard time understanding the DCQE, and it's fruitless to argue with such people. There is no simple "explanation" of what is "happening", there is Quantum Mechanics and there are the various interpretations of it, and they are the best explanation you CAN have.

I do not think Zeilinger is an obscure source. As I told you already beforehand, my explanation is not at odds with QM. I use the usual picture Glauber introduced in his definitions. It seems you just do not bother to understand the references given to you. However, the basic result of complementarity of single- and two-photon interference in such experiments which is one of the main points the Dopfer thesis was cited for, is already given in Phys. Rev. A 48, 1023–1027 (1993) By Jaeger et. al. The equivalence between two-photon Fourier optics and classical Fourier optics has also been pointed out in "Random delayed-choice quantum eraser via two-photon imaging", G. Scarcelli et al., Eur. Phys. J. D 44, 167-173 (2007) where the following is expressed:
"As for the entanglement, this experiment has strikingly shown a fundamental point that is often forgotten: for entangled photons it is misleading and incorrect to interpret the physical phenomena in terms of independent photons. On the contrary the concept of “biphoton” wavepacket has to be introduced to understand the nonlocal spatio-temporal correlations of such kind of states. Based on such a concept, a complete equivalence between two-photon Fourier optics and classical Fourier optics can be established if the classical electric field is replaced with the two-photon probability amplitude. The physical interpretation of the eraser that is so puzzling in terms of individual photons’ behavior is seen as a straightforward application of two-photon imaging systems if the nonlocal character of the biphoton is taken into account by using Klyshko’s picture."

I assume you will call this paper also irrelevant.

To measure the events separately and compare timestamps you would need two detectors synchronised sufficiently accurately to a cpu and an operating system that could reliably record the timestamps.

Most coincidence counters indeed work in a start-stop geometry and measure timestamps. This is not as complicated as you make it sound. In fact the timestamps you get from the electronics are usually more exact than the time resolution of photo diodes is. As an alternative, you could also use a streak camera in single photon counting mode using either two cameras or two different regions of the same camera. You can get timestamps with a resolution as good as 1.4 ps this way. At least that was the best I got.

Crackpots pick a particular experiment which might appeal to some type of obfuscated classical analysis, it takes moderately intelligent people like the undergraduates in Walborn's group ( http://arxiv.org/abs/quant-ph/0106078 ) to put together an experiment which much more simply shows the crackpots are clearly wrong.

I'm not going to argue about dumb irrelevant classical phase relationships in other convoluted setups, I've explained several times that coincidence counters don't do phase matching.

The coincidence counters are required because of the probabilistic nature of QM, this is assumed obvious in the peer reviewed papers,

I also explained the Walborn experiment to you, but all you said was it was "irrelevant" without any closer explanation. By the way coincidence counters do not need to do phase matching. I do not know where you got that idea from. Most probably it is a strawman argument. It is also interesting what you assume other people assume as obvious.

I assume I should stay out of this discussion. You repeatedly insult me without showing any arguments or publications and tell me the publications I link are irrelevant without telling me why. As you already showed elsewhere that you easily and often insult other people (https://www.physicsforums.com/showthread.php?t=495469"), I do not see much sense in discussing with you. Feel free to answer if you want to discuss something, but please just ignore this post if you just want to declare all as irrelevant that does not match your liking or if you want to claim that I said stuff I never said (like DCQE is classical).

 

[unusualname]

Cthuga, you clearly stated that the coincidence counters were required to ensure spatial coherence, I had to end two discussions with you as I couldn't understand you and just stated I had a different understanding.

I explained that they were just counting coincidences of entangled pairs, and eventually had to point out that this was mostly due to probabilistic nature of QM (malus' law in polarizers) (In practice, isolating background noise may also be important)

In the peer reviewed papers they seem to take this as obvious so as not to bother clarifying it, eg in The Walborn paper they just mention that detection times are doubled once the polariser is in place (I think they assume people will know why)

The fact that Zeilinger had a phd student whose thesis (you claim supports your view) was once online (and is now only available on a web archive link apparently) isn't really great evidence that you were correct is it?

I repeatedly asked for peer-reviewed references, as I would be quite interested to read an analysis of the DCQE which trivially reduces it to phase relationships between the different paths the SPDC pairs follow.

Really, I'm fully willing to submit to appropriate evidence, can you provide it?

Otherwise, anytime someone mistakenly posts some compliment about your apparent explanation of this incredible experiment I'll feel free to point out it's not mainstream science, ok?

 

[Cthugha]

Cthuga, you clearly stated that the coincidence counters were required to ensure spatial coherence, I had to end two discussions with you as I couldn't understand you and just stated I had a different understanding.

No, I never said that. Maybe that wording was part of a larger set of sentences and is rippe dout of context. Coincidence counting can be used to pick a spatially coherent subset by placing one detector in the Fourier plane and thus destroying any position info, yes.

I explained that they were just counting coincidences of entangled pairs, and eventually had to point out that this was mostly due to probabilistic nature of QM (malus' law in polarizers) (In practice, isolating background noise may also be important)

In the peer reviewed papers they seem to take this as obvious so as not to bother clarifying it, eg in The Walborn paper they just mention that detection times are doubled once the polariser is in place (I think they assume people will know why)

This is simply not true. No peer-reviewed paper I know of mentions Malus' law as the reason why coincidence counters are needed. If it was that way, FTL signaling would be possible using schemes that do not make use of polarizers or introduce other losses. By the way even Walborn himself does not follow your argument as he finishes his overview article about Quantum erasure (American Scientist, vol 91, p. 336 (2203)) saying:
"Even so, we are making progress. We understand now that quantum entanglement, a necessary part of the act of measurement itself, rather than the “quantum uncertainty” involved in the measurement, is responsible for complementarity in the double-slit experiment. This may seem like a subtle point, but it will make many physicists sleep more soundly at night."

The fact that Zeilinger had a phd student whose thesis (you claim supports your view) was once online (and is now only available on a web archive link apparently) isn't really great evidence that you were correct is it?

As I said before I do not really care much about the great mystery how the info how photon B has to behave when photon A gets detected gets passed along. If I had to, I would prefer to follow Scarcelli's and Shih's view presented above. I just analyzed the other part: What happens if you need to take conservation laws into account and how important is complementarity.

I repeatedly asked for peer-reviewed references, as I would be quite interested to read an analysis of the DCQE which trivially reduces it to phase relationships between the different paths the SPDC pairs follow.

Really, I'm fully willing to submit to appropriate evidence, can you provide it?

I have given you plenty of references at least aiming at the relevance of these points. One of the best ones discussing shortly the importance of conditional interference fringes (which is exactly the point I was trying to get along) is given by Walborn himself. See "Spatial correlations in parametric down-conversion" by Walborn et al. (Physics Reports Volume 495, Issues 4-5, October 2010, Pages 87-139), also available at Arxiv: http://arxiv.org/abs/1010.1236" and references therein. The non-local dependence of spatial coherence is discussed in section 4.1. The sections on spatial entanglement and the section about conditional interference patterns (6.1) might also be interesting. Section 6.1 basically gives my point of view, however, using an experimental setup that is more pedagogical and involving two double slits.

Otherwise, anytime someone mistakenly posts some compliment about your apparent explanation of this incredible experiment I'll feel free to point out it's not mainstream science, ok?

No, not ok. Pointing out something is not mainstream is one thing. Calling someone who does not follow your (btw. also not mainstream) opinion a crackpot and his posts bollocks something entirely different.

 

[unusualname]

No, I never said that. Maybe that wording was part of a larger set of sentences and is rippe dout of context. Coincidence counting can be used to pick a spatially coherent subset by placing one detector in the Fourier plane and thus destroying any position info, yes.

No it can't, the coincidence counters don't have nearly enough resolution (and the position of the cc is very variable in these experiments)

This is simply not true. No peer-reviewed paper I know of mentions Malus' law as the reason why coincidence counters are needed. If it was that way, FTL signaling would be possible using schemes that do not make use of polarizers or introduce other losses. By the way even Walborn himself does not follow your argument as he finishes his overview article about Quantum erasure (American Scientist, vol 91, p. 336 (2203)) saying:
"Even so, we are making progress. We understand now that quantum entanglement, a necessary part of the act of measurement itself, rather than the “quantum uncertainty” involved in the measurement, is responsible for complementarity in the double-slit experiment. This may seem like a subtle point, but it will make many physicists sleep more soundly at night."

As I keep explaining the mainstream papers assume basic physics understanding so don't point out every single point of the experiment for schoolchildren or similar.

As I said before I do not really care much about the great mystery how the info how photon B has to behave when photon A gets detected gets passed along. If I had to, I would prefer to follow Scarcelli's and Shih's view presented above. I just analyzed the other part: What happens if you need to take conservation laws into account and how important is complementarity.

ie you think QM is not mysterious and can be explained quite rationally by your type of arguments, or appeal to some obscure reference which no one but you would think to appeal to, in particular no one doing the experiments thinks to refer to.

I have given you plenty of references at least aiming at the relevance of these points. One of the best ones discussing shortly the importance of conditional interference fringes (which is exactly the point I was trying to get along) is given by Walborn himself. See "Spatial correlations in parametric down-conversion" by Walborn et al. (Physics Reports Volume 495, Issues 4-5, October 2010, Pages 87-139), also available at Arxiv: http://arxiv.org/abs/1010.1236" and references therein. The non-local dependence of spatial coherence is discussed in section 4.1. The sections on spatial entanglement and the section about conditional interference patterns (6.1) might also be interesting. Section 6.1 basically gives my point of view, however, using an experimental setup that is more pedagogical and involving two double slits.

All your references are your own references for your own argument, no professional experimenter in delayed choice experiments has your references in their papers, probably because they are not remotely relevant.

No, not ok. Pointing out something is not mainstream is one thing. Calling someone who does not follow your (btw. also not mainstream) opinion a crackpot and his posts bollocks something entirely different.

Well I was nice to you on a couple of occasions, but can't believe people think your arguments still have credibility, and here you are still trying to promote your analysis.
You clearly have some good knowledge of optics, and I wish you well in your work, you just have a bad understanding of QM, delayed choice erasers are supposed to explain why classical optics and naive intuition doesn't work, and in particular why the type of analysis you have attempted doesn't work.

 

[SpectraCat]

No it can't, the coincidence counters don't have nearly enough resolution (and the position of the cc is very variable in these experiments)

What do you mean they don't have enough resolution .. I think you are confused about what is being discussed. The spatial resolution is provided by the stepping of the movable detector, and it is clearly sufficient to observe interference, since interference is observed in the experiments. The coincidence counter is just a complicated electronic circuit .. the only resolution relevant to the coincidence counter is the temporal resolution. Once again, that certainly seems to be sufficient to observe interference. Please explain which of these resolutions (spatial or temporal) you think is insufficient, and why.

All your references are your own references for your own argument, no professional experimenter in delayed choice experiments has your references in their papers, probably because they are not remotely relevant.

Cthugha has now given you at least three separate peer reviewed references that support his claim, all by different authors. In what way are these all "his own references"? Is he the secret PI for all of these groups? One of the references was by Walborn for crying out loud, and another was specifically about the DCQE, as indicated by the presence of the phrase, "delayed choice quantum eraser" in the title! Why are you being so dismissive of these references which you yourself requested?

Well I was nice to you on a couple of occasions, but can't believe people think your arguments still have credibility, and here you are still trying to promote your analysis.
You clearly have some good knowledge of optics, and I wish you well in your work, you just have a bad understanding of QM, delayed choice erasers are supposed to explain why classical optics and naive intuition doesn't work, and in particular why the type of analysis you have attempted doesn't work.

There is nothing naive about Cthugha's analysis, and there is nothing wrong with his understanding of quantum mechanics, at least not as evidenced by his posts on here. Both he and I have explained to you repeatedly why his analysis is NOT classical OR trivial, and in fact ASSUMES quantum mechanical entanglement. You have never even addressed our arguments, or provided a shred of evidence why they are wrong or misguided. You just keep relying on your own dogmatic beliefs and suppositions. That is hardly scientific.

Just as an FYI .. Cthugha is a published author in this field, and I am a professor of chemical physics, who has published over 30 papers in peer reviewed journals ... all of them deal with quantum mechanics, (although not this specific subfield, about which I am still learning.) What are your credentials, that you feel so qualified to blithely claim that we don't know what we are talking about?

 

[unusualname]

What do you mean they don't have enough resolution .. I think you are confused about what is being discussed. The spatial resolution is provided by the stepping of the movable detector, and it is clearly sufficient to observe interference, since interference is observed in the experiments. The coincidence counter is just a complicated electronic circuit .. the only resolution relevant to the coincidence counter is the temporal resolution. Once again, that certainly seems to be sufficient to observe interference. Please explain which of these resolutions (spatial or temporal) you think is insufficient, and why.

I mean that the coincidence counters are not accurate enough wrt the photon frequencies/wavelengths so that you could suggest they are responsible for ensuring any type of classical coherence (like Cthuga has in the past, he seems to be backtracking now). The timing is not perfect, the circuit just records coincidences at the two detectors within a sufficiently small time window.

The stepping motor is required to record the interference pattern over a large space, with better technology they wouldn't need this, for example a very large CCD screen would suffice.

Cthugha has now given you at least three separate peer reviewed references that support his claim, all by different authors. In what way are these all "his own references"? Is he the secret PI for all of these groups? One of the references was by Walborn for crying out loud, and another was specifically about the DCQE, as indicated by the presence of the phrase, "delayed choice quantum eraser" in the title! Why are you being so dismissive of these references which you yourself requested?

Because none of the references claim to "explain" the DCQE by appeal to a classical phase analysis, you know, the one from Cthugha that I've been arguing is wrong for the last year.

There is nothing naive about Cthugha's analysis, and there is nothing wrong with his understanding of quantum mechanics, at least not as evidenced by his posts on here. Both he and I have explained to you repeatedly why his analysis is NOT classical OR trivial, and in fact ASSUMES quantum mechanical entanglement. You have never even addressed our arguments, or provided a shred of evidence why they are wrong or misguided. You just keep relying on your own dogmatic beliefs and suppositions. That is hardly scientific.

The thing that's wrong with Cthugha's analysis is that it attempts to explain a QM effect using classical phase analysis. The mathematics is ok, and you can no doubt draw classical waves on a diagram of the DCQE setup, but it has no relevance to the explanation, which requires an understanding of QM probabilistic effects (to account for the coincidence counter) and bizarre non-locality and/or non-separability to account for the delayed eraser effect.

Just as an FYI .. Cthugha is a published author in this field, and I am a professor of chemical physics, who has published over 30 papers in peer reviewed journals ... all of them deal with quantum mechanics, (although not this specific subfield, about which I am still learning.) What are your credentials, that you feel so qualified to blithely claim that we don't know what we are talking about?

Einstein was a genius with several published papers, but I would argue with him in the same way if he posted an incorrect analysis of a QM experiment here.

There are clearly many otherwise very competent scientists around who don't accept the stunning non-classicality of QM. I haven't got Bohr's ability or patience to continually counter intricate analyses based on classical concepts, so maybe I should give in and let you guys get on with it. When the next post comes up discarding 80 years of QM understanding and praising Ctugha's "solution" to the DCQE I'll just let it go.

 

[Cthugha]

No it can't, the coincidence counters don't have nearly enough resolution (and the position of the cc is very variable in these experiments)

Whether or not you can do this depends solely on the detector size or the size of the pinholes put in front of the detector, the size of the beam and the focus length of the lens used. As the detectors are quite small this is not a problem. I have myself done filtering in momentum-space by placing a 1 mm pinhole in a beam and I can assure you that it is possible to filter out a small wavevector range this way, effectively increasing spatial coherence. And yes, this was published in a peer-reviewed journal. Do you want to see the reference?

ie you think QM is not mysterious and can be explained quite rationally by your type of arguments, or appeal to some obscure reference which no one but you would think to appeal to, in particular no one doing the experiments thinks to refer to.

Yanhua Shih who is a really highly cited author and has been discussed on these forums also on other occasions is also obscure?

All your references are your own references for your own argument, no professional experimenter in delayed choice experiments has your references in their papers, probably because they are not remotely relevant.

You keep asking for references and I give you some. You immediately call any of them irrelevant. Given the time you took to write an answer here, you are either already familiar with these papers (which should mean that they are somewhat relevant to DCQE) or you do not even bother to look at them which makes a discussion pointless. I have given you a review article by Walborn himself. The guy who performed the best experiment on DCQE in your opinion and he explicitly gives a simple phase analysis of conditional interference patterns in equation 96 which is explicitly based on the theory presented in section 3 of the same paper where it is explicitly derived how to get two-photon coincidence count rates and why they depend among others on coherence properties, spatial properties of the pump field and the positions of both detectors.

Is Walborn also obscure or a crackpot? So please explain me where his arguments are wrong.

I mean that the coincidence counters are not accurate enough wrt the photon frequencies/wavelengths so that you could suggest they are responsible for ensuring any type of classical coherence (like Cthuga has in the past, he seems to be backtracking now). The timing is not perfect, the circuit just records coincidences at the two detectors within a sufficiently small time window.

No, I am not backtracking. You can pick a spatially coherent subset if you place the detector correctly.


edit:

I mean that the coincidence counters are not accurate enough wrt the photon frequencies/wavelengths so that you could suggest they are responsible for ensuring any type of classical coherence (like Cthuga has in the past, he seems to be backtracking now). The timing is not perfect, the circuit just records coincidences at the two detectors within a sufficiently small time window.

How is the timing relevant for spatial coherence? Spatial coherence is mostly determined by the angular size of the source as "seen" by the detector.

 

[SpectraCat]

Because none of the references claim to "explain" the DCQE by appeal to a classical phase analysis, you know, the one from Cthugha that I've been arguing is wrong for the last year.




The thing that's wrong with Cthugha's analysis is that it attempts to explain a QM effect using classical phase analysis. The mathematics is ok, and you can no doubt draw classical waves on a diagram of the DCQE setup, but it has no relevance to the explanation, which requires an understanding of QM probabilistic effects (to account for the coincidence counter) and bizarre non-locality and/or non-separability to account for the delayed eraser effect.

You are simply wrong, as I have posted several times ... Cthugha's analysis is NOT classical! Both he and I have explained why, and you simply pretend not to see it I guess, because you have never once addressed our comments.

Einstein was a genius with several published papers, but I would argue with him in the same way if he posted an incorrect analysis of a QM experiment here.

There are clearly many otherwise very competent scientists around who don't accept the stunning non-classicality of QM. I haven't got Bohr's ability or patience to continually counter intricate analyses based on classical concepts, so maybe I should give in and let you guys get on with it. When the next post comes up discarding 80 years of QM understanding and praising Ctugha's "solution" to the DCQE I'll just let it go.

Again with the blanket claims and appeals to dogma ... I can only conclude that you have no idea what you are talking about, because you seem unable to give a substantive refutation of any of the points that have been explained to you. You also seem incapable of understanding that the analysis is not classical but quantum mechanical, and requires the "stunning non-classicality of QM" for the most significant aspect, namely way that the observed interference pattern arises from the well-defined phase relationship between the entangled photons. The explanation is NOT really intricate, it is fairly straightforward and sensible, and yet you cannot seem to come up with a specific physical argument to rebut any single point of it. You can only make vague and incorrect claims about "resolution" of coincidence counters, and then mis-characterize the explanation as classical in character, and call us names for thinking it is correct, when our familiarity with both the concepts and relevant experimental techniques is clearly more well-developed than your own.

The really galling thing is that I started this "conversation" thinking that you might have something substantive to offer that would grow my understanding of the DCQE. Instead you started with the insults, and I allowed myself to get dragged into a silly argument with someone doesn't really seem to understand the DCQE, or QM for that matter. This discussion is now pointlessly going in circles, with no possible further benefit to those reading it (if anyone is left). I will leave it to the readers to decide who has done a better job supporting their argument, and withdraw from this thread.

 

[unusualname]

No, I am not backtracking. You can pick a spatially coherent subset if you place the detector correctly.editow is the timing relevant for spatial coherence? Spatial coherence is mostly determined by the angular size of the source as "seen" by the detector.

It's not relevant, that's MY point, YOU're the one that seemed to think it might be responsible for ensuring some kind of classical phase condition on individual photons. I don't know what you mean so I can't really give a sensible response except to point out you're wrong.

Coincidence counters count coincidences within a time window, nothing more special than that, in fact if you alter the length of travel of the p-photons so as to get the delayed eraser you will never have perfect coincidences, will you?

You have simply posted references to quantum optics papers that analyse something not relevant to explaining the DCQE by classical phases, which is why I keep ignoring them.

Are you still suggesting your classical phase analysis solves any "mystery" in the DCQE? Because that's what it sound like to me.

If you have changed your mind and realize that the analysis of the phases doesn't explain delayed erasure then make that clear please.

 

[Cthugha]

Are you still suggesting your classical phase analysis solves any "mystery" in the DCQE? Because that's what it sound like to me.

I have given you Walborn's opinion on the topic. He uses phases to explain conditional interference patterns. I fully agree with him. By the way it is rather strange to talk about classical phases. When quantifying the em field you also get fields with phases. I already gave you a reference on that which you did not bother to read.

I don't know what you mean

Yes, that was my impression from the beginning of this topic.

Just read up on spatial coherence. It seems to me that you do not even know what spatial coherence means. Do you know how spatial coherence and the visibility of a double slit interference pattern are connected? Then you can easily generalize that to two-photon states. Walborn does all of that in his review paper. All you need to do is read it. So I challenge you again to point out Walborn's error in equation 96 of the paper I linked earlier where the conditional interference patterns are explained in terms of phase relationships.

Please read up on it and/or post some publications in support of your view on the topic or stop trolling.

 

[unusualname]

I have given you Walborn's opinion on the topic. He uses phases to explain conditional interference patterns. I fully agree with him. By the way it is rather strange to talk about classical phases. When quantifying the em field you also get fields with phases. I already gave you a reference on that which you did not bother to read.



Yes, that was my impression from the beginning of this topic.

Just read up on spatial coherence. It seems to me that you do not even know what spatial coherence means. Do you know how spatial coherence and the visibility of a double slit interference pattern are connected? Then you can easily generalize that to two-photon states. Walborn does all of that in his review paper. All you need to do is read it. So I challenge you again to point out Walborn's error in equation 96 of the paper I linked earlier where the conditional interference patterns are explained in terms of phase relationships.

Please read up on it and/or post some publications in support of your view on the topic or stop trolling.

No, you please explain why coincidence counters are used. This is a much more simpler point of the experiment and one that has a simple answer, and one that you have obfuscated by appealing to weird results and papers from all sorts areas.

Once I understand your explanation of the coincidence counters I will be able to continue, otherwise this is going to go the same way as the other discussions.

 

[Cthugha]

No, you please explain why coincidence counters are used. This is a much more simpler point of the experiment and one that has a simple answer, and one that you have obfuscated by appealing to weird results and papers from all sorts areas.

Once I understand your explanation of the coincidence counters I will be able to continue, otherwise this is going to go the same way as the other discussions.

So I should explain it again? Why? It is all in Walborn's overview article. Results are not weird because you dislike them.

However, there are several answers to this question.

You use coincidence counters because you want to identify a two-photon interference pattern. This means that it is present only in the two-photon coincidence count rate, but not in the single photon count rate. Those two are complementary. In detail, you will only be able to see an interference pattern with perfect visibility if you have a momentum eigenstate which is equivalent to having no which-way information and also equivalent to having a high degree of spatial coherence. The most commonly used way of destroying which-way information lies in using a lens and placing the detector in the Fourier plane which means that a detection event at some position of the detector could come from any position of the crystal, but corresponds to some well defined momentum/wave vector. If you move this detector around, you get a different wave vector. So in order to single out a momentum eigenstate (which is the same as having high spatial coherence) you need a detector which does not spread across the whole Fourier plane, but is small enough to pick a momentum eigenstate.

If you do so, it is clear that all detection events at this position correspond to some momentum eigenstate. All detections on the other side (showing up in the coincidence counts) will also belong to some momentum eigenstate (high spatial coherence). As the visibility of a double slit interference pattern is proportional to the spatial coherence of the light beam used, this will give a conditional interference pattern. If you move the first detector out of the Fourier plane, you will notice the visibility of the interference pattern go down as you do not choose a momentum eigenstate subset anymore. Spatial coherence goes down and so does the visibility of the interference pattern. If you move the first detector around, you will notice that the interference pattern moves around as it is now the conditional interference pattern belonging to a different wave vector. To perform DCQE you can now play tricks and insert polarizers wave plates and whatever, but essentially this does not make the experiment more mysterious than entanglement already is.

Originally Posted by unusualname
The delay is the delay after the s-photons are measured/detected.

Why should a polariser placed in another galaxy affect the s-photon detections, there is a delay of several years before the p-photons will even reach the eraser?

(yes you will have to wait years to do the coincidence match, but there will be an interference pattern if the eraser was in place and there won't be if it wasn't in place, how did the s-photon's "know" that years before)

i am trying to grasp this. maybe cthuga/spectracat can explain how sub-samples

so is the mach zender interference/non-interference also explained by phase?

Hmm, how can I explain this more pedagogically. The polariser placed in a different galaxy does not modify the s-photon detections. There is no retrocausation or such stuff. Have a look at section 6.1. of the arxiv paper from Walborn I linked earlier. That explains the basics of subsampling way better (and with pictures) than I could do by just typing text. Once you understand that it is not a big step to understanding DCQE.

 

[SpectraCat]

i am trying to grasp this. maybe cthuga/spectracat can explain how sub-samples

so is the mach zender interference/non-interference also explained by phase?

It is because the polarizer placed in the p-beam doesn't actually affect the s-photon detection events. It only affects the *coincidence measurements* which are not even generated until after *BOTH* photons have been detected. If you look only at the s-photon detections for the both cases, without considering the coincident statistics, then there is absolutely no difference for sets taken with and without the polarizer in the p-branch. In other words, there is NO interference observed in the detections for the s-photons in any case. The interference fringes are only evident in the coincident measurements.

Notice also, that for the case where the eraser (i.e. the polarizer in the p-beam) is in place, there are two different interference patterns that are observed, depending on whether the polarizer angle is set to match the quarter-wave plate for slit one or for slit two. The two patterns of fringes and anti-fringes (to use the terminology from Walborn's paper) are 180º out of phase ... this is because the of the well-defined phase relationship between the two-photon states, as explained by Cthugha.

 

[unusualname]

So I should explain it again? Why? It is all in Walborn's overview article. Results are not weird because you dislike them.

However, there are several answers to this question.

You use coincidence counters because you want to identify a two-photon interference pattern. This means that it is present only in the two-photon coincidence count rate, but not in the single photon count rate.

So what you mean is the pattern in the coincidence counts shows interference.

Those two are complementary. In detail, you will only be able to see an interference pattern with perfect visibility if you have a momentum eigenstate which is equivalent to having no which-way information and also equivalent to having a high degree of spatial coherence. The most commonly used way of destroying which-way information lies in using a lens and placing the detector in the Fourier plane which means that a detection event at some position of the detector could come from any position of the crystal, but corresponds to some well defined momentum/wave vector. If you move this detector around, you get a different wave vector. So in order to single out a momentum eigenstate (which is the same as having high spatial coherence) you need a detector which does not spread across the whole Fourier plane, but is small enough to pick a momentum eigenstate.

Well I don't think you're correct there, in the Walborn experiment they adjust the p-photon arm by a couple of meters, no worrying about planes there. In recent experiments they have done this stuff across Canary Islands, where I think it would be difficult to accurately find the "Fourier Plane". And I believe fibre optics are/will be used which makes the idea of your planes not really relevant.

If you do so, it is clear that all detection events at this position correspond to some momentum eigenstate. All detections on the other side (showing up in the coincidence counts) will also belong to some momentum eigenstate (high spatial coherence). As the visibility of a double slit interference pattern is proportional to the spatial coherence of the light beam used, this will give a conditional interference pattern. If you move the first detector out of the Fourier plane, you will notice the visibility of the interference pattern go down as you do not choose a momentum eigenstate subset anymore. Spatial coherence goes down and so does the visibility of the interference pattern. If you move the first detector around, you will notice that the interference pattern moves around as it is now the conditional interference pattern belonging to a different wave vector. To perform DCQE you can now play tricks and insert polarizers wave plates and whatever, but essentially this does not make the experiment more mysterious than entanglement already is.

Yeah, I have no doubt the pattern moves around, but we're investigating delayed eraser so all we really want is no pattern/some pattern as we remove/put in place the eraser.



The correct answer to the question "Why are coincidence counters used?" is that QM is probabilistic. Even if you could remove all background effects and have an efficient entangled pair source you still have the fact that ~50% of the p-photons will pass through the eraser probabilistically. There is no deterministic way round it, not by phase matching or other weird calculation, otherwise FTL signalling would be possible since you wouldn't need a coincidence match to determine if the eraser was in place or not.

Hmm, how can I explain this more pedagogically. The polariser placed in a different galaxy does not modify the s-photon detections. There is no retrocausation or such stuff. Have a look at section 6.1. of the arxiv paper from Walborn I linked earlier. That explains the basics of subsampling way better (and with pictures) than I could do by just typing text. Once you understand that it is not a big step to understanding DCQE.

No I already understand that QM is either non-local and/or non-separable so I have no problem interpreting the experiment.

You seem to have found a different interpretation that doesn't require non-locality and/or non-separability. You should try to publish this discovery, really.

 

[unusualname]

It is because the polarizer placed in the p-beam doesn't actually affect the s-photon detection events. It only affects the *coincidence measurements* which are not even generated until after *BOTH* photons have been detected. If you look only at the s-photon detections for the both cases, without considering the coincident statistics, then there is absolutely no difference for sets taken with and without the polarizer in the p-branch. In other words, there is NO interference observed in the detections for the s-photons in any case. The interference fringes are only evident in the coincident measurements.

Notice also, that for the case where the eraser (i.e. the polarizer in the p-beam) is in place, there are two different interference patterns that are observed, depending on whether the polarizer angle is set to match the quarter-wave plate for slit one or for slit two. The two patterns of fringes and anti-fringes (to use the terminology from Walborn's paper) are 180º out of phase ... this is because the of the well-defined phase relationship between the two-photon states, as explained by Cthugha.

The problem is that Cthugha's analysis uses classical phases, so it would be unlikely to apply across galaxies, or even the Canary islands with accuracy.

Do you not agree that QM is non-local and/or non-separable?

 

[SpectraCat]

Quo

The problem is that Cthugha's analysis uses classical phases, so it would be unlikely to apply across galaxies, or even the Canary islands with accuracy.

No it doesn't .. I have explained this many times, as has Cthugha, yet you persist to claim that it is true without any support for your position. By the way, what do you mean by "classical phase"? Phase is phase .. it has the same interpretation in both classical and quantum mechanics as far as I can tell.

Do you not agree that QM is non-local and/or non-separable?

Of course ... where did you get the idea that I wouldn't agree with that?

 

[unusualname]

No it doesn't .. I have explained this many times, as has Cthugha, yet you persist to claim that it is true without any support for your position. By the way, what do you mean by "classical phase"? Phase is phase .. it has the same interpretation in both classical and quantum mechanics as far as I can tell.

er, you're being funny right?. No, in QM a phase is assigned to a complex probability amplitude that evolves according to the Schrödinger eqn., in classical EM it is assigned to a wave described my Maxwell's equations. In measurements the intensity predicted by Maxwell's eqns matches the probability predicted by quantum (field) theory, but this is misleading, the Maxwell EM wave is not an ontological wave traveling through space with a well defined phase at all times (so that you might think you can naively interpret phase diagrams for single photons)

Of course ... where did you get the idea that I wouldn't agree with that?

The fact that you think DCQE can be explained by a classical phase argument.

 

[San K]

So I should explain it again? Why? It is all in Walborn's overview article. Results are not weird because you dislike them.


Hmm, how can I explain this more pedagogically. The polariser placed in a different galaxy does not modify the s-photon detections. There is no retrocausation or such stuff. Have a look at section 6.1. of the arxiv paper from Walborn I linked earlier. That explains the basics of subsampling way better (and with pictures) than I could do by just typing text. Once you understand that it is not a big step to understanding DCQE.

I have not been able to find the relevant/reference Walborn paper. Can you please paste the link again?


is it this one? http://arxiv.org/abs/quant-ph/0106078

navigating forums is a pain because only 10-15 posts show up per page (click) instead of say 200

edit: it must be this one ----> http://arxiv.org/abs/1010.1236

 

[San K]

It is because the polarizer placed in the p-beam doesn't actually affect the s-photon detection events. It only affects the *coincidence measurements* which are not even generated until after *BOTH* photons have been detected. If you look only at .

what do you mean by "generated" above? did you mean filtered?

because the s-photon position has been generated, the s-quantum has been registered.

the only thing that is to be done is filtering out from the noise (via coincidence counter) to get the proper sub-sample.

 

[San K]

Cthugha's analysis is clearly correct at least about one thing.
Postselection by coincidence counter has a key role in appearance of interference pattern.

That can be easily seen if you replace polarizer in idler beam with polarization beam splitter. Then you will have fringe and antifringe pattern at the same time just by looking at coincidences between signaling detector and one of the two detector at different outputs of PBS.

Ah...this explains it well...good one

 

[unusualname]

Cthugha's analysis is clearly correct at least about one thing.
Postselection by coincidence counter has a key role in appearance of interference pattern.

That can be easily seen if you replace polarizer in idler beam with polarization beam splitter. Then you will have fringe and antifringe pattern at the same time just by looking at coincidences between signaling detector and one of the two detector at different outputs of PBS.

Ah...this explains it well...good one

Yes he's correct in that one thing.

 

[SpectraCat]

what do you mean by "generated" above? did you mean filtered?

because the s-photon position has been generated, the s-quantum has been registered.

the only thing that is to be done is filtering out from the noise (via coincidence counter) to get the proper sub-sample.

No, the coincidence counter is not a simple noise filter ... it is required for proper identification of the photons that were generated as entangled pairs. Since the photon's travel different distances through the apparatus, they arrive at their respective detectors at different times. To keep it simple, consider a case where the A photon travels 2.9979 m to its detector, and the B photon travels twice as far to its detector. In that case there will be a delay of 1.0000 ns between emission and detection of the A photon, and a delay of 2.0000 ns for the B photon. The coincidence counter compensates for these delays so that the proper pair of photon detection events (separated by 1.0000 ns in this case) are paired in the analysis.

So, in order to see the interference pattern in these experiments, you have to wait until both the s-detector and p-detector events for each entangled pair have been registered before they can be properly compared via the coincidence counter. If the polarizer is in place (and set to the appropriate angle), then you will see the interference pattern revealed as an oscillation in the coincidence counts as a function of the s-detector position.

 

[San K]

No, the coincidence counter is not a simple noise filter ... it is required for proper identification of the photons that were generated as entangled pairs. Since the photon's travel different distances through the apparatus, they arrive at their respective detectors at different times. To keep it simple, consider a case where the A photon travels 2.9979 m to its detector, and the B photon travels twice as far to its detector. In that case there will be a delay of 1.0000 ns between emission and detection of the A photon, and a delay of 2.0000 ns for the B photon. The coincidence counter compensates for these delays so that the proper pair of photon detection events (separated by 1.0000 ns in this case) are paired in the analysis.

So, in order to see the interference pattern in these experiments, you have to wait until both the s-detector and p-detector events for each entangled pair have been registered before they can be properly compared via the coincidence counter. If the polarizer is in place (and set to the appropriate angle), then you will see the interference pattern revealed as an oscillation in the coincidence counts as a function of the s-detector position.

ok... so instead of "generated" we can say "identified/paired" (by the coincidence counter)

because when you said generated, one could assume...as "created"...it can give the sense that position is generated/created...

once s-quantum is registered at Ds, its position is locked, it won't change, right?

thus s-photon/position is not generated but identified (via pairing through coincidence counter)

 

[SpectraCat]

er, you're being funny right?. No, in QM a phase is assigned to a complex probability amplitude that evolves according to the Schrödinger eqn., in classical EM it is assigned to a wave described my Maxwell's equations.

Yeah, that's what I thought .. you don't understand what you are talking about. Phase is just an expression of the relative position of a wave in its cycle ... that's why it is expressed as an angle. So any system with wave-like properties will have a phase, regardless of whether it is quantum or classical. Coherence in any system can be expressed as the persistence of a well-defined phase relationship in time and/or space. So, the spatial coherence that is observed in the double-slit experiment (of which the DCQE is just an extension), occurs because the photon (or massive particle) is interfering with itself, and thus the different paths through the apparatus always have a well-defined phase relationship. The DCQE is more complicated, because it involves phase relationships of the two-photon entangled state as well, but that can still be accounted for, as Cthugha showed in his analysis.

Also, since photons are intrinsically quantum mechanical objects, it seems silly to talk about the phase relationships that Cthugha is presenting as classical. The polarization of photons is ALSO quantum mechanical, however it is analogous to the Jones vector for the classical description of EM radiation.

In measurements the intensity predicted by Maxwell's eqns matches the probability predicted by quantum (field) theory, but this is misleading, the Maxwell EM wave is not an ontological wave traveling through space with a well defined phase at all times (so that you might think you can naively interpret phase diagrams for single photons)

I have no idea what you tried to express above .. what is a "phase diagram" for a single photon?

The fact that you think DCQE can be explained by a classical phase argument.

Nope, I sure don't ... for the umpteenth time, if Cthugha's argument were classical, then a) we would not be talking about photons, and b) there could never be a well-defined phase relationship between the entangled photons, because there is no way of describing that classically.

 

[SpectraCat]

ok... so instead of "generated" we can say "identified/paired" (by the coincidence counter)

because when you said generated, one could assume...as "created"...it can give the sense that position is generated/created...

once s-quantum is registered at Ds, its position is locked, it won't change, right?

Yes, of course.

thus s-photon/position is not generated but identified (via pairing through coincidence counter)

I never said the s-photon position was generated .. I said that the interference pattern is only evident in the coincidence counts, which cannot be generated until detection events for both photons have been registered.

 

[unusualname]

Yeah, that's what I thought .. you don't understand what you are talking about. Phase is just an expression of the relative position of a wave in its cycle ... that's why it is expressed as an angle. So any system with wave-like properties will have a phase, regardless of whether it is quantum or classical. Coherence in any system can be expressed as the persistence of a well-defined phase relationship in time and/or space. So, the spatial coherence that is observed in the double-slit experiment (of which the DCQE is just an extension), occurs because the photon (or massive particle) is interfering with itself, and thus the different paths through the apparatus always have a well-defined phase relationship. The DCQE is more complicated, because it involves phase relationships of the two-photon entangled state as well, but that can still be accounted for, as Cthugha showed in his analysis.

haha, what more can I say, maybe you can use a similar analysis for GHZ states.

Also, since photons are intrinsically quantum mechanical objects, it seems silly to talk about the phase relationships that Cthugha is presenting as classical. The polarization of photons is ALSO quantum mechanical, however it is analogous to the Jones vector for the classical description of EM radiation.

yes of course it is, you guys are right there is no mystery in the DCQE, what was I thinking?

I have no idea what you tried to express above .. what is a "phase diagram" for a single photon?

Nope, I sure don't ... for the umpteenth time, if Cthugha's argument were classical, then a) we would not be talking about photons, and b) there could never be a well-defined phase relationship between the entangled photons, because there is no way of describing that classically.

No there isn't, well done.

 

[Cthugha]

edit: it must be this one ----> http://arxiv.org/abs/1010.1236

Yes, I mean this one. Sorry, I thought it was easy to find as I mentioned it only a few posts earlier. Besides section 6, also section 4.2 about ghost interference might help you understand the details better.

Well I don't think you're correct there, in the Walborn experiment they adjust the p-photon arm by a couple of meters, no worrying about planes there. In recent experiments they have done this stuff across Canary Islands, where I think it would be difficult to accurately find the "Fourier Plane". And I believe fibre optics are/will be used which makes the idea of your planes not really relevant.

As I said before you need to create a momentum eigenstate and using the Fourier plane is one of the possibilities. You can also go to far field conditions like in the double slit quantum eraser experiment. The result is the same. You decrease the angular size of the source, get a better defined momentum of your subset and increase spatial coherence. You just need the Fourier plane if you don not wan t to work at far field conditions.

Yeah, I have no doubt the pattern moves around, but we're investigating delayed eraser so all we really want is no pattern/some pattern as we remove/put in place the eraser.

Fine, then you agree with my explanation because if you do not use coincidence counting you will just measure a superposition of all these moced patterns which gives no pattern at all. From this point on the inserting/removing the eraser thing is trivial.

The correct answer to the question "Why are coincidence counters used?" is that QM is probabilistic. Even if you could remove all background effects and have an efficient entangled pair source you still have the fact that ~50% of the p-photons will pass through the eraser probabilistically. There is no deterministic way round it, not by phase matching or other weird calculation, otherwise FTL signalling would be possible since you wouldn't need a coincidence match to determine if the eraser was in place or not.

This is easily refuted because coincidence counting is also needed in experiments without DCQE or polarizers which just rely on basic conditional interference patterns or ghost imaging. The simple fact that relative phases leading to two-photon interference require coincidence counting, is all there is to it. This is not specific to DCQE. Again this is a result explained around equation 96 in Walborn's paper. You still did not tell me where he is wrong. The thing that prevents FTL signaling is the single-photon phase which is not well defined on average and therefore prevents information transfer. It is generally accepted that two-photon interference and single photon-interference are complementary due to the reasons I gave and information contained in coincidence counts therefore cannot be carried by one of these photons alone. I already gave you the references. Do you have any supporting your claim?

No I already understand that QM is either non-local and/or non-separable so I have no problem interpreting the experiment.

You seem to have found a different interpretation that doesn't require non-locality and/or non-separability. You should try to publish this discovery, really.

You do not seem to grasp where non-locality or non-seperability stems from. It comes into play because of the second and the first photon ending up in "compatible" states according to the rules of entanglement even if the second detection is not in the light cone of the first one and that is all there is to it. Most DCQE experiments indeed are not a proof of nonlocality as no Bell tests are performed. The first experiments on DCQE really showing that came in 2004 according to Walborns review paper.

No, in QM a phase is assigned to a complex probability amplitude that evolves according to the Schrödinger eqn., in classical EM it is assigned to a wave described my Maxwell's equations. In measurements the intensity predicted by Maxwell's eqns matches the probability predicted by quantum (field) theory, but this is misleading, the Maxwell EM wave is not an ontological wave traveling through space with a well defined phase at all times (so that you might think you can naively interpret phase diagrams for single photons)

You are wrong here. The phase is (somewhat) well defined in each run of the experiment. It is, however, usually not defined well on average, that means the phase will be different in every run of the experiment. Besides that you can map two-photon Fourier optics to classical Fourier optics using Klyshko's picture as mentioned in the Scarcelli et al. paper I already cited.

 

[unusualname]

OK, Cthuga I'm out, otherwise I may incur further warnings and infractions.

I would suggest you guys put together your explanations of the DCQE (since they are not widely known) in a paper, even just on arXiv so at least it may be argued on a more professional level.

 

[Drakkith]

The correct answer to the question "Why are coincidence counters used?" is that QM is probabilistic. Even if you could remove all background effects and have an efficient entangled pair source you still have the fact that ~50% of the p-photons will pass through the eraser probabilistically. There is no deterministic way round it, not by phase matching or other weird calculation, otherwise FTL signalling would be possible since you wouldn't need a coincidence match to determine if the eraser was in place or not.

I don't see what the 50% has anything to do with a counter. Take the eraser away and you would still need the counter to determine which photons were entangled, correct? It looks like its only job is to count when events are detected at the detectors. Nothing else. Take the counter away, run the experiment, and take the times from each detector for the detection events and compare them to match up the events. You just became a coincidence counter. Correct?

Also, can someone tell me what this purpose of these DCQE's are? Are they to determine if the photons can communicate with each other once one interacts and has to go to a set quantum state after the other one has already been detected?

 

[Cthugha]

Take the counter away, run the experiment, and take the times from each detector for the detection events and compare them to match up the events. You just became a coincidence counter. Correct?

Yes, modern coincidence counters really allow to do so and also offer a mode which gives only timestamps of detections from each photo diode and allows to do the analysis by hand. However, this mode is rarely used as the count rates from each diode are usually on the order of 10^6 per second at least and recording timestamps for every detection piles up huge amounts of data which need to be written to a hard disk quickly.

Also, can someone tell me what this purpose of these DCQE's are? Are they to determine if the photons can communicate with each other once one interacts and has to go to a set quantum state after the other one has already been detected?

Well, from a historical point of view quantum erasers and its delayed choice version were introduced by Scully in 1982. Back then this experiment was aimed at answering the question whether uncertainty or complementarity is more fundamental. So they were aiming at showing that it is not the uncontrolled disturbance introduced by a position measurement in the common double slit experiment that causes the interference pattern to disappear in the double slit experiment and thought of a reversible way to mark the way. The delayed choice version of the quantum eraser was introduced in the same paper - mainly because Aspect's idea of delayed choice experiments in general were intensely debated at that time.

 

[San K]

So they were aiming at showing that it is not the uncontrolled disturbance introduced by a position measurement in the common double slit experiment that causes the interference pattern to disappear in the double slit experiment and thought of a reversible way to mark the way.

it is not the uncontrolled disturbance but rather the way (rules/methods) the sub-samples of photons are chosen/identified ? ...that causes interference pattern to appear/disappear

 

[zonde]

yeah, you are probably right, it would be interesting to see an experiment done this way rather than using a coincidence counter, afaik there is no such published experiment.

There is. Weihs et al experiment http://arxiv.org/abs/quant-ph/9810080"

"Each observer station featured a PC which stored the tables of time tags accumulated in an individual measurement. Long after measurements were finished we analyzed the files for coincidences with a third computer."

 

[Drakkith]

Well, from a historical point of view quantum erasers and its delayed choice version were introduced by Scully in 1982. Back then this experiment was aimed at answering the question whether uncertainty or complementarity is more fundamental. So they were aiming at showing that it is not the uncontrolled disturbance introduced by a position measurement in the common double slit experiment that causes the interference pattern to disappear in the double slit experiment and thought of a reversible way to mark the way. The delayed choice version of the quantum eraser was introduced in the same paper - mainly because Aspect's idea of delayed choice experiments in general were intensely debated at that time.

What did they find out?

 

[unusualname]

yeah, you [SpectraCat] are probably right, it would be interesting to see an experiment done this way rather than using a coincidence counter, afaik there is no such published experiment.

There is. Weihs et al experiment http://arxiv.org/abs/quant-ph/9810080"

"Each observer station featured a PC which stored the tables of time tags accumulated in an individual measurement. Long after measurements were finished we analyzed the files for coincidences with a third computer."

Thanks zonde, I assume this should be easier now than in 1998, but the lengths the experimenters go to close "loopholes" is impressive.

 

[zonde]

There is something puzzling for me about Walborn Quantum eraser experiment.

Without quarter wave plates coincidence postselection increases spatial coherence. Otherwise interference is just a single beam interference.
At least that's how it seems from Walborn explanation (http://arxiv.org/abs/1010.1236" 4.1)

But in that case placing polarizer in idler beam when there are no quarter wave plates before slits should change nothing. Interference pattern should still be there.

Does it seems right?

 

[SpectraCat]

There is something puzzling for me about Walborn Quantum eraser experiment.

Without quarter wave plates coincidence postselection increases spatial coherence. Otherwise interference is just a single beam interference.
At least that's how it seems from Walborn explanation (http://arxiv.org/abs/1010.1236" 4.1)

But in that case placing polarizer in idler beam when there are no quarter wave plates before slits should change nothing. Interference pattern should still be there.

Does it seems right?

Yes, that is correct. Note also that with the QWP's in place, the angles of the QWP's seem to define a preferred basis for the polarizer to recover the interference patterns. The authors only report results for the polarizer in the QWP basis (45 and 135 degrees), but it would be interesting to see how changing the angle of the polarizer changes the experimental results. From an (admittedly casual) analysis of the mathematical treatment earlier in the paper, it seems like moving the polarizer angle away from 45 towards 90 would cause the interference fringes to gradually lose intensity, and eventually disappear at 90 (or 0) degrees. The reason I find this somewhat striking (assuming it is correct) is that it seems to work in the opposite sense of other experiments where a particular polarization basis is associated with "which path" information, in that interference is not observed in those cases until the polarizer angle is chosen to mix the basis states.

 

[zonde]

Yes, that is correct. Note also that with the QWP's in place, the angles of the QWP's seem to define a preferred basis for the polarizer to recover the interference patterns. The authors only report results for the polarizer in the QWP basis (45 and 135 degrees), but it would be interesting to see how changing the angle of the polarizer changes the experimental results. From an (admittedly casual) analysis of the mathematical treatment earlier in the paper, it seems like moving the polarizer angle away from 45 towards 90 would cause the interference fringes to gradually lose intensity, and eventually disappear at 90 (or 0) degrees. The reason I find this somewhat striking (assuming it is correct) is that it seems to work in the opposite sense of other experiments where a particular polarization basis is associated with "which path" information, in that interference is not observed in those cases until the polarizer angle is chosen to mix the basis states.

Well, I am quite sure that source determines preferred basis. Source produces H and V modes in certain basis. QWP mixes H and V modes so that interference (at detector) happens between them. And polarizer at +45° or -45° mixes H and V modes in similar way as two QWPs.
When you rotate polarizer you get more of one mode and less of the other. Finally at 0° and 90° you get pure H or V mode (so no interference between modes after polarizer).

That's how I see it.

 

[SpectraCat]

Well, I am quite sure that source determines preferred basis. Source produces H and V modes in certain basis. QWP mixes H and V modes so that interference (at detector) happens between them. And polarizer at +45° or -45° mixes H and V modes in similar way as two QWPs.
When you rotate polarizer you get more of one mode and less of the other. Finally at 0° and 90° you get pure H or V mode (so no interference between modes after polarizer).

That's how I see it.

No .. the source is entangled, so there is no preferred basis for detection until the QWP's are added. This is the fundamental results showed by the Aspect experiments, and since confirmed many times.

 

[San K]

There is something puzzling for me about Walborn Quantum eraser experiment.

Without quarter wave plates coincidence postselection increases spatial coherence. Otherwise interference is just a single beam interference.
At least that's how it seems from Walborn explanation (http://arxiv.org/abs/1010.1236" 4.1)

But in that case placing polarizer in idler beam when there are no quarter wave plates before slits should change nothing. Interference pattern should still be there.

Does it seems right?

Well then in this case you are getting which-way (via polarizer in idler) and also getting interference pattern...

 

[San K]

Well then in this case you are getting which-way (via polarizer in idler) and also getting interference pattern...

edit: on second thoughts...ah well you are not getting which-way...because you cannot get which-way by just a polarizer, you need to compare with s-photon

 

[San K]

in conclusion (to the title of this topic):

the position of the s-photon (or the p-photon or anything in the world for that matter) on the detector/screen never changes...once registered...(i.e. the past doesnot/cannot be changed)

the interference pattern is created/emerges when we "sample/filter out" the detections/photons lying on the blank spaces between the fringes...

or in other words...

the non-interference pattern is blob...out of the blob...we filter out (via coincidence counter) the markings/photons that don't lie on the interference fringes...

the non-interference blob contains the fringes hidden within it as a sub-set/sub-sample...

on a separate but related note:

also you can go from an (original) non-interference pattern to a interference pattern

but not vice-versa, i.e.

you cannot get/go from an (original) interference pattern to a non-interference pattern

because: the interference pattern is a sub-set of the non-interference pattern

Walborn does not discuss this, nor does he mention why they do the experiment only one-way...i.e. from non-interference blob to interference fringes...

 

[unusualname]

It's easier to explain if you just accept a QM explanation, and not try to make it "intuitive", in case you disagree perhaps try to explain this experiment:

Experimental realization of Wheeler's delayed-choice Gedanken Experiment (Science, 2007)

 

[San K]

It's easier to explain if you just accept a QM explanation, and not try to make it "intuitive", in case you disagree perhaps try to explain this experiment:

Experimental realization of Wheeler's delayed-choice Gedanken Experiment (Science, 2007)

Cthuga/SpectraCat and I are advocating non-local phenomena with sub-sampling as explanation for the patterns in DCQE.

you said, that you agreed with Cthuga/SpectraCat and San K.

I am ok with the explanation above, did you go through it?

There is nothing surprising in the paper, you mentioned, as I see it.

When we try which-way the photon wave function collapses and it follows a non-interference path.

The wave function can collapse well after the slits. It collapses when we try to detect the photon.

In my, long held, hypothesis:

the photon, always carries both options - particle and wave.

It depends upon which aspect we want the photon to manifest.

 

[unusualname]

Cthuga/SpectraCat and I are advocating non-local phenomena with sub-sampling as explanation for the patterns in DCQE.

you said, that you agreed with Cthuga/SpectraCat and San K.

I am ok with the explanation above, did you go through it?

However I will go through the link you provided.

If you want to assume an explicit non-local explanation then fine, but be aware that it is just an assumption and many people don't like explicit non-local interpretations of QM.

 

[unusualname]

There is nothing surprising in the paper, you mentioned, as I see it.

When we try which-way the photon wave function collapses and it follows a non-interference path.

The wave function can collapse well after the slits. It collapses when we try to detect the photon.

In my, long held, hypothesis:

the photon, always carries both options - particle and wave.

It depends upon which aspect we want the photon to manifest.

Yeah, ok, but that's not an explanation it's just an advertisement of your philosophical pondering, and doesn't really make any sense to me. But QM is tricky and people have all kinds of weird ideas all over the place ;-)

 

[San K]

If you want to assume an explicit non-local explanation then fine, but be aware that it is just an assumption and many people don't like explicit non-local interpretations of QM.

ya agreed...:), Einstein was one of them...

 

[SpectraCat]

Cthuga/SpectraCat and I are advocating non-local phenomena with sub-sampling as explanation for the patterns in DCQE.

Yeah, ok, but that's not an explanation it's just an advertisement of your philosophical pondering, and doesn't really make any sense to me. But QM is tricky and people have all kinds of weird ideas all over the place ;-)

Just to clarify, there is a phase-based quantum explanation of the 2007 Aspect delayed-choice experiment as well, and just as with the DCQE, it serves to dispel much of the apparent mysticism associated with the popular analysis.

The 2007 Aspect experiment can be explained using an even simpler phase-based argument than the DCQE experiments. In this case, there is only one-photon. Interaction with the first 45º-polarized photon into a superposition state, which becomes entangled with the two spatial paths in the interferometer .. one polarization state (S or P) travels down each arm. At the detector, we have two choices based on the setting of an electo-optic modulator (EOM)

1) in the open configuration, the (EOM) does not affect the two orthogonally polarized beams, and they are sent through to a Wollaston-prism which collapses the spatial-entanglement, with a 50-50 probability distribution between the two detection channels, irrespective of the phase of the interferometer.

2) in the closed configuration, the (EOM) acts like a second beamsplitter in a Mach-Zender interferometer, converting the spatially-entangled state back into a polarization-entangled state in a *phase-sensitive fashion*. That is, since the quantum state is coherent, there is a well-defined phase relationship between the two polarization components as they travel through the two arms of the interferometer. These two paths have different lengths (controlled by tilting an optic on the detection stage in this experiment), and therefore the two output beams have different phases, and interfere with each other, causing their associated polarization components to have different contributions to the final superposition, and this gives rise to the different intensities in the S & P channels after the Wollaston-prism, depending on the phase difference between the interferometer arms.

There are two important things to notice about this interpretation. First, it is inherently NON-LOCAL .. it relies on the entanglement of the photon along two spatially separated paths through the interferometer. Second, it relies on the COHERENCE of the quantum superposition throughout the apparatus. Both of those are features that are exclusive to quantum mechanics, and cannot be explained by a classical interpretation.

Another important thing to notice about the explanation is that it completely removes any question of whether there is a causality violation in the experiment. Since the photon always travels through both arms of the interferometer, there is no paradox about "which path" information being selected "after" the photon has "chosen a path". With all due respect to Prof. Wheeler, it is my opinion that those are features of a classically-based MIS-interpretation of the experiment.

 

[unusualname]

You're still not getting it SpectraCat. You are trying to explain what is "happening" via some convoluted argument about phase relationships which doesn't, and can't possibly explain what is "happening".

You are not dispelling any "mysticism" you are simply promoting an interpretative explanation.

 

[SpectraCat]

You're still not getting it SpectraCat. You are trying to explain what is "happening" via some convoluted argument about phase relationships which doesn't, and can't possibly explain what is "happening".

You are not dispelling any "mysticism" you are simply promoting an interpretative explanation.

Nope .. I am definitely "getting it". As I understand it, the principal question with delayed choice is whether or not it violates causality. My analysis above, which is not really convoluted but is rather simple once you understand it, uses the principles of quantum mechanics to show unequivocally why the answer is, "no, delayed choice does not violate causality".

[EDIT] By the way, you characterized this as an "alternative explanation" .. what other explanations are there that are consistent with QM?

[EDIT2] Also, in what way does the explanation I gave fail to explain what happens in the experiment?