DCQE - how does/can the pattern change?

[Drakkith]

No Malus' law is not a red herring it is the main reason why coincidence counters are needed.

Where is the PBS you describe in the DCQE experiments?

Even in the case of large distances for the p-photons (which hasn't been done in practice beyond a few meters for the DCQE btw) you will still have ~50% of p-photons not reaching the detector, even if you use fibre optics to reduce background noise and an efficient entangled photon source.

Can we forego the counter and just use timestamps to determine which photons at S matched up at P?

 

[Drakkith]

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 in there will be aqn 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)

According to everything I've read in the last few hours, it never had anything to do with photons knowing anything. It is all the counter. The results from the experiments that you have linked and I have looked up have all had their final data based on Coincidence Counts, not counts from each detector individually. That is, unless I've misunderstood something, which is entirely possible.

 

[unusualname]

According to everything I've read in the last few hours, it never had anything to do with photons knowing anything. It is all the counter. The results from the experiments that you have linked and I have looked up have all had their final data based on Coincidence Counts, not counts from each detector individually. That is, unless I've misunderstood something, which is entirely possible.

Coincidence counters are always needed to extract an interference pattern. But if you don't think there is anything unusual about a distant eraser changing the results of the coincidence count then well done, maybe you just get QM so naturally it doesn't require further contemplation for you!

 

[unusualname]

Can we forego the counter and just use timestamps to determine which photons at S matched up at P?

Yes, but i don't know if the technology exists (photons are damn fast!)

 

[Drakkith]

Coincidence counters are always needed to extract an interference pattern. But if you don't think there is anything unusual about a distant eraser changing the results of the coincidence count then well done, maybe you just get QM so naturally it doesn't require further contemplation for you!

Do you mean they are required for experiments of this type? I know you can get an interference pattern just by shining a laser through a double slit, you don't even need a detector or counter.

I don't see anything odd about the eraser because I don't think it is actually affecting anything other than what we decide to look at. If I put a polarizer that only let's Y polarized photons through, I have chosen to only look at those. Because of entanglement, the only photons that arrive at the P AND the S detectors within the timeframe of the counter are the ones where Y polarized photon went to P and X polarized photon went to S. All the Y photons that actually made it to the S detector are never recorded because there was no corrosponding input from the other detector to the counter.

Is it wrong to say that we KNOW that all the photons going through POL1 to detector P are Y polarized? Of course until the photons actually enter POL1 we cannot say which photon is X or Y.

Yes, but i don't know if the technology exists (photons are damn fast!)

Sure, but I think we have the means to accurately keep the time in two detectors synced close enough together to accurately distinguish the time between two pairs of photons hitting the detectors. It really isn't the speed of the photon, it is the time between strikes.

 

[SpectraCat]

Yes, but i don't know if the technology exists (photons are damn fast!)

Of course the technology exists, what do you think a coincidence counter *is*? It is a way of "time-stamping" detection events so that you can correlate them with very precise delays expected for different travel distances.

Conceptually it works as follows .. although I believe the actual coincidence counters may register coincidences in real time .. that difference is not significant. Once the detection events from the two detectors have been registered, you subtract the respective delays (which are usually constant for all detections) and compare the two data streams. Any detection events that match within a pre-defined time window (typically chosen to be the temporal resolution of the recording system) are considered to be coincident.

 

[Drakkith]

Of course the technology exists, what do you think a coincidence counter *is*? It is a way of "time-stamping" detection events so that you can correlate them with very precise delays expected for different travel distances.

Conceptually it works as follows .. although I believe the actual coincidence counters may register coincidences in real time .. that difference is not significant. Once the detection events from the two detectors have been registered, you subtract the respective delays (which are usually constant for all detections) and compare the two data streams. Any detection events that match within a pre-defined time window (typically chosen to be the temporal resolution of the recording system) are considered to be coincident.

So why filter the results at all? What is the point? I fail to see any significance in the fact that causing only X or Y polarized photons through one side of the experiment will cause different effects for the other when the counter is filtering the results anyways.

I understand the whole thing about entangled photons being in both states at the same time or whatever. I don't see this experiment as any evidence for that though.

 

[unusualname]

Of course the technology exists, what do you think a coincidence counter *is*? It is a way of "time-stamping" detection events so that you can correlate them with very precise delays expected for different travel distances.

Conceptually it works as follows .. although I believe the actual coincidence counters may register coincidences in real time .. that difference is not significant. Once the detection events from the two detectors have been registered, you subtract the respective delays (which are usually constant for all detections) and compare the two data streams. Any detection events that match within a pre-defined time window (typically chosen to be the temporal resolution of the recording system) are considered to be coincident.

have you any experience of timing and computers?

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.

For example, in linux/unix it is nontrivial to get professional music software working with a latency of less than 10ms (which is absolute minimum tolerance for musicians) without applying real-time patches to the kernel.

A coincidence counter is a self contained electronic circuit, which works on physical principles.

However, this is really not a big argument point for me, as I've often suggested the exact scenario you mention to deal with very distant erasers. It would be easier to try to introduce larger delays between the entangled pair emissions so the time window dosen't have to be so accurate.

 

[SpectraCat]

No Malus' law is not a red herring it is the main reason why coincidence counters are needed.

You are right that it is required if PBS's are not used.

[EDIT: What I meant was that you are right that coincidence counting MUST absolutely be used if you are losing half your photons due to Malus' Law .. not sure if my initial phrasing captured that]

Where is the PBS you describe in the DCQE experiments?

Sorry .. I was answering the question in general, not in the context of the DCQE. It doesn't seem like PBS's have been used in DCQE experiments, at least not the Walborn or Kim & Scully experiments. This may be because the PBS's could interfere with the DCQE results ... if the different paths in the DCQE are polarization sensitive, then recording the results with a PBS could be equivalent to obtaining which path information, which would destroy the interference pattern. I'm actually interested to search the literature now and see if this has been tested .. maybe Dr. Chinese knows the answer.

Even in the case of large distances for the p-photons (which hasn't been done in practice beyond a few meters for the DCQE btw) you will still have ~50% of p-photons not reaching the detector, even if you use fibre optics to reduce background noise and an efficient entangled photon source.

Well, if the distances are different by a few cm, then it will change the coincidence timing rather drastically. A 1 cm difference in distance correlates to about a 33 ps difference in timing, which is easily within the limits of modern single-photon counting detectors. So, I would say the proper accounting of the different delay timing is also quite important for a proper interpretation of the experiment, which was my point.

 

[SpectraCat]

have you any experience of timing and computers?

Yes, lots ... and also with ps-synchronization of delay generators and photodiode detectors as well.

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.

I don't think that's really true .. you only need a single timestamp at the begninning of each stream if you have a reliable data recording device. Once you have the initial timestamp (which could be taken from a radio signal from an atomic clock broadcast, or similar), then you use that as your initial trigger and start counting time bins. A fast digital oscilloscope (I have seen them with up to 12 GHz bandwidth, or about 83 ps per bin .. I guess there may even be faster ones available) can be used to record the data stream on either end. The coincidence comparisons can be done later, as I pointed out. Usually this is not required, since the experiments are done in the same lab, but I don't see why it shouldn't be possible.

With regard to operating systems and computer hardware, there are real-time operating systems which are expressly designed for applications where real-time synchronization is essential ... they have WAY fewer latency issues than conventional operating systems. We used one of these to control the free-electron laser at the institute where I did my first post-doc. I think it was called LynxOS. If I recall correctly, there is a real-time version of Linux available as well. Perhaps that would help with your timing issues for the music software?

However, this is really not a big argument point for me, as I've often suggested the exact scenario you mention to deal with very distant erasers. It would be easier to try to introduce larger delays between the entangled pair emissions so the time window dosen't have to be so accurate.

I am not sure that is even required, if you use the setup I described above.

 

[unusualname]

Do you mean they are required for experiments of this type? I know you can get an interference pattern just by shining a laser through a double slit, you don't even need a detector or counter.

yes

I don't see anything odd about the eraser because I don't think it is actually affecting anything other than what we decide to look at. If I put a polarizer that only let's Y polarized photons through, I have chosen to only look at those. Because of entanglement, the only photons that arrive at the P AND the S detectors within the timeframe of the counter are the ones where Y polarized photon went to P and X polarized photon went to S. All the Y photons that actually made it to the S detector are never recorded because there was no corrosponding input from the other detector to the counter.

Is it wrong to say that we KNOW that all the photons going through POL1 to detector P are Y polarized? Of course until the photons actually enter POL1 we cannot say which photon is X or Y.

Yes they have a known polarisation once they go through POL1 but they have UNKNOWN polarisation before they go through POl1.
But how did the s-photons corresponding to these photons in the coincidence counts "know" which photons would pass through POL1 if the s-photons were detected maybe several years earlier?

 

[unusualname]

Yes, lots ... and also with ps-synchronization of delay generators and photodiode detectors as well.



I don't think that's really true .. you only need a single timestamp at the begninning of each stream if you have a reliable data recording device. Once you have the initial timestamp (which could be taken from a radio signal from an atomic clock broadcast, or similar), then you use that as your initial trigger and start counting time bins. A fast digital oscilloscope (I have seen them with up to 12 GHz bandwidth, or about 83 ps per bin .. I guess there may even be faster ones available) can be used to record the data stream on either end. The coincidence comparisons can be done later, as I pointed out. Usually this is not required, since the experiments are done in the same lab, but I don't see why it shouldn't be possible.

With regard to operating systems and computer hardware, there are real-time operating systems which are expressly designed for applications where real-time synchronization is essential ... they have WAY fewer latency issues than conventional operating systems. We used one of these to control the free-electron laser at the institute where I did my first post-doc. I think it was called LynxOS. If I recall correctly, there is a real-time version of Linux available as well. Perhaps that would help with your timing issues for the music software?



I am not sure that is even required, if you use the setup I described above.

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.

 

[Drakkith]

yes



Yes they have a known polarisation once they go through POL1 but they have UNKNOWN polarisation before they go through POl1.
But how did the s-photons corresponding to these photons in the coincidence counts "know" which photons would pass through POL1 if the s-photons were detected maybe several years earlier?

I personally would say they didn't nor did they need to. However that makes me feel like I just stabbed some great QM law in the back lol.

I'm guessing this comes down to whether or not the emitted photons are actually in the X or Y polarization when emitted, or if they are in "both" states and only go to one when forced to. I don't know the various interpretations of QM well enough to say either way really. All I know is that the experiment, 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.

 

[unusualname]

I personally would say they didn't nor did they need to. However that makes me feel like I just stabbed some great QM law in the back lol.

I'm guessing this comes down to whether or not the emitted photons are actually in the X or Y polarization when emitted, or if they are in "both" states and only go to one when forced to. I don't know the various interpretations of QM well enough to say either way really. All I know is that the experiment, 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.

Exactly, Bell tests have shown that it is not the case, especially the modern ones like GHZ experiments. Until measurements are made we can't say a photon has a definite property.

This is highly non-intuitive, and maybe even unsettling, but it is what experiments force us to conclude.

Nature must have a non-classical description, maybe involving both non-locality and non-realism, and we haven't figured it out yet.

 

[Drakkith]

What have the bell tests shown? I'll go look them up now, but it would help if I knew exactly how the results supported this view.

I mean, I agree that we cannot know the state until we have measured it or influenced it, but to me that doesn't mean that it in both or neither states until that time. I know that at least one major interpretation says that, so I feel silly for disagreeing, but I'd really need to see exactly why they say that.

Edit: Reading some stuff on Bells Theorem now...

 

[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.