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DCQE - how does/can the pattern change?

  1. Jun 2, 2011 #1
    in DCQE - Delayed Choice Quantum Eraser

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

    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 [Broken]

    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?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Jun 2, 2011 #2


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    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.
    Last edited: Jun 2, 2011
  4. Jun 2, 2011 #3


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    I did have a question though. The graphs from here: http://grad.physics.sunysb.edu/~amarch/ [Broken]
    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?
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  5. Jun 2, 2011 #4
    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.....
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  6. Jun 2, 2011 #5


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    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.
  7. Jun 2, 2011 #6


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    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 :wink:.

    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.
  8. Jun 2, 2011 #7
    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?
    Last edited: Jun 3, 2011
  9. Jun 3, 2011 #8
    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.
  10. Jun 3, 2011 #9
    hmmm........so who is right......cthuga/spectracat or unusual name?....eeny meeny miny moe
  11. Jun 3, 2011 #10
    here's a clue, coincidence counters aren't accurate enough to do phase matching.
  12. Jun 3, 2011 #11
    i am surprised.....its a matter of time till we find who is the right/better physicist...till then......let the debate begin.....
  13. Jun 3, 2011 #12
    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.
  14. Jun 3, 2011 #13


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    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.
  15. Jun 3, 2011 #14
    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,
    Last edited by a moderator: Jun 3, 2011
  16. Jun 3, 2011 #15
    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?
    Last edited by a moderator: Jun 3, 2011
  17. Jun 3, 2011 #16
    The coincidence counts match SPDC pairs that have both travelled 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)

    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.
    Last edited by a moderator: Jun 3, 2011
  18. Jun 3, 2011 #17
    Unusualname - one experiment, where phase relationships and interference is discussed, comes to mind.....


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

  19. Jun 3, 2011 #18


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    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.
  20. Jun 4, 2011 #19


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    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.
  21. Jun 4, 2011 #20
    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.
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