Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Double slit experiment with electrons - detector at one slit

  1. Mar 28, 2014 #1

    I'm new here, but I've been researching this subject for several months now. I'm especially interested in the double-slit experiment performed with one electron at a time, particularly with the detection apparatus turned ON.

    I've found this statement, or something very much like it, in numerous blogs and websites (Starts with a Bang, Empirical Zeal, Dr. Quantum, etc.):

    “…we can do one more experiment: this time, we shoot electrons one-at-a-time at this wall, but at each slit, we shine a bit of light, and detect which slit the electron goes through. As each electron is fired, one (but never both) of the detectors goes off, telling you which slit the electron went through. But — and here’s the crazy part — the pattern on the screen now shows no interference, and instead we just get two separate peaks corresponding to the two “classical”, particle-like paths the electrons could have taken.” scienceblogs.com/startswithabang/2009/06/01/a-tale-of-two-slits/‎

    Here is an illustration of the result: Double slit experiment eletrons detector on.jpg

    So, here is my question, and it is at the very nub of my gist, and possibly even at the heart of quantum physics:

    Where is the PROOF, i.e., a link to the actual experiment which shows that when we detect, or record, or measure the electrons going through the slits, we get two “bands” on the screen? I have never seen a single reference to any experiment which clearly demonstrates this. If and when I do, I will be convinced that this actually does happen, and I can die a happy man. Otherwise, it is just unsubstantiated conjecture (and I'm sure it's not). Believe me, I really DO want to be convinced. So, can anyone point me the way to the actual, physical experiment where the detectors are turned on, the interference pattern collapses and two bands, or peaks, appear on the screen??? I'm 64, so I'd like to solve this final piece of the puzzle some time before I kick the bucket.

    Thanks to anyone who has the answer!

  2. jcsd
  3. Mar 28, 2014 #2


    User Avatar

    Staff: Mentor

    Google for "Demonstration of Single-Electron Buildup of an Interference Pattern" and you'll eventually find a paper by Tonomura, Matsuda, Kawaska, and Ezawa with both a discussion of the electron case and some discussion of previous experiments in this area.
  4. Mar 29, 2014 #3
    I don't think this paper shows the disappearing of interference when the path of electrons is measured. I have not yet seen that either.
  5. Mar 29, 2014 #4
    Thanks for this reference Nugatory, but I think DParlevliet is right. This paper does not show the disappearing of interference when the path of electrons is measured. It only shows the buildup of an interference pattern using single electrons. It does not show the "two bands". And I believe it's Kawasaki, not Kawaska.

    The search goes on...
  6. Mar 29, 2014 #5


    User Avatar
    Science Advisor
    Gold Member

    How about an atomic interferometer? See "Origin of quantum-mechanical complementarity probed by a 'which-way' experiment in an atom interferometer": Nature 395, 33-37 (3 September 1998) | doi:10.1038/25653; (1998).

    http://www.atomwave.org/rmparticle/ao%20refs/aifm%20pdfs%20by%20group%20leaders/rempe%20%20pdfs/Rempe%20decoherence%201998.pdf [Broken]
    Last edited by a moderator: May 6, 2017
  7. Mar 29, 2014 #6


    User Avatar
    Science Advisor
    Gold Member

  8. Mar 30, 2014 #7
    Double slit experiment with a laser pointer

    This is my own simple experiment using a 5 mW green laser pointer (532 nm) and three 0.5mm pencil leads placed about 0.3 mm apart. First I pointed the laser at the wall, which produced a bright point of light. Next, I shone the laser beam through the pencil leads at a distance of about 30 cm, and got a very distinct diffraction pattern on the wall. Over a distance of about 2.5 m it spread out some 15 cm. That’s pretty amazing for a laser beam, which can appear as a small spot on a wall 30 m away. I was surprised and very pleased to get such good results!

    I realize this is only a replication of Thomas Young's original experiment in the early 1800s, but it did demonstrate to me the wave properties of light, and had implications for more sophisticated experiments with electrons.

    Here are the apparatus and the results:
  9. Mar 30, 2014 #8
    Thanks UltrafastPED, I think this is what I've been looking for!
    Last edited by a moderator: May 6, 2017
  10. Mar 31, 2014 #9


    User Avatar
    Science Advisor
    Gold Member

    BTW, nice setup for your laser pointer diffraction setup!

    The last time I did a diffraction experiment it was with electrons through an ultrathin film of polycrystalline platinum (about 10 nm, or 25 atoms thick). You shoot a few electrons at a time through the film, and after a bit you can see the diffraction pattern for the polycrystalline platinum - you get rings because the little crystallites are oriented in all directions, as with powder x-ray diffraction.

    The "few at a time" are distributed over a fairly large area: a pulse of 1,000 electrons is spread over a 200 micron diameter circle, and a pulse duration of about 300 femtoseconds, so on average each electron is only interacting with itself.

    Or if you crank up the electron photo-emission rate, you can put lots of electrons through on each pulse, and the pattern develops fast enough to see with the naked eye on the phosphor screen.

    However, I don't look to see "which atomic layer" the diffraction came from - you would need a very tiny probe for this! - so I don't have any examples of "which way" from my many experimental runs.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook