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Are probability coefficients source for semitransparence

  1. Jan 18, 2016 #1
    If I have a 60-40 beam splitter, I have a semitransparent plate that will classicaly reflect 60% of the light and transmit 40%.

    On quantum level, photons take both trajectories at the same time, but with different probability coefficients.

    So is quantum superposition of trajectories for photons that make an object look semitransparent?

    The wave gets split due to quantum tunneling, but only part of the probability amplitude of the photon leaks through the thin reflective barrier, so as a function of the thickness, we have more or less transmitted light.

    Is semitransparence due to classical scattering or due to quantum superposition? Are interference patterns visible manifestations of quantum superposition?
     
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  3. Jan 18, 2016 #2

    DrChinese

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    Most beam splitters alter the trajectory of some light passing through it, based on its polarization. I would not call that semitransparent.

    Quantum tunneling is a different effect, unrelated to beam splitters.

    The thickness of the barrier normally has nothing directly to do with the amount of light reflected in an optical lens. Its wavelength relative to the thickness is an indirect factor in some cases.
     
  4. Jan 18, 2016 #3

    Nugatory

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    You've marked this thread as A-level but it's clear that you're not looking for an answer at that level so I have taken the liberty of resetting the level to I (no familiarity with quantum field theory and quantum electrodynamics).

    You have to be very careful with statements like "take both trajectories at the same time". It's a popular description of what happens in the double-slit experiment, but it's easy to read too much into it - especially with photons which have no clearly defined positions or trajectories except when they are interacting with matter.
    That's one way of thinking about it (and it's also quite different from saying that the photon takes both trajectories). You may want to get hold of Feynmann's book "QED: The strange theory of light and matter" which is an excellent layman-friendly explanation of what's going on.

    Usually not. The interference patterns you observe with light (for example, in the double-slit experiment) are a classical phenomenon caused by ordinary classical interference of ordinary classical electromagnetic radiation, and were observed and well understood by classical physicists long before quantum mechanics came along. The interference pattern is only an example of quantum superposition when you do the experiment with single particles (which is quite a bit a more demanding) so that the pattern emerges from individual detections.
     
  5. Jan 18, 2016 #4
    I asked my teacher about all this and he told me that you get different probability coefficients for different beamsplitter reflectivities.
    For example a beamsplitter with 90% reflectivity and 10% transmisivity gives you a 0.9 |R> + 0.1 |T> superposition state for a single photon.

    You may not have understood what the role of beamsplitters was. They actually do a fine tunning of the probability coefficients found in superposition.

    And he also told me that interference patterns exist only when the which-path information is not known, only when the wave character of photons is not disturbed by measurement.
    If the trajectory of single particles or classical waves is measured, the interference pattern disappears like any quantum superposition does. There is no classical interference pattern that remains whatever you do to it.
     
  6. Jan 18, 2016 #5

    Nugatory

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    Yes, of course. Nothing that DrChinese or I have said above disagrees with that. You will find the Feynmann book I recommended very helpful in understanding this behavior.
    This is also true; if there is only one path open a classical electromagnetic wave will not produce an interference pattern. However, that does not have anything to do with quantum interference, and does not change the fact that the answer to your question "Are interference patterns visible manifestations of quantum superposition?" is still "usually not" because classical interference patterns are not quantum mechanical phenomena at all (except in the trivial sense that classical E&M is the many-photon limit of quantum electrodynamics).
     
  7. Jan 18, 2016 #6

    DrChinese

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    That is generally true, but is an oversimplification.

    You can measure photon polarization (by placing a polarizer in front of each slit), for example, and choose to either eliminate or not eliminate double slit interference (by choice of relative settings). In other words, it is not the act of having it go through the polarizer (presumably disturbing it in your example); it is whether or not you could potentially extract which-path information. "Disturbance" itself is not the determining factor.
     
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