Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Delayed Choice Quantum Eraser - double-photon explanation

  1. Mar 7, 2012 #1
    It is claimed that DCQE is equivalent to a single-photon double-slit setup. However, In this experiment, the 351.1nm Argon ion pump laser beam is divided by a double-slit, which means the actual photon rate is 1013 times higher than in a single-photon setup.

    Consequently, it makes it possible for both A and B regions to generate photon pairs simultaneously enough to allow corresponding photons from different pairs to interfere further in the set.

    I believe that the interpair interference explains the experimental results. If this hasn’t been discussed yet, I’d like to develop this idea step by step, so please comment.
  2. jcsd
  3. Mar 7, 2012 #2
    it's just a "filtering" trick.

    the interference pattern is "hidden/embedded" in the non-interference pattern
  4. Mar 7, 2012 #3
    Not sure what you mean. Do you support or oppose quantum eraser explanation? :smile:
  5. Mar 7, 2012 #4
    there are many explanations for the quantum eraser..:), i support the one which says superimposed/embedded...as explained in the paper by kim yoon.
  6. Mar 7, 2012 #5
    Let's not go that far for now. What do you think of photons generated at A and B at the same time?
  7. Mar 7, 2012 #6
    Photons aren't "generated" into two spots, it's a wave, waves span out over distance, what's the big deal? When you measuring something all your doing is making it's probability equal to a specific point, so there's nothing that happens in-between, the photon's position is just equal to being in a specific point once it interacts with the screen. This is why quantum mechanics is where math meets reality. Things work because they mathematically act that way or they "appear" in a given place without going through the intervening space from a previous measurement is because of the fact that their probability is equal to a non-zero number in that place in that distance.
  8. Mar 7, 2012 #7
    I don't think you're referring to the experiment.
  9. Mar 8, 2012 #8
    Are you talking about that double slit experiment? Because that's not two photons, that's the same photon interfering with itself.
  10. Mar 8, 2012 #9


    User Avatar
    Science Advisor

    At first, you should quote which DCQE experiment you are talking about. Your quote makes me assume you mean the following one: http://arxiv.org/abs/quant-ph/9903047.

    Now let us start at the very beginning.
    This is not claimed anywhere. You see an equivalent pattern. That does not mean the experiments are equivalent.
  11. Mar 8, 2012 #10
    Well, not in original paper, but not anywhere, too. There are many comments emphasizing simultaneous emissions from A and B are not possible as only one photon at a time passes through the double slit. Thus my original point, I believe simultaneous emissions do occur. Do you agree?
  12. Mar 8, 2012 #11


    User Avatar
    Science Advisor

    Ehm, where is what claimed, exactly? It does not make any sense to discuss someone said sometime without knowing what is exactly meant.

    Again: Who claims that where? By the way, that does not have anything to do with DCQE anyway. The physics of single-photon interference patterns is well understood and the physics of two-photon interference patterns is important for DCQE, but very different from single photon interference.

    Not really.

    Where do you get this number from? The original paper does not even give the intensity or coherence properties of the pump beam, so one actually cannot judge the actual photon number rate.
  13. Mar 8, 2012 #12
    It was in the articles and presentations I looked through.

    So, it's both yes and no at the same time, got it :smile:.
    Joking aside, what are the two interfering photons you are referring to?

    My bad, that was my quick estimate, and I just considered a 1 W laser, which is probably too much, and it appears I even made a mistake by one order. 1 W creates 1018 photons, and SPDC (that is used for one-photon source) rate is 106, so it is 1012 difference.
  14. Mar 8, 2012 #13


    User Avatar
    Science Advisor

    Do you by chance have a link or something? There is everything from valid peer-reviewed publication to obscure crackpottery about DCQE.

    The entangled photons after the down-conversion process are in a state that lacks first-order coherence, but has phase sensitive correlations that can end up yielding so-called two-photon interference patterns. Such patterns show up ONLY in coincidence counting, but never on one detector alone.

    That is hard to judge. I do not know whether the laser power is given in the manuscript and whether the laser is pulsed or cw. Also, the efficiency of the SPDC crystal will depend on its thickness and how well it is adjusted. 10^12 photons (if equidistant) would correspond to a mean difference of 300 microns between consecutive photons which is already quite much. That should be long compared to the coherence length of the SPDC photons.
  15. Mar 8, 2012 #14
    I could try to find the links again, but they're really irrelevant here.

    So, is it correct that it is not a real interference but something that just produces an interference-like pattern?

    I know, but the key word is "mean", and the rate is high.
    Now that I'm thinking about it, since the distance between A and B is quite big, the emissions must not be simultaneous for interference to occur somewhere down the set. However, my original question still stands -- is it the interpair interference what causes the interference patterns? That is something for me to think about further, but do you agree that technically this interference is possible?

    Update: since Glan-Thompson prism was used to split signals and idlers, the distance between A and B gets compensated, so that if A and B are emitted at the same time, they can interfere later. Thoughts?
    Last edited: Mar 9, 2012
  16. Mar 9, 2012 #15


    User Avatar
    Science Advisor

    Two-photon interference patterns are just as real as single-photon interference patterns. It is just a different mechanism. There is no singular mechanism which is "the real" interference mechanism.

    I still do not get your point. The photons are either inside the same coherence volume and therefore indistinguishable per se, so it is meaningless to talk about "interpair" interference or they are not from one coherence volume. In the latter case they are distinguishable and do not interfere at all.
  17. Mar 9, 2012 #16
    To me there is an interference when photons (either single or double) pass through a double slit and interact with each other or themselves. On the contrary, in the experiment, the entangled photons never interact, the only connection between them is the coincidence circuit. Can you still call this an interference even though the result chart looks like interference? I hope we'll not get at interpretations or philosophy here.

    My point is that if two coherent photons are generated at A and B, they can interfere (actually, not just by means of a coincidence circuit) further in the set. And to me it looks like this real interference (namely, on BS) is what causes interference patterns when D1 or D2 is used, but not with D3. Also it simply and clearly explains the phase shift between D1 and D2 charts.
  18. Mar 9, 2012 #17


    User Avatar
    Science Advisor

    Interference occurs whenever you have two or more different, but completely indistinguishable ways to get from some initial to some final state. Photons do not have to meet somewhere to interfere. That has been tested by Pittman, Shih and others.

    If what you see was a single photon interference pattern created by coherent photons from A and B, you should be able to see the patterns without coincidence counting by just moving D1 and D2 around. This is not the case.
  19. Mar 9, 2012 #18
    Are you referring to "Optical imaging by means of two-photon quantum entanglement"? This is so good that you mentioned it, as it makes it easier for me to explain my idea--see below.

    If they moved them, which I believe they did not in the original experiment, they wouldn't get the pattern, but not because interference isn't happening there. The reason is that in reality A and B photons each time have different directions, and the resulting image is a sum of multiple interference patterns randomly shifted along the detector scanning axis, which makes it a simple hump.
    Now if you are able to only select photons having specific direction, you'll get a normal interference pattern out of this mess. And you are--you have the movable D0 that catches entangled photons, and entangled means their directions are correlated with the photons interfering on BS.
    It works almost like in the experiment mentioned above, only instead of letters aperture you have an interference pattern in front of the fixed detector.
    Last edited: Mar 9, 2012
  20. Mar 9, 2012 #19
    By the way, I'm not sure if passing the double slit and hitting the BBO with signal-idler generation collapses photon's wave function. If it doesn't, things are even simpler because simultaneity doesn't apply and interference on BS is obvious.
  21. Mar 9, 2012 #20
    well said!

    can you elaborate (on the experiment) or post the paper?

    aps wont allow...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook