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B Single-source and multi-source interference

  1. Aug 13, 2015 #1
    [Mentor's note: This thread was split out from another longer one because it is an interesting topic in its own right]

    Something that I've been confused about when it comes to interference between photons is the distinction between two different quantities that can interfere constructively or destructively:
    1. There is a quantum amplitude describing the occurrence of a photon at a particular location at a particular time. If there are multiple ways that a photon can arrive at that location, then the amplitudes add, and depending on the relative phases, produce constructive or destructive interference.
    2. Classically, light is a fluctuating electromagnetic field, and at every point and time, there is an associated vector quantity, the polarization, which describes how the electric and magnetic fields point. Light from different sources that combine at a point will interfere constructively if their electric and magnetic fields point in the same directions and destructively otherwise.
    Effect #1 only makes sense quantum-mechanically, and the interference involves a particle interfering with itself. Effect #2 is true classically, and it does not seem to require self-interference.

    Is there some sense in which effect #2 is also self-interference? That doesn't seem right to me, because you can produce interference effects from two different light sources, where it doesn't make sense (or at least not in any simple way) to think that it's the same photon interfering with itself.
    Last edited by a moderator: Jan 11, 2017
  2. jcsd
  3. Jan 10, 2017 #2
    Aren't all photons just the same field? If you combine two half-photons with the same polarisation you'll get one photon, right? And if they are different, what would you get?
  4. Jan 10, 2017 #3


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    There is no such thing as a "half-photon" and no such thing as combining photons.

    We do have some pretty decent "What is a photon?" threads - you might want to search for them.
  5. Jan 11, 2017 #4
    Thanks, I have read the top hit before, but I guess it takes a while to take in all the details. Does it make the answer to stevandrayl's question clear?
  6. Jan 11, 2017 #5


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    This may be so but the only way to tell that there is a photon at a certain location is to have a photon detector there.

    Presumably the 'arrival' of two out-of-phase photons will be the same as no photon. What happens in the intermediate case might be interesting.

    Ballentine says that photon detectors respond to the square of the electric field component, so the square of the sum of two electric fields could show interference.

    My understanding is that 'self-interference' is the consequence of a (quantum) superposition. As in the Hong-Ou-Mandel setup.

    @Devin Bayer : the section in the attached slide presentation ( by A. Neumaier) called 'What is a photon' is very good.

    Attached Files:

    Last edited: Jan 11, 2017
  7. Jan 12, 2017 #6
    Definitely. But that is classical interference, not quantum interference. It certainly doesn't depend on the photon concept, since it was predicted by non-quantum Maxwell equations.

    But is classical interference of electromagnetic fields somehow understood in terms of quantum interference, or is it a separate phenomenon? I'm not sure. It is something I ought to know the answer to, but I don't.
  8. Jan 12, 2017 #7


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    Some people might argue that all light interference is both quantum and classical because light straddles these definitions.
    What I meant to mention was the single photon in a Mach-Zehnder interferometer experiment by Grangier and in the attached ( basic level) paper.

    Is this classical ?

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  9. Jan 12, 2017 #8


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    It can't be a completely separate phenomenon since the classical model of the EM field is just an approximation derived from the quantum model.
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