DCQE - how does/can the pattern change?

[San K]

in DCQE - Delayed Choice Quantum Eraser


such as the one listed here -----> http://grad.physics.sunysb.edu/~amarch/

let's say we know which-way for s photons...thus we should get two columns/bands of dots...one for each slit...a Zero interference pattern...and it would look like this-----> [PLAIN]http://www.colorado.edu/physics/2000/schroedinger/images/sgt_gunn.gif

now we put eraser in front of p-photon...erasing which-way info...now later when we do co-incidence count ...we see the pattern of s is not two bands...but many bands...corresponding to an interference pattern.

how did the pattern change from two columns, to multiple columns of interference bands...that too after the s-photons had already registered their position?...

i mean...where did the interference fringes come from? since there are no sub-samples for the fringes...all there is simply two bands as shown in the animation above

what am i missing here?

is it that...when we do which way for s-photons...within those two bands...are hidden the multiple bands/fringes?
...i.e. the sub samples (containing the fringes) are hidden within those two bands?

 

[Drakkith]

The animation above doesn't even depict a diffraction pattern. See here: http://upload.wikimedia.org/wikipedia/commons/c/c2/Single_slit_and_double_slit2.jpg (Or not much of one at least)

The first image is of a single slit diffraction, while the bottom is the double slit diffraction with interference. When you know which slit the photons go through the interference bands dissappear but the diffraction pattern remains. The image I linked doesn't show it, but a double slit with no interference would be the sum of the two patterns I think. It wouldn't be two columns like your animation is presenting.

 

[Drakkith]

I did have a question though. The graphs from here: http://grad.physics.sunysb.edu/~amarch/
are showing counts per 400 seconds based on the distance from 0 mm that the detector is moved. Why does moving the detector back and forth affect the number of counts on the incident counter?

 

[San K]

I did have a question though. The graphs from here: http://grad.physics.sunysb.edu/~amarch/
are showing counts per 400 seconds based on the distance from 0 mm that the detector is moved. Why does moving the detector back and forth affect the number of counts on the incident counter?

i can only take a guess, since I am not a physicist.

the detector is small, it is not able to capturing all (or even most of) the photons, it is simply caputuring the photons that fall within its width at a particular position.

the images you showed are good and answer my question partially, but there would be two colums and this can be concluded from the images you showed...the single slit shows one...a double slit (without interefernce) would show two...bars...

 

[Drakkith]

i can only take a guess, since I am not a physicist.

the detector is small, it is not able to capturing all (or even most of) the photons, it is simply caputuring the photons that fall within its width at a particular position.

the graphs you showed are good and answer my question partially, but there would be two colums and this can be concluded from the graphs you showed...the single slit shows one...a double slit (without interefernce) would show two...bars...

I don't think the detector is that small. I think it has to do with the interference, but I'm not sure how.

Yeah, I think the pattern for 2 slits without interference would be like that bottom picture but without the interference bands. But I'm not sure.

 

[SpectraCat]

The only issue with the DCQE experiment is the way in which it is explained .. the kind of narrative like the one on the site in the OP makes it seem like something mystical is happening, but it is just smoke and mirrors. I always suspected that was the case, but I didn't fully understand why until I read the explanations posted on here by Cthugha .. I encourage you to search for them, read them carefully ... then go away, come back, and read them again .. repeat as necessary until the lightbulb comes on .

A very brief summary is that the effect of "erasure" is that it changes which photons can participate in the coincidence counting for each position of the Ds detector. There is a definite phase relationship between the photons that can be coincidently detected when no "which-path" information is available. It is this phase relationship that is responsible for the interference pattern. When you modify the double-slit side of the apparatus so that which path information can be obtained, you lose that well-defined phase relationship, and so the interference pattern *seems* to disappear. In reality, it is still there, but it is superposed with a similar interference pattern that is 180º out of phase, so the interference fringes are not observable.

I am fairly sure I have reproduced the sense of Cthugha's excellent explanations with the above summary, but of course you should check it for yourselves to verify the details. One summary is here: https://www.physicsforums.com/showthread.php?t=320334, but if you search for Cthugha's posts you can find more complete descriptions.

 

[San K]

The only issue with the DCQE experiment is the way in which it is explained .. the kind of narrative like the one on the site in the OP makes it seem like something mystical is happening, but it is just smoke and mirrors. I always suspected that was the case, but I didn't fully understand why until I read the explanations posted on here by Cthugha .. I encourage you to search for them, read them carefully ... then go away, come back, and read them again .. repeat as necessary until the lightbulb comes on .

A very brief summary is that the effect of "erasure" is that it changes which photons can participate in the coincidence counting for each position of the Ds detector. There is a definite phase relationship between the photons that can be coincidently detected when no "which-path" information is available. It is this phase relationship that is responsible for the interference pattern. When you modify the double-slit side of the apparatus so that which path information can be obtained, you lose that well-defined phase relationship, and so the interference pattern *seems* to disappear. In reality, it is still there, but it is superposed with a similar interference pattern that is 180º out of phase, so the interference fringes are not observable.

I am fairly sure I have reproduced the sense of Cthugha's excellent explanations with the above summary, but of course you should check it for yourselves to verify the details. One summary is here: https://www.physicsforums.com/showthread.php?t=320334, but if you search for Cthugha's posts you can find more complete descriptions.

good link, and explanation, of Cthuga's ...thanks SpectraCat


Do the two bands (formed in case there is non-interference) also contain the interference bands within them?

 

[unusualname]

...There is a definite phase relationship between the photons that can be coincidently detected when no "which-path" information is available. It is this phase relationship that is responsible for the interference pattern...

No, no.

There is no classical explanation of the DCQE by appealing to classical phase relationships.

I'm fed up correcting this idea promulgated on a forum 80+ years since this sort of stuff was well understood.

 

[San K]

No, no.

There is no classical explanation of the DCQE by appealing to classical phase relationships.

I'm fed up correcting this idea promulgated on a forum 80+ years since this sort of stuff was well understood.

hmmm...so who is right...cthuga/spectracat or unusual name?...eeny meeny miny moe

 

[unusualname]

hmmm...so who is right...cthuga/spectracat or unusual name?...eeny meeny miny moe

here's a clue, coincidence counters aren't accurate enough to do phase matching.

 

[San K]

here's a clue, coincidence counters aren't accurate enough to do phase matching.

i am surprised...its a matter of time till we find who is the right/better physicist...till then...let the debate begin...

 

[unusualname]

i am surprised...its a matter of time till we find who is the right/better physicist...till then...let the debate begin...

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).

If you think peer reviewed journals would allow publication of experiments like the DCQE without mentioning the (according to crackpots) important role of the coincidence counters in measuring classical phases then you should join a forum with crackpots who discuss how QM has been "wrong" and classical physics can stiil be right in detail that might appeal to a gone-awry mind.

 

[SpectraCat]

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).

If you think peer reviewed journals would allow publication of experiments like the DCQE without mentioning the (according to crackpots) important role of the coincidence counters in measuring classical phases then you should join a forum with crackpots who discuss how QM has been "wrong" and classical physics can stiil be right in detail that might appeal to a gone-awry mind.

Please provide a detailed refutation of the explanation of the DCQE experiment given by Cthugha in the post I referenced. I have worked through it myself, and I don't believe that there are any errors. It is also consistent with all of the experimental data, so I don't really understand what your objection could be, but that may be because I still have something to learn about this experiment. If so, then I would happy to learn it from you.

 

[unusualname]

Please provide a detailed refutation of the explanation of the DCQE experiment given by Cthugha in the post I referenced. I have worked through it myself, and I don't believe that there are any errors. It is also consistent with all of the experimental data, so I don't really understand what your objection could be, but that may be because I still have something to learn about this experiment. If so, then I would happy to learn it from you.

No, you please provide a peer reviewed reference which supports this analysis.

Cthugha has very carefully derived some mathematical phase relationships which have no bearing on the physical explanation of the experiment.

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,

 

[San K]

No, you please provide a peer reviewed reference which supports this analysis.

Cthugha has very carefully derived some mathematical phase relationships which have no bearing on the physical explanation of the experiment.

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

Unusual name- thanks for your posts.

some of us don't know who is right yet, but just to understand this better and get started

what do co-coincident counters do?

1. do they match via timing?
2. can they match entangled pairs? how?
3. can they check for spin (and hence opposite spin)?

I am aware that two randoms photons can get selected in the same time bin and thus the accuracy of the co-incidence counter is not 100%.Are you also saying that the "phase difference" analysis by Cthuga is not a valid explanation because if it was then Walborn's group (and the paper you cite) would have mentioned it?Also what, in your opinion, is the explanation for the DCQE observations/results?

 

[unusualname]

Unusual name- thanks for your posts.

some of us don't know who is right yet, but just to understand this better and get started

what do co-coincident counters do?

1. do they match via timing?
2. can they match entangled pairs? how?
3. can they check for spin (and hence opposite spin)?

I am aware that two randoms photons can get selected in the same time bin and thus the accuracy of the co-incidence counter is not 100%.


Are you also saying that the "phase difference" analysis by Cthuga is not a valid explanation because if it was then Walborn's group (and the paper you cite) would have mentioned it?


Also what, in your opinion, is the explanation for the DCQE observations/results?

1,2.
The coincidence counts match SPDC pairs that have both traveled through the entire experimental apparatus, the timing window is short enough to ensure matches, see published experimental details to ensure resolution window (unless it's assumed obvious)

3.
Who the heck cares about spin or any other variable?

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

My explanation for the DCQE is that nature is non-local and non-real.

 

[San K]

No, you please provide a peer reviewed reference which supports this analysis.

Cthugha has very carefully derived some mathematical phase relationships which have no bearing on the physical explanation of the experiment.

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,

Unusualname - one experiment, where phase relationships and interference is discussed, comes to mind...

http://spie.org/etop/2007/etop07expI.pdf

Also the Mach Zehnder description below talks about phase difference and interference

http://en.wikipedia.org/wiki/Mach–Zehnder_interferometer

 

[SpectraCat]

No, you please provide a peer reviewed reference which supports this analysis.

Cthugha has very carefully derived some mathematical phase relationships which have no bearing on the physical explanation of the experiment.

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, so maybe not made clear for you idiots. (sorry for the abuse, but at least fourth time I've had to explain this in a year)

First off, there's no call for any kind of abuse, and especially not calling people who are discussing things in good faith "idiots" or "crackpots". I am not a crackpot, and neither is Cthugha. That is quite an accusation, and you should be very careful before throwing it around so freely.

You and Cthugha have tangled before, and I have read those threads ... in my opinion he made his points much more clearly than you did, and supported them better with literature references as well. Everything he mentions is consistent with a QUANTUM explanation, not a simple classical one .. where do you think the well-defined phase of the two-photon state comes from, if not the fact that it is a pure quantum state, and therefore has a coherent phase-relationship between the photons? Furthermore, his explanation is completely consistent with the experimental results. He is not denying any of the claims or interpretations made in the papers themselves, he is only pointing out that there is less mysticism associated with the observed effects that is generally attributed to them by laymen and in the popular media.

Finally, it is worth pointing out that no one claimed that "coincidence counters do phase matching", so I don't know where you came up with that phrasing ... rather, Cthugha's point is that the interference that is observed arises from the well-defined phase relationship between the entangled photons. Unless you can come up with a detailed rebuttal to his arguments, I will remain convinced that he is correct, and you are the one who is confused.

 

[zonde]

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

My explanation for the DCQE is that nature is non-local and non-real.

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.

 

[unusualname]

First off, there's no call for any kind of abuse, and especially not calling people who are discussing things in good faith "idiots" or "crackpots". I am not a crackpot, and neither is Cthugha. That is quite an accusation, and you should be very careful before throwing it around so freely.

You and Cthugha have tangled before, and I have read those threads ... in my opinion he made his points much more clearly than you did, and supported them better with literature references as well. Everything he mentions is consistent with a QUANTUM explanation, not a simple classical one .. where do you think the well-defined phase of the two-photon state comes from, if not the fact that it is a pure quantum state, and therefore has a coherent phase-relationship between the photons? Furthermore, his explanation is completely consistent with the experimental results. He is not denying any of the claims or interpretations made in the papers themselves, he is only pointing out that there is less mysticism associated with the observed effects that is generally attributed to them by laymen and in the popular media.

Finally, it is worth pointing out that no one claimed that "coincidence counters do phase matching", so I don't know where you came up with that phrasing ... rather, Cthugha's point is that the interference that is observed arises from the well-defined phase relationship between the entangled photons. Unless you can come up with a detailed rebuttal to his arguments, I will remain convinced that he is correct, and you are the one who is confused.

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.

These sort of debates had merit maybe in the 1930s but in 2011 to have people still bewildered by QM is just tiring. Go and read a popular discussion of QM like Gribbin's "In Search Of Schrödingers Cat", it's a non technical explanation of what is accepted by all correctly thinking scientists today.

 

[unusualname]

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.

 

[SpectraCat]

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.

Of course I think QM is correct, and so does Cthugha as far as I can tell. Neither of us has said anything that would suggest otherwise. Cthugha's analysis doesn't refute QM .. it requires it, as I said in my last post (which you didn't address). Specifically, it requires that entangled photons maintain a well-defined phase relationship over large (possibly spacelike, although that is not specifically addressed) separations. No attempt is made to explain that phenomenon, or interpret it, or rationalize it in terms of classical or local realistic arguments as you have implied, it is just accepted in the spirit of "Shut up and calculate", and then used to explain the experimental observations.

I am tired of seeing you post blanket "refutations" of this stuff without a shred of supporting detail. You just say things like, "Well, if you think <X>, then you're never going to understand <Y>, so I won't bother explaining it." You don't seem to understand Cthugha's arguments at all, because you insist on characterizing them as classical, and you have never said why any specific aspect of what he proposes violates any particular physical principle. Also, if you think spatial coherence somehow necessarily implies a classical explanation, perhaps you should review the double-slit experiment.

Finally, the "obscure Ph.D. thesis" was from Anton Zeilinger's group, and quantum optics is hardly irrelevant to what we are discussing. The dissertation in question also has not disappeared from the web .. it can be found http://web.archive.org/web/20070714025355/www.quantum.univie.ac.at/publications/thesis/bddiss.pdf" . It is true that Ph.D. theses do not undergo peer review in the same fashion as journal articles, but they have certainly been reviewed by the student's advisor and Ph.D. committee. Perhaps you think Anton Zeilinger also doesn't understand the DCQE?

 

[unusualname]

Of course I think QM is correct, and so does Cthugha as far as I can tell. Neither of us has said anything that would suggest otherwise. Cthugha's analysis doesn't refute QM .. it requires it, as I said in my last post (which you didn't address). Specifically, it requires that entangled photons maintain a well-defined phase relationship over large (possibly spacelike, although that is not specifically addressed) separations. No attempt is made to explain that phenomenon, or interpret it, or rationalize it in terms of classical or local realistic arguments as you have implied, it is just accepted in the spirit of "Shut up and calculate", and then used to explain the experimental observations.

I am tired of seeing you post blanket "refutations" of this stuff without a shred of supporting detail. You just say things like, "Well, if you think <X>, then you're never going to understand <Y>, so I won't bother explaining it." You don't seem to understand Cthugha's arguments at all, because you insist on characterizing them as classical, and you have never said why any specific aspect of what he proposes violates any particular physical principle. Also, if you think spatial coherence somehow necessarily implies a classical explanation, perhaps you should review the double-slit experiment.

Finally, the "obscure Ph.D. thesis" was from Anton Zeilinger's group, and quantum optics is hardly irrelevant to what we are discussing. The dissertation in question also has not disappeared from the web .. it can be found http://web.archive.org/web/20070714025355/www.quantum.univie.ac.at/publications/thesis/bddiss.pdf" . It is true that Ph.D. theses do not undergo peer review in the same fashion as journal articles, but they have certainly been reviewed by the student's advisor and Ph.D. committee. Perhaps you think Anton Zeilinger also doesn't understand the DCQE?

I'm not going to further argue with you, I could make up uncountable numbers of convoluted analyses of the DCQE and demand that you refute them. If the argument appears even vaguely in any peer reviewed literature then can you post a reference, thanks.

 

[Drakkith]

i can only take a guess, since I am not a physicist.

the detector is small, it is not able to capturing all (or even most of) the photons, it is simply caputuring the photons that fall within its width at a particular position.

the images you showed are good and answer my question partially, but there would be two colums and this can be concluded from the images you showed...the single slit shows one...a double slit (without interefernce) would show two...bars...

Ok, at this link: http://arxiv.org/PS_cache/quant-ph/pdf/0106/0106078v1.pdf which unusualname gave, I can see in figure 1 that the detector is moved to the left and right. I thought they were moving it back and forward. And you were right, the slit to the detector is only .3 mm wide.

 

[Drakkith]

Alright, let me see if I have this correct.

Initially you have two entangled photons traveling to separate detectors, each linked to a coincidence counter. One detector we call S. The other we call P. Without the slits, the number of photons detected is at max with detector S right in the middle of the "beam". Moving it left or right results in a steady dropoff of detections.

Now we place a double slit between detector S and the source. Now, since the photons traveling to detector S interfere, moving the detector left or right slightly will show the interference pattern at each point as it would appear if you had a large detector capable of detecting the entire field of the beam instead of very small slit which only enables the detection of a small part at a time. (Meaning that the greater number of detections corresponds with a bright constructive interference band, and a lesser number of detections corresponds to a darker band)

So now we place a polarizer in each slit. One circularly polarizes them +45 and the other -45 depending on their initial linear polarization. Each slit results in the same interference pattern, only offset by 90 degrees. The built up inerference pattern is approximately the sum of the two different patterns, which looks like there isn't any interference pattern at all. (Page 5, http://arxiv.org/PS_cache/quant-ph/pdf/0106/0106078v1.pdf)

Alright, so now we place a polarizer between detector P and the source. If we set this polarizer to +45, then because of the entanglement the only photons on path S that can be detected at the same time as the ones at P are the photons that are originally polarized oppositely of P. If P is linearly polarized in the Y direction, which gets absorbed by the +45 polarizer, then it cannot get through to the P detector. The S photon that is polarized in the X direction gets through, strikes the detector, but because his twin didn't get detected, S is ignored by the coincidence counter. (I'm guessing that due to the doubled detection time once POL1 was placed in path P. Page 5, same link as above)

This results in only Y polarized photons striking the S detector at the same time as the P detector detects photons. So the resulting interference pattern observed at is what you would see if only Y polarized photons were allowed through the double slit at all.

If this is pretty much correct, I see absolutely nothing weird here. It looks like you have pre-selected which photons can or can't get to detector P, and via the coincidence counter determined which photons will be counted at detector S.

Edit: Spelling

 

[Drakkith]

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

According to http://en.wikipedia.org/wiki/Coincidence_counting_(physics ), and various other sites I have looked at, the counter is required not because of the probabilistic nature of QM, but because of the background noise and to make sure that the two particles you detect are indeed the ones that are entangled. Is this incorrect?

 

[unusualname]

Alright, let me see if I have this correct.

Initially you have two entangled photons traveling to separate detectors, each linked to a coincidence counter. One detector we call S. The other we call P. Without the slits, the number of photons detected is at max with detector S right in the middle of the "beam". Moving it left or right results in a steady dropoff of detections.

Now we place a double slit between detector S and the source. Now, since the photons traveling to detector S interfere, moving the detector left or right slightly will show the interference pattern at each point as it would appear if you had a large detector capable of detecting the entire field of the beam instead of very small slit which only enables the detection of a small part at a time. (Meaning that the greater number of detections corresponds with a bright constructive interference band, and a lesser number of detections corresponds to a darker band)

So now we place a polarizer in each slit. One circularly polarizes them +45 and the other -45 depending on their initial linear polorization. Each slit results in the same interference pattern, only offset by 90 degrees. The built up inerference pattern is approximately the sum of the two different patterns, which looks like there isn't any interference pattern at all. (Page 5, http://arxiv.org/PS_cache/quant-ph/pdf/0106/0106078v1.pdf)

Alright, so now we place a polarizer between detector P and the source. If we set this polarizer to +45, then because of the entanglement the only photons on path S that can be detected at the same time as the ones at P are the photons that are originally polarized oppositely of P. If P is linearly polarized in the Y direction, which gets absorbed by the +45 polarizer, then it cannot get through to the P detector. The S photon that is polarized in the X direction gets through, strikes the detector, but because his twin didn't get detected, S is ignored by the coincidence counter. (I'm guessing that due to the doubled detection time once POL1 was placed in path P. Page 5, same link as above)

This results in only Y polarized photons striking the S detector at the same time as the P detector detects photons. So the resulting interference pattern observed at is what you would see if only Y polarized photons were allowed through the double slit at all.

If this is pretty much correct, I see absolutely nothing weird here. It looks like you have pre-selected which photons can or can't get to detector P, and via the coincidence counter determined which photons will be counted at detector S.

Yes but what goes through POL1 ( the eraser) is probabilisitic. You have to double the detection time because only 50% of photons (approx) get through (Malus' law).

And there is no way to "know" which p photons go through and which don't.

And you can place POL1 and the p-photon detector distantly so that the corresponding s-photons are already measured (detected) before the p-photon can have even reached POL1 (so you might think the presence of POL1 should now not affect the s-photon measurements)

 

[unusualname]

According to http://en.wikipedia.org/wiki/Coincidence_counting_(physics ), and various other sites I have looked at, the counter is required not because of the probabilistic nature of QM, but because of the background noise and to make sure that the two particles you detect are indeed the ones that are entangled. Is this incorrect?

it's partially correct, but you can reduce background noise and use efficient entangled pair sources almost to make these effects negliglible.

The main reason for the coincidence counters is that only ~50% of a randomly polarised source of photons will pass through a polarizer (Malus' Law) and you can't know which will and which won't, so you must use coincidence counters. (similar probabilistic laws apply to wave plates and other apparatus)

People get confused because classically we couldn't measure individual photon detections.

 

[Drakkith]

Yes but what goes through POL1 ( the eraser) is probabilisitic. You have to double the detection time because only 50% of photons (approx) get through (Malus' law).

And there is no way to "know" which p photons go through and which don't.

And you can place POL1 and the p-photon detector distantly so that the corresponding s-photons are already measured (detected) before the p-photon can have even reached POL1 (so you might think the presence of POL1 should now not affect the s-photon measurements)

Maybe I'm missing some key concept here, but I don't see why knowing or not knowing which individual photons go through POL1 and into the P detector has any bearing on this. If the only time there is a count added by the coincidence counter is if the times for both detectors match up, then simply not letting Y polarized light through POL1 immediately means that you will never count any X polarized light at detector S. The coincidence counter doesn't get a count from detector P, so it ignores any count from S that were initally X polarized, correct?

Edit: Spelling

 

[Drakkith]

it's partially correct, but you can reduce background noise and use efficient entangled pair sources almost to make these effects negliglible.

The main reason for the coincidence counters is that only ~50% of a randomly polarised source of photons will pass through a polarizer (Malus' Law) and you can't know which will and which won't, so you must use coincidence counters. (similar probabilistic laws apply to wave plates and other apparatus)

People get confused because classically we couldn't measure individual photon detections.

If you can get the background down to such low levels, why do you even need a coincidence counter? Wouldn't it be simpler to just record every detection? I understand that only 50% of the photons will get through a polarizer. (Ugh, been spelling polarizer wrong for a bit now) Why wouldn't you just count all the photons going into the detectors and look at the timestamp of each detection?

 

[SpectraCat]

it's partially correct, but you can reduce background noise and use efficient entangled pair sources almost to make these effects negliglible.

The main reason for the coincidence counters is that only ~50% of a randomly polarised source of photons will pass through a polarizer (Malus' Law) and you can't know which will and which won't, so you must use coincidence counters. (similar probabilistic laws apply to wave plates and other apparatus)

People get confused because classically we couldn't measure individual photon detections.

That is not really correct. Coincidence counting is required because you have to correlate specific detection events at separate detectors with precise delay times in order to know with a high degree of certainty that two photons were generated as an entangled pair. Say for example you are sending one photon to Alice, who is 3 m away in the lab where the pair is generated, and the other photon to Bob, who is 150 m away in another building at the end of a fiber optic cable. So for a given entangled pair, the photons will arrive at the two detectors at different times ... you must have some way of knowing how to properly pair the separate detection events, and this is called coincidence counting.

The point about Malus' Law is something of a red herring, since most modern experiments with entanglement use polarizaing beam splitters (PBS's). A PBS sends photons with one polarization (say |H>) along one path to one detector, and those with the opposite polarization (say |V>) along a separate path to a separate detector. Both of those detectors are hooked to the coincidence counter, so both the |H> and |V> detection events can be captured in a single apparatus.

 

[unusualname]

If you can get the background down to such low levels, why do you even need a coincidence counter? Wouldn't it be simpler to just record every detection? I understand that only 50% of the photons will get through a polarizer. (Ugh, been spelling polarizer wrong for a bit now) Why wouldn't you just count all the photons going into the detectors and look at the timestamp of each detection?

Well that's what coincidence counters do! They measure all photons, and if two are detected within a certain time window the electronics signal a "coincidence". You could just use separate detectors and compare timestamps afterwards to recover your interference pattern if the technology was accurate enough, but you'll have to investigate the practical limits of the technology yourself to see if this is possible.

But, actually this type of coincidence counter will be needed to do experiments where the eraser and p-photon detector is placed so distant (much beyond the 2 meters in the Walborn experiment) that the time between p-photon and s-photon detections may be a second or more (if we can use satellites for example)

 

[Drakkith]

Well that's what coincidence counters do! They measure all photons, and if two are detected within a certain time window the electronics signal a "coincidence". You could just use separate detectors and compare timestamps afterwards to recover your interference pattern if the technology was accurate enough, but you'll have to investigate the practical limits of the technology yourself to see if this is possible.

But, actually this type of coincidence counter will be needed to do experiments where the eraser and p-photon detector is placed so distant (much beyond the 2 meters in the Walborn experiment) that the time between p-photon and s-photon detections may be a second or more (if we can use satellites for example)

Ok, I understand and agree with that. I don't understand why any of this is called "Delayed Choice Quantum Eraser". It looks to me like you are preselecting which photons can and can't be detected. To me it all comes back down to the coincidence counter. No matter when the first photon is detected, if the counter doesn't receive a count from both detectors the detection doesn't matter. It is thrown out. The choice seems to be made by the detectors and counter after the fact, not before by the photons. Nothing is being "erased", it just isn't even being counted.

 

[unusualname]

That is not really correct. Coincidence counting is required because you have to correlate specific detection events at separate detectors with precise delay times in order to know with a high degree of certainty that two photons were generated as an entangled pair. Say for example you are sending one photon to Alice, who is 3 m away in the lab where the pair is generated, and the other photon to Bob, who is 150 m away in another building at the end of a fiber optic cable. So for a given entangled pair, the photons will arrive at the two detectors at different times ... you must have some way of knowing how to properly pair the separate detection events, and this is called coincidence counting.

The point about Malus' Law is something of a red herring, since most modern experiments with entanglement use polarizaing beam splitters (PBS's). A PBS sends photons with one polarization (say |H>) along one path to one detector, and those with the opposite polarization (say |V>) along a separate path to a separate detector. Both of those detectors are hooked to the coincidence counter, so both the |H> and |V> detection events can be captured in a single apparatus.

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.

 

[unusualname]

Ok, I understand and agree with that. I don't understand why any of this is called "Delayed Choice Quantum Eraser". It looks to me like you are preselecting which photons can and can't be detected. To me it all comes back down to the coincidence counter. No matter when the first photon is detected, if the counter doesn't receive a count from both detectors the detection doesn't matter. It is thrown out. The choice seems to be made by the detectors and counter after the fact, not before by the photons. Nothing is being "erased", it just isn't even being counted.

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)