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Help with Quantum Eraser experiments

  1. Mar 25, 2013 #1
    I read an article I found on the internet the other day about some Chinese scientists who have set a new speed record for Quantum Entanglement or (Spooky action at a distance). I have since been fascinated by Quantum Mechanics and am trying to understand it (as best I can). I am not a scientist in any way shape or form and as such don't have access to the equipment or the knowledge to experiment myself.

    I would really appreciate it if Anybody with an understanding of the "Quantum Eraser" could take a look at these 4 experiments and enlighten me as to what results I should see at each of the detectors in each of the following 4 experiments in terms of Interference pattern or No Interference Pattern.

    I may be (probably) completely off the mark here but I am particularly interested in the results of experiment 2 as from what I can tell this should provide "Which Way" information without affecting the polarization.

    http://sphotos-b.ak.fbcdn.net/hphotos-ak-ash4/481900_4695680957061_1075943993_n.jpg [Broken]
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Mar 25, 2013 #2


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    It might help, if you explain your drawings.

    I suppose the blue rectangle and black square are a laser and a nonlinear crystal creating entangled photon pairs. Are these momentum-entangled with arbitrary polarization?

    What are black-and-white things? Double slits? Polarizers? Double slits with polarizers? Do the two thin lines represent a beam splitter? Also what kind of detector do you intend to use? Small ones? Large bucket detectors? What kind of interference patterns are you after? A simple spatial double slit pattern? If so, do you scan the position of some detectors to measure it and if so, for which detectors do you scan position?

    All of that may sound like nifty details, but they become important.
  4. Mar 25, 2013 #3
    Thanks for the reply Ctugha. Sorry for my ignorance, as I said I am not a scientist. Yes the blue rectangle and black square are as you suggested. The black and white things are Double Slits "Without" polarizers and the two thin lines are mirrors. Sorry you've lost me on the detectors. I was thinking of anything that can detect weather an interference pattern is present or not.
  5. Mar 26, 2013 #4


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    Well - it is complicated.

    It is not even completely clear, what one will see in experiment 1. Typically, entangled photons will not show a double slit interference pattern because they are pretty incoherent. However, one can always go to far field conditions (double slit far away from the crystal). In this case coherence increases (for the same reason light from very distant stars is pretty coherent), but doing so breaks entanglement. So I suppose, you are not interested in this kind of condition and want to keep entanglement. In this case, you will not see an interference pattern in experiment 1 on any of the two detectors. You might be able to recover some pattern by postselection (just considering detections at detector B which correspond to simultaneous detection events at some well defined position of detector A), but on the bare detectors, there will be no pattern.

    In experiment 2, no detector can show a pattern. You need a superposition at two possible paths to a single detector to be able to create an interference pattern.

    In experiment 3, one might be able to see a pattern on detector B, if the slits of the first double slit are very narrow. This is equivalent to increasing the distance between the crystal and the double slit as mentioned for experiment 1 and wll increase coherence. It will also break entanglement.

    In experiment 4, you might see a pattern on detector B and C for the same reasons as in experiment 3.

    For these cases in experiment 3 and 4, one might also be able to see an interference pattern using wider slits, if one again uses postselection as mentioned in the discussion of experiment 1.

    I am sorry that this is most likely not the discussion of the experiment you wanted to get, but these tiny details are very important for double slit and quantum eraser experiments.
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