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Hi Folks,I am currently reading Brian Greene's 'The Fabric of the

  1. Aug 3, 2011 #1
    Hi Folks,

    I am currently reading Brian Greene's 'The Fabric of the Cosmos' which is excellent.

    I am trying to get my head around the Delayed Choice Quantum Eraser in this book. Is there a website link I could follow up that explains it well? I have already looked at


    Perhaps there is some applet one can download?


  2. jcsd
  3. Aug 4, 2011 #2
    Re: Dcqe

    Getting your "head around" THAT discussion may be impossible...depending on your objective.

    Greene, Page 199....

    There is another description here:


    but don't expect it to make anymore common sense than Greene's discussion.

    It's akin to special relativity: Either you accept the experimental results which support the theory ( in SR, that space and time are not fixed and immutable) and use those results and theory in your "new" reality via new ways of thinking or you don't. You will need to accept a new "logic".

    I've read that book two or three times now and still find new perspectives upon subsequent reading and discussions in these forums. If this stuff was obvious, everybody would be a quantum theorist!
  4. Aug 4, 2011 #3
    Re: Dcqe

    Thanks Naty1,

    I have printed that link ready for attack later. I havent the book with me right now but I will indicate which part I dont understand and will reply later.

  5. Aug 4, 2011 #4
    Re: Dcqe

    I have read through some material and I find it mind boggling the weirdness of these experiments.

    What amazes me is the very fact of having 'which path' information on the idler photons and then 'erasing' this information will still recover the interference pattern despite the fact that we have not gone near the signal photon.

    The nature of the photon appears to depend on what the experimental setting is prior to hitting the screen regardless of whether the photon is a particle or a wave 'after' leaving the down converter.

    This interpretation is correct, right?

  6. Aug 4, 2011 #5


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    Re: Dcqe

    I am not sure if you got properly what happens in this experiment. Many (most...) people misunderstand it.

    If you have quartervawe plates in a lower branch, we never see the pattern on the screen. The photon distribution is uniform (or rather wide blob). The pattern appears only in a correlation between upper (eraser) and lower (double-slit) detectors. You may picture it as the blob you see on a lower screen is a sum of two fringed patterns, displaced by half of the fringe span. Photons creating one pattern are perfectly correlated with photons vertically polarized in the upper branch. Those making second pattern - are correlated with photons hororizontally polarized in upper branch.
    Both cases you may explain with simplest high-school wave optics. No mystical erasure is needed, no action on distance, no mysterious particle-wave duality, no 'nature of photons'....

    The only remaining mystery is entanglement: however you measure polarisation of photons in both branches, the result is always the same if axes of both polarisators are parallel.

    To make it even easier: use normal, untangled light, and put your 'eraser' polariser just by the source. Then analyse what pattern would you see on the screen, if QWP are installed behind slits. Easy. Turn around your polariser.
    What entanglement does is the same: correlation between entangled particles ensures that whenever upper detector receives vertically polarized photon, the one in lower branch is also vertical. So restricting the analysis to those events, when upper detector clicks with vertical photon, is equivalent to putting a vertical filter by the source in lower branch.
    Last edited: Aug 4, 2011
  7. Aug 4, 2011 #6
    Re: Dcqe

    Hi xts,

    Unfortunately, you have gone beyond my capacity as a layperson at this stage. So is it possible that Brian Greene's description is too simplistic such that it invites misinterpretations of this experiment?

  8. Aug 4, 2011 #7


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    Re: Dcqe

    I don't know Greene's book, so I can't judge it. But virtually all popular text on this issue I ever read were misleading.
    It is not a matter of oversimplification. Rather contrary - it is building mysterious structure where it really is not needed.
    As Occam's advocate I always prefer simpler explanations...
    I also don't like sensational language used in this field: 'teleportation', 'erasure' (working on a distance), etc. - it is awfully misleading especialy for laypeople.
  9. Aug 4, 2011 #8


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    Re: Dcqe

    OK. So once again:
    Imagine you have just a source of light, then linear polarisator, then double-slit with QWP installed behind each slit, then screen - as in QE experiment. If there is no polarisator - there is no pattern on the screen (large blob).If you put the polarisator and turn it axis, fringes appear.
    Let's say they appear as the polarisator is set to vertical. As you turn polarisator to horizontal, fringes appear again - but shifted: now you have light in places previously dark and vice versa.
    To explain this behaviour you don't need QM - just 19th century wave optics.
    So you know, that if incoming light is polarised vertically you have pattern, if it is polarised horizontally - you have antipattern.
    Now you remove the polarizator. Pattern vanishes.
    You put polarizator into upper branch (eraser branch). Nothing happens - you still see no pattern.
    But now you start to count individual photons in both branches. You got a long list of pairs: (was the photon in upper branch vertical? | photon position on the screen in lower branch)
    And you take only those cases, where in upper branch the photon was vertical, and plot a dot on the lower screen in the place hit by photon. But only for those cases, when upper one was vertical. Those dots form a pattern - exactly the same, as you saw when you put vertical filter in the lower branch.

    Nothing mysterious, weird, spooky, no 'nature of photons' is needed...

    The only remaining mystery is that this special crystal, sending two photons at once, always send identically polarised photons in both branches. (actually entanglement involves deeper correlation, but for now that's allowable simplification - let DrChinese forgive me!)
  10. Aug 5, 2011 #9
    Re: Dcqe

    For a start go with Greene's discussion and not xts

    xts seems to believe in "elements of reality", ie he believes photons have a definite value of a property such as polarisation before it is measured.

    He must have missed the last several decades of experimental physics which shows this is not the case.

    People like Wheeler, Aspect and Zeilinger aren't idiots.

    The point of modern quantum optics experiments is that they cannot be explained by "19th century wave optics", and anyone who keeps suggesting this here, on physicsforums, ought to be warned methinks.

    Once you accept a QM explanation it is simple enough to see that no retrocausality occurs (eg Demystifying the Delayed Choice Experiments )

    In fact the experiment is more confusing if you try to understand it using classical concepts (unless you make up your own rules of nature)

    But don't be surprised that this stuff is "mystifying", no one really understands QM, for an authoritative overview see a review article like Franck Laloe's

    Do we really understand Quantum Mechanics?
  11. Aug 5, 2011 #10


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    Re: Dcqe

    Oooch? Do I? I think you totally misunderstood me.
    What I am trying to advocate in this and several parallel threads is an extreme 'non-realistic' approach. I am trying to convince people that it just makes no sense to even think about such 'entities' as 'photon path', 'photon nature', property existence when not measured, etc....

    I have much less realistic approach than Wheeler had! Nothing against Aspect and Zeilinger...
  12. Aug 5, 2011 #11
    Re: Dcqe

    Your explanation above sounds pretty realist to me. 19th century wave optics assumes classical realism you know.
  13. Aug 5, 2011 #12


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    Re: Dcqe

    19th century wave optipcs is essentially equivalent to QM. It is absolutely non-realistic! There are no photons, paths, nothing like that. Just a propagation of waves. No particles carrying any properties.
  14. Aug 5, 2011 #13


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    Re: Dcqe

    I wouldn't say any of these are realists!
  15. Aug 7, 2011 #14
    Re: Dcqe

    ok guys,

    Thank you for your inputs, I appreciate it. I will look at those links. Cheers
  16. Aug 8, 2011 #15
    Re: Dcqe

    Actually, the dcqe in Greene's book isn't the one with 1/4 wave plates, it's the other one, where a beam splitter 'erases' the which-path information in the idler photons.

    It's important to note that there is a classical connection (a wire) between the idler detectors and signal detectors, because you need to do coincidence counting to reveal the interference pattern when the which-path information is 'erased.' Greene mentions this requirement in the text, but it is missing from his diagram of it, so some people have gotten the (erroneous) impression that you could use this to signal information faster than the speed of light.

    Basically, without the final beam splitter, all you get is a big smudge of photons. With the beam splitter, the signal photons show a set of fringes associated with idlers leaving the splitter to the left, and another set of fringes associated with the idlers going to the right.
  17. Sep 18, 2011 #16
    Re: Dcqe

    Greene's book is indeed excellent.

    Best "explanation" of the actual DCQE experiment I have found so far is http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

    One question I have which someone here may be able to help me with, as follows:

    The raw position data (without correlation with any of the other detectors) collected at D0 should show an interference pattern, correct?

    The various sets of correlated data (obtained via the coincidence counter) are D0-D1, D0-D2, D0-D3 and D0-D4 (depending on whether the idler photon ends up at D1, D2, D3 or D4) - but each set of correlated data is a subset of the raw D0 data - correct?

    In other words, if we add up the position data from all of the subsets, we should end up with the raw (uncorrelated) distribution, which should show an interference pattern?

    But D0-D3 and D0-D4 correlation data show no interference pattern, and the interference patterns from D0-D1 and D0-D2 are out of phase, so adding them would destroy the pattern.

    So how can we add together all the subsets of data and still end up with a raw data interference pattern?

    I'm confused. can anyone help me?
  18. Sep 18, 2011 #17


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    Re: Dcqe

    That's very common misunderstanding of the "quantum eraser" and similar experiments.
    The full set of data, as collected, never shows any pattern.
    The data may be just divided into two subsets, each of them exhibiting a pattern (they differ in phase), using data collected in other branch as a selection trigger.
    No timing issues need to be considered. You may just collect independently two series of data in both detectors, store them and compare them offline a week later to produce fringed plots.
    Last edited: Sep 18, 2011
  19. Sep 18, 2011 #18
    Re: Dcqe

    Thanks for the reply, but not sure I follow you.

    The raw signal photon data at D0 (prior to any correlation of these data with idler photon detection data) MUST surely show an interference pattern - because this (the raw data at D0, without any other inputs) is precisely the standard 2-slit experiment, with no which-path information.

    Indeed, this is exactly what the following website claims:


  20. Sep 18, 2011 #19


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    Re: Dcqe

    That's not true.
    Pure D0 (without any coincidence) is a double slit with each slit lit by independent incoherent source. Or - if you prefer - it is a double slit with spatially extended incoherent light source.
  21. Sep 18, 2011 #20
    Re: Dcqe

    Now you have me really confused. If the source illuminating the double slit is incoherent, how can it ever result in observation of an interference pattern, either with or without inclusion of the coincidence-counter data?
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