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Double slit experiment / Quantum interferometer

  1. Apr 10, 2008 #1
    I would really appreciate if someone could answer what maybe a really simple question. I'm no physics expert but I do read a lot and I have a keen interest.

    My question is related to light interference and the double slit experiment.
    I know that by observing which slit a photon of light passes through, the wave breaks down and the interference pattern no longer appears on the screen behind.
    How is the photon observed passing the slit?
    I read an interferometer is used but I don't understand quite how this is done.

    Sorry if this is very basic stuff.

  2. jcsd
  3. Apr 10, 2008 #2
    try reading "The Feynman lectures on physics Volume III" he gives a very good explanation of the interferometer. Basically you put a light source behind the 2 slits and between them, when an electron goes through a slit, it will trigger the light source to emit a photon, and depending on the position of the photon emitted, we'll know which slit the electron goes through.
    when we put the light source there, the interference disappears.
  4. Apr 13, 2008 #3
    Thanks for the quick reply.
    I'll go and do some reading.
  5. Apr 23, 2008 #4
    So the electron has an effect on the light source such that it deflects photons towards the detector, so detecting which slit the electron passes.
    Are there any other ways to detect the passing electron, that don't have any effect on the electron, maybe detecting the electrons magnetic field? Would this be too difficult?

    Is there a similar way to detect photons rather than electrons passing through a slit?
  6. Apr 23, 2008 #5
    If the detector has no effect, how would it know when it detected?
    I aways thought it was the effects that were actually what was being detected.
    Indicators that the electron was present, based on an effect.
  7. Apr 26, 2008 #6

    The classical double-slit experiment is used to show the wave nature of light. It isn't possible to observe the "photon" as it passes through the slits. There are people who will argue that with some very modern experiments using entangled beams, we can in fact tell which slit the photon went through. But that was never the point of the double slit experiment.

    There is also a thought experiment, made famous by Feynmann, using electrons instead of light. Feynman shows how observing the position of the electron upsets the interference pattern. He talks about different ways of doing this, including illuminating the slits or measuring the recoil of the barrier. The uncertainty principle comes in when you find the once your measurement system becomes accurate enough to determine which slit the electron went through, the interference pattern is destroyed.

    I'm not sure the experiment has ever been done with electrons. Feynmann presents it as a thought experiment.
  8. Apr 26, 2008 #7


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    Well, yes and no. There are obviously situations where NOT detecting something also gives us information and this "nondetection" will therefore have an effect on the system we are observing.
    The most famous example of this is a single ion (trapped in an ion trap and manipulated/probed using lasers) in a V configuration (TWO excited states with a common ground state), one transition is fluorescent meaning we know that the system was relaxing from this state if we detect a photon; the other state has a longer lifetime and is dark.
    Hence, nondetection of a photon means that the ion was in this state so this is a case of observation by nondetection.

    The formalism for handling these situations (known as quantum jumps) hasn't been around for very long, much of it is only about 10-15 years old. The problem was that "classical" QM can't really handle single systems very well. ยด

    Martin Plenio and Peter Knight wrote a good review a few years ago, I think it is available on the arXiv.
  9. Sep 5, 2010 #8
    you don't need to interact directly with a particle to know something about it. Free interaction measure are possible. The point in quantum mechanics is that no matter if you interact directly or not at the moment that you know something about the particle the state of the particle collapse to the eigenstate corresponding to the eigenvalue that you know. In the case of the DSE doesn't matter which method you use to know which slit the electron took you will always lose the interference pattern. The reason is that a the beginning you knew the energy of the electron (depends of the source that is known to the experimentalist) and then you have a low uncertainty about the momentum of the electron. This relativity low uncertainty about the momentum and high uncertainty about the position are the responsible of the interference pattern. Now if you measure directly or not the position of the particle then the state collapse to a position eigenstate and by the uncertainty principle the state have a high uncertainty about the momentum and the interference pattern disappear.
  10. Sep 14, 2010 #9
    hi i resently read about the double slit test and the results you get from it (when fireing the photon one at a time you can get it appering in random locations in the space between the wall with slits in and detector) and i was thinking that when the reading is taken could it just be the photon is going much faster than the detector is scaning and also that the photon is rebounding off of the detector and the wall so the detector only detects the photon when it has rebounded a few times and ether the half life of the photon is so short that it soon disintergrates or it in the end up flowing out of the other slit.
    p.s. please correct me if anypoint in my thory is incorrect and i will rethink it
  11. Sep 14, 2010 #10
    A direct measurement of the position of the photon destroy the photon, because normally the detector consist of a photon gun and the photon is his own anti-particle, then the collision is inelastic, also in the case of the detector panel, it absorb the photon. But I think there is a experiment that answer your question the name of the experiment is the wheeler delayed choice experiment. You can wiki or goggle the experiment to see the details. The point is that the experiment show that is not what happen during the period of time that the photon was going through the slit what affect the result,is the measurement the one that affect the experiment and this measurement can be made at any time after the photon went trough the slit. Then I can measure which slit the photon took a long time after the photon "made the decision". For example in theory is possible to shoot one photon every one year and use a distance between the "double slit" and the detector panel of .7 light years and the result would be the same.

    Now also answering your question in QM is possible that the photon took any possible path and we can calculate the result of the experiment using that fact. The method is the path integral method and can be found in any good QM book. Also if you have time I recommend you the book "QED,The Strange Theory of Light and Matter - R. Feynman" in which Feynman discus the method with some examples. Then yes in theory is possible that the photon took random paths. Then what you said is a possibility, the point is that we need to consider all the possible paths. The only problem is that the photon is no going to rebound in the detector, because the photon is always a light velocity and if the photon rebound that mean that for a moment the photon stop. What happen is that the detector absorb the photon and is possible that the detector emit other photon. Also how you define the decay of a photon? In vacuum we can consider photon like stable particles, the half life is really long.
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