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The Bell-state Quantum Erasor

  1. Sep 20, 2013 #1
    I've been studying the Bell-state Quantum Eraser from every source I can find, including the Walborn paper. I get contradictory information concerning what happens in the Dp arm where a polarizing filter is placed before the detector to lose which-way information and regain the interference pattern. In one source it explains that the polarizer is a diagonal polarizer and that it polarizes the Dp photon to a diagonal orientation causing its entangled partner photon, the one going towards the double slits and Ds, to become diagonal too. So, then the diagonally polarized Ds photons will go through the twin quarter-wave polarizers before the double slits giving ambiguous results, causing either right-hand or left-hand polarization from the same filter, thus destroying which-way information. On the other hand, the Walborn paper states: "[recovering the interference pattern] can be done by placing a polarizer in the path of beam p and orientating it at +45 degrees to select |+>p or at - 45 degrees to select |->p. The interference pattern is recovered through the coincidence detection of photons s and p. I have no idea what he is talking about. Can someone explain this part of the Bell-state quantum eraser in clear and simple terms? Thanks
     
  2. jcsd
  3. Sep 26, 2013 #2
    I do wish that someone would answer your question.

    While Googling delayed choice quantum eraser I came across this video which has a step by step explanation of the set up and the explanations for each step. I don't know if it will help with the confusion between your two sources, but maybe seeing the set up in the video will help. The set up starts at 3:28 and goes through 12:06. As for offering this, I do so disregarding the portion after that.
    http://m.youtube.com/watch?v=hSRTvK...uri=%2Fwatch%3Fv%3DhSRTvKgAs9c%26autoplay%3D1
     
  4. Sep 27, 2013 #3
    Oh, thank you for your efforts, but this is not the Bell-state quantum eraser. Though this configuration is more complicated, but is not ambiguous at any step, thus actually easy to understand.
     
  5. Sep 27, 2013 #4

    meBigGuy

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    I think the second explanation is saying the same thing as the first. Place a diagonal polarizer in the path and then record coincident detections. The first explanation just assumed you recorded coincident detections. Coincident detections are just to filter for only entangled pairs. But you should already understand that since you understand the delayed quantum eraser. So maybe I'm missing your issue?
     
    Last edited: Sep 27, 2013
  6. Sep 27, 2013 #5
    In one explanation, it's about changing the polarity of the photon at the Ds detector by putting in a diagonal filter for its idler photon to pass through. In the other explanation, it seems to more about what you say, that coincident detections are just being filtered. "[recovering the interference pattern] can be done by placing a polarizer in the path of beam p and orientating it at +45 degrees to select |+>p or at - 45 degrees to select |->p. The interference pattern is recovered through the coincidence detection of photons s and p." So, if we are only reading coincidence pairs from one slit or the other, how does that recover the interference pattern?
     
  7. Sep 27, 2013 #6

    meBigGuy

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    They are simply using coinicidence detection between the idler and the target to only count entangled pairs. The first experiment could have said, "only measure coincidence detections of entangled pairs", but they just assume you are only measuring entangled pairs. The first is a summary, the second is a formal paper.

    It isn't coincidence from one slit or the other, it is coincidence of photon to upper detector and lower detector.
     
  8. Sep 27, 2013 #7
    But, when the polarizer at Dp is put in place its polarization apparently matches one or the other of the quarter wave-place polarizations, so now one is measuring coincidence pairs from one side or the other only, cutting out half of the pair information. If previously there was a solid band because wave function had collapsed, then for some reason just getting half the information is supposed to bring it back? Let's see, I'm registering hits only from one slit and not the other, so surly that would still produce just a single band. But, if one includes the idea that by putting in the diagonal polarizer at Dp it also changes the entangled idler photons at Ds to diagonal, then I must assume that diagonal photons can go through either quarter wave with mixed results allowing for the light to be the same from side to side and the fringes reappear.
     
  9. Sep 27, 2013 #8
    "Finally, a linear polarizer is introduced in the path of the first photon of the entangled pair, giving this photon a diagonal polarization (see Figure 2). Entanglement ensures a complementary diagonal polarization in its partner, which passes through the double-slit mask. This alters the effect of the circular polarizers: each will produce a mix of clockwise and counter-clockwise polarized light, regardless of which slit the second photon passes through. Thus the second detector can no longer determine which path was taken, and the interference fringes are restored."

    http://upload.wikimedia.org/wikiped...3_pol.svg/250px-2SlitApparatus_w3_pol.svg.png
     
  10. Sep 27, 2013 #9

    meBigGuy

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    "But, when the polarizer at Dp is put in place its polarization apparently matches one or the other of the quarter wave-place polarizations, "

    I don't think so -- it is diagonal.
     
  11. Sep 27, 2013 #10
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