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I A question about two-slits experiment

  1. Jun 3, 2016 #1
    hello, I heard that in 2012 someone did an two-slits experiment in which they can detect which slit the photon passes without destroying the interference pattern. Is this news a fact or not ? if it is a fact, anyone could give me the paper to read ? thanks.
  2. jcsd
  3. Jun 3, 2016 #2


    Staff: Mentor

  4. Jun 3, 2016 #3
    Dear athosanian,

    That's in somewhat loose wording was Dr. Aephrain Steinberg (University of Toronto - Canada) has done. More rigorously, you can read it in "Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer" - Science 3 June 2011: Vol. 332 no. 6034 pp. 1170-1173 DOI: 10.1126/science.1202218
    The abstract was:

    "A consequence of the quantum mechanical uncertainty principle is that one may not discuss the path or “trajectory” that a quantum particle takes, because any measurement of position irrevocably disturbs the momentum, and vice versa. Using weak measurements, however, it is possible to operationally define a set of trajectories for an ensemble of quantum particles. We sent single photons emitted by a quantum dot through a double-slit interferometer and reconstructed these trajectories by performing a weak measurement of the photon momentum, postselected according to the result of a strong measurement of photon position in a series of planes. The results provide an observationally grounded description of the propagation of subensembles of quantum particles in a two-slit interferometer.

    Read more at: http://phys.org/news/2011-06-quantum-physics-photons-two-slit-interferometer.html#jCp
  5. Jun 4, 2016 #4
  6. Jun 5, 2016 #5
  7. Jun 5, 2016 #6

    Thanks a lot, Tollendal, you are really very helpful !
  8. Jun 5, 2016 #7
  9. Jun 5, 2016 #8
    actually speaking, it is a little disappoint to me. but it is also interesting.
  10. Jun 11, 2016 #9
  11. Jun 12, 2016 #10


    Staff: Mentor

    Last edited: Jun 12, 2016
  12. Jun 12, 2016 #11
    Well that's not very helpful. The article I linked to in post #9 says the trajectory (which slit) of no single photon was observe. But several other articles claim otherwise, e.g. http://phys.org/news/2011-06-quantum-physics-photons-two-slit-interferometer.html#jCp. I listened to Steinberg for over an hour and though he intimated he could observe which slit and still retain interference (I don't believe it) there was a lot of vagueness with regards to weak measurements. I've tried plowing through the technical details, but it's very unpleasant and I lose the forest for the trees. Here (https://www.perimeterinstitute.ca/news/are-weak-values-quantum-dont-bet-it) Combes seems skeptical about the whole enterprise.

    I'm convinced of your knowledge and insight, but am I too dense for you to pass on some enlightenment and clear the fog?
  13. Jun 12, 2016 #12


    Staff: Mentor

    Scratching my head about exactly what your issue is.

    What weak measurements are is well known. They give the average of an ensemble of observations. There are a few technicalities involved and you can consult texts on it:

    You mentioned the double slit experiment. Applying weak measurements to observing which slit it goes through you can still get interference because it only weakly couples to the system so doesn't affect the interference pattern. But the result is not which slit it goes though - its the average trajectory:
    'Weak measurement followed by post-selection did not allow simultaneous position and momentum measurements for each individual particle, but rather allowed measurement of the average trajectory of the particles that arrived at different positions. In other words, the experimenters were creating a statistical map of the full trajectory landscape'

    That's basically all there is to it. If you call them quantum or not is just semantics - IMHO they are quantum but that's just my opinion.

    Last edited: Jun 12, 2016
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