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Double slit experiment: counter?

  1. Nov 25, 2008 #1
    I've been thinking about the double slit experiment - the one with single photons going thru two slits forming an interference pattern never the less.

    Now, one thing i was unable to find clarification for is the claim that placing a detector even in just one of the slits to find out thru which slit a photon passed will result in the disappearance of the interference pattern.

    The question is - how does such detector work?
    How can one detect a photon without destroying it?
  2. jcsd
  3. Nov 25, 2008 #2


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    Well, in the kind of experiment you describe, the photon will usually be destroyed by detecting it.

    However, in some cases it is possible to detect photons without destroying them. Usually one uses some resonator, for example some cavity, in which photons go back and forth and prepare some atom in a very well defined spin state. Now the atom falls through the cavity perpendicular to the photons moving back and forth and the spin state of the atom after leaving the cavity will depend on the number of photons because the spin precession will be a bit faster in presence of photons. If you do this several times, you will get a nondestructive photon number measurement. However, these are so called weak measurements, so this means you do not change the photon states if you are in a photon number eigenstate already. The first measurement however might change the photon state from some undefined state to a photon number eigenstate.
  4. Nov 26, 2008 #3
    Hi Cthuga Your description was both interesting and informative but I dont see how it could be useful in a double slit experiment.
    Leaving the original question [which I have wondered about myself] unanswered.
    Is there any answer or is the original proposition
    merely an assumption for the purposes of a thought experiment???
  5. Nov 26, 2008 #4
    By placing a detector at only one slit. No detection means the photon passed through the other slit.
  6. Nov 26, 2008 #5

    Understood. With that the question becomes --- What kind of detector????
    More broadly----- What kind of detector is there that would not interfere with or change that part of the wave form passing through the detection slit , of a photon that actually passes through the other slit????
  7. Nov 26, 2008 #6
    "I've been thinking about the double slit experiment"
    Don't we all!!!

    In simple terms, the whole point of quantum uncertainty/weirdness is that you cannot under any circumstances get perfect measurements; you cannot "detect" a photon without "destroying" it, that is ,changing its state.

    Or maybe you can: what's your definition of "destroyed"?

    The delayed choice and quantum eraser experiments suggest far more subtley than is being implied in your question. (Are you related to yourself? the answer is unimportant; the way it makes you consider "related" is the type subtle situation implied in your question).

    The eraser experiment for example, suggests that even after you have "measured" (destroyed?) the photon if the signature is removed (erased) the interference pattern returns! In other words, if it's "tagged" and appears as a particle and the tag is subsequently removed, voila, it becomes a wave again. Seems like "information" is at the heart of your question.

    Your question implies a basis in our intuitive sense of time and reality; they don't apply to quantum weirdness!!! Biran Greene's FABRIC OF THE COSMOS provides nice discussion of all these situations...and provides "explanations" as well as they are understood...

    Good question, no simple answer....
  8. Nov 26, 2008 #7


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    I do not think I get the second part of the question right, but one could for example use a polarizer to detect, which way the photon went. This has the cool advantage, that you can destroy the which way information again.
  9. Nov 26, 2008 #8
    Particularly, by using polarisation-rotators and (polarisation dependant) beam-splitters, none of the photons need to be destroyed either.
  10. Nov 27, 2008 #9
    It is not a simple question of destroy or not destroy. If you view a photon as a wave form spread out in space and consequently passing through BOTH slits at once then how do you detect passage through either slit without CHANGING that part of the waveform.
    I am not up on polarizers but from what I do know it is very questionable that they do not change the photons that pass through. Polarization rotators obviously do. As do polaization dependant beam splitters which do not destroy but effect a selection based on polarization.
    So if a polorizer, filter or other device is used for detection ,,if it simply changes the polarity of the part of the wave passing through it then it has changed the overall picture of the events.
    Back to the question of detectors. DO you know if it is the case that these actual experiments, utilized polarization as the means of detection???
    Also how does a polarity based detector actually detect or indicate the passage of a photon ? Thanks
  11. Nov 27, 2008 #10


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    I still do not get it. What makes you think that the detection is supposed to leave the quantum state unchanged? It is not supposed to. Also I need to add, that it is not necessary to actually detect, which way a photon took in such experiments. Is is sufficient that the photon carries that information, for example in its polarization. That sure changes the photon state, but this is no problem at all.

    Take for example linearly polarized light and a double slit. You will see some interference pattern. Put a lambda/4 plate behind one slit, so that there will be circularly polarized light behind one of the slits. The interference pattern will disappear. Now you would say this changed the overall picture of the events. Now put in a second plate in after the first, which converts the circularly polarized light to linearly polarized light again. The which-way-information is destroyed and the interference pattern will reappear, so using these plates does not seem to change the overall picture of events.

    Some of the first quantum erasers used this trick. Just use google to find some experiments.
  12. Nov 29, 2008 #11
    All the documentation I have read on the Delayed choice quantum eraser experiments suggest a "choice"(a 50/50 defractor) being made before the idler photon is detected, but after the signal photon is detected? So going back to the original Double slit experiment, this suggests that the state of the photon between the slits and the backplate is not defined and can change? I've also heard a physicist talk about how a choice to view/erase slit data can be made "after" the photon as hit/appeared on the backplate and that the backplate results(i.e. wave or particle cluster) coincides with the choice to either view or erase the slit data(assuming the backplate data had not been viewed before the choice had been made). Has an experiment ever been carried out to determine this? or did I miss something?
    If the above hypothesis is correct, dosen't this indicate some kind of "time travel" or some kind of worm hole interference or even maybe dare I say, fate? I'm just a laymen btw.
  13. Nov 30, 2008 #12
    Hi blandrew It sounds like you are talking about the parametric down conversion crystal experiments , is this correct????
    I have not read about a choice being made after the photon has actually arrived at the detector and I have done some searching on the web. Do you know of any sources you could steer me to???
    What you are talking about reminds me of the 30 year study done at Princeton U testing the effects of human mental activity on Random Event Generators . They apparently got statistically significant results for activity performed after the events but before the event results were viewed. I dont completely dismiss statisical results but at the same time they are not completely convincing.
    From what I know of the entangled photon experiments the results are obtained through statisical correspondence of two separated detectors rather than direct interference on a single plate , but I could be misinformed . The writing I have found so far was less than comprehensive regarding the methods or results.
    If the EPR experiments ever prove totally valid,,, Ie. -- If reorientation of polarity at one location results in interference or not at a removed location, then faster than light information transmission would be possible and according to the princibles of SR would be traveling back in time.
    Thanks Cheers
  14. Nov 30, 2008 #13
    Hi Austin0, thanks for the response. The experiment I'm refering to is the "delayed choice quantum eraser" experiment proposed by Scully and Druhl in 1982 and carried out in 1999. I think it's the same one you mention. What led me to search for such an experiment was a lecture I watched on youtube by Ross Rhodes called "The double slit experiment" most of what is said is familiar ground to anyone with an academic or otherwise interest in this subject, however, in clip 6(of 8) Rhodes talks about delayed choice in reference to detection of photons at the slits. In summery, unless I missunderstood him, the delay to choose whether or not to view the data from the slit detectors can be made "after" the photons are detected at the backplate(time being no relevance, so could be days after). And the result will be in keeping with a wave or particle cluster depending on the choice made(i.e. choose to view the data = particle cluster, choose to erase the data = interference cluster). The only experiment that I've manage to find that resembles this hypothesis is the forementioned "delayed choice quantum eraser". However, the delay in this instance is referenced to a time after the photons have passed the double slits but before any detection at the backplate(proxy), or at least in the case of the signal photon. If I've understood the experiment correctly, the "choice" is made by a defractor that will channel the idler photon either to a path that will ultimately reveal which slit it(the photon before it is split into a signal and idler photon) passed through, or to a path that will mask that information(random 50/50). And the results match with what we would expect from a "knowing or not knowing" theory. This is not the same as a delayed choice in which the delay is after the detection of the signal(or the idler) photon at the backplate. And it seems to me that an experiment to varify Rhodes theory would be simpler than splitting photons. Which is why I think that I've either misunderstood something or hit on something. Hope all that didn't sound too amateurish and you can make some sense out of it :)
  15. Dec 2, 2008 #14
    The answer to this question is dependent on what you believe, not on physics!!!

    What you are refering to is refered to as the which way information. Afshar http://axion.physics.ubc.ca/rebel.html has perferomed an experiment that does not destroy the which way information. Unless you are a Copehagian in which case the photon has no physical reality does not travel in any real path or have classical path aspect or reality in the normative sense. As a Copenagen will only allow you to specify a photon was emitted and a photon was dedected, but will not allow statements about how it got to the detector, you can not disprove their belief with physical experiments.

    The basics of the experiment are given at the web site http://axion.physics.ubc.ca/rebel.html so you might want to see for yourself what you believe about this expiriment. I think it is brilliant.

    The problem is that once one stops believing in physics and become a mystical physicist you can not prove there is even a reality.
  16. Mar 3, 2009 #15
    Once a while my interest in physics, as today, erupts.I was astonished by a lecture on Utube by Mr. Ross Rhodes, as appears later, from the Rhodes University. In 8 parts, in all about 60 minutes, he explaines bright and clear about the double slit experiment In part 6 he leaves no shadow of a doubt that a day after the experiment with the use of detectors, erasing the gathered ,untouched, not peeped at thus virginlike data of the detectors before looking at the result at the plates would result in an interfering-pattern on these plates, while if one chooses not to errase the data collected by the detectors before looking at the receiving plate for the first time ,a particle-pattern would reveal itself on the plate. This seemes like devine intervention and after a carride I tried to find confirmation of this Wonder. Yet, no positive found.Though, dissapointingly, much more down to earth interpretations of this experiment. Mr Rhodes is a relegious person, as the place(a church) and the end of the lecture will reveal. I would be thankfull if you could confirm 100% my suspicion he is wrong. Alhas, confirmation would be, after the thrill, a bit of a blow. All this reminds me of my own believe when I was 6,7,8 years old. You can have as much intercourse as you like, but without being married, babies will not emerge. Therefor the blessing of God and a stork is needed.(YouTube - The Double Slit Experiment Part 6 of 8)
  17. Mar 3, 2009 #16


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    Yes, Rhodes makes it sound convincing and easy to understand, but nevertheless most of what he says is wrong. The so-called delayed choice quantum eraser experiments work differently than he says.
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