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I Quantum eraser

  1. Nov 17, 2016 #1
    hi guys, i'm new to this beautiful forum and i signed up to ask a question that drives me nuts for weeks. im german so my english sucks but, i learned about this experiment with the quantum eraser. basically the experiment goes like this. you shoot photons through a double slit and behind the slits you have a crystal that splits them in 2 identical twins. one twin goes at a wall where the pattern will be seen the other goes to a detector. of course when one twin photon is detected the other one looses its wavefunction. so far so quantum weird. now you put 2 more detectors behind the 2 first ones and the photon has a 50% chance to hit those instead of the 2 first ones. the second detectors are build up so that you cant say what slit the photon took. so whenever a photon hits the first detector it's twin has no wave function whenever the photon hits the second detector the photon does have a wavefunction and an interference pattern emerges. my question is how do we know which photon at the "wall" is the twin of which photon in the detectors? and even more important why was there no experiment with only the 2. pair of detectors?`everytime i research it the experiment is build that half the photons are detected and half not. than people say the photons that got detected formed a clunk pattern while the photons that went to the second detector formed an interference pattern. but how can you tell which photon belonged to which twin? :D :D
    i'm confused and i can't find it on google because i only find articles about how weird the whole thing is.
    well thanks for reading it's probably a stupid question i know :D
  2. jcsd
  3. Nov 17, 2016 #2


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    Staff: Mentor

    You are thinking abouy Kim's experiment? https://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser#The_experiment_of_Kim_et_al._.282000.29
    If a photon arrives at what you're calling "the wall" at the same time that one of the four detectors was triggered by another photon, we assume they are twins. This works because the rate at which pairs are created is small enough that there is usually only pair of photons in the apparatus at a time.

    It is also important to understand that there is no "wall" on which a visible interference pattern is formed. Instead, there is a detector that is moved back and forth across the path of the first photon. Whenever that detector is triggered, we record its position and whether its twin had triggered the "we know which slit" or the"we don't know which slit" pair of detectors. We're looking for areas where that detector is more or less likely to be triggered at the same time that one or the other of our two pairs of detectors is triggered, and that only becomes apparent when we've looked through the recorded data after the experiment has run for a while.
  4. Nov 17, 2016 #3


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    The quantum eraser is one of the most difficult to understand. There are numerous variations on it, Nugatory has linked you to one of the best.

    Key is to recognize, as he says, that it takes information from 2 photons to "see" the interference pattern. Looking at one side or the other does not appear unusual.
  5. Nov 19, 2016 #4
    thanks a lot for you answer :) i guess because the photon arives at "the wall" earlier than at the detectors (to measure if the imformation travels back in time) i haven't though about the timing. do they just calculate the difference in?
  6. Nov 19, 2016 #5


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  7. Nov 21, 2016 #6
    ahhh, i'm stupid. the photons arive one at a time, of course you know which one is which one lol
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