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Photon emission

  1. Feb 9, 2006 #1
    Hey all,

    I'm currently in my second QM class, and I have a question about the emission of photons. Last semester, as we studied the wavefunction of a hydrogen electron, the professor briefly mentioned that the energy levels correspond to the observed energies of photons emitted from hydrogen atoms. In general, however, an electron will be in a linear combination of the eigenstates. Since the photon energies are only measured to be discrete, does this mean that the emission of a photon corresponds with a measurement of the electron's energy?

    NB: I haven't learned QED yet, so please give me a dumbed-down answer. Thanks. :biggrin:
  2. jcsd
  3. Feb 10, 2006 #2

    Meir Achuz

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    It measures what the electron's energy was before the radiation since
  4. Feb 11, 2006 #3


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    Yes! Picture it this way: the superposition of electron eigenstates, through the interaction with the EM field, will result in a superposition of the EM photon states, where each of the terms corresponds to each of the eigenstate-transitions in the hydrogen atom.
    And now you measure, and see one of those photons (application of the measurement postulates).

    Symbolic example:

    |H-atom> = |psi3> + |psi5>
    (say, superposition of the third and fifth excited state, just for the sake of it).

    Interaction with EM field (from the vacuum state) gives you:

    |H-atom+EM field> = a |psi0> |photon3> + b |psi0> |photon5> + c |psi1> |photon2> + ...

    where the first state in each term gives you the "final" state of the H-atom, and the second state in each term gives you the EM field state (kind of photon you have).

    Now you do a "photon" measurement, so, with probability |a|^2, you have the first state (ground state + photon3), with probability |b|^2 you have the second state (ground state + photon5), ...
  5. Feb 11, 2006 #4
    Ah, so the photon itself is in a superposition of eigenstates. For some reason, I never considered that possibility. Does that mean that you can determine the state of the electron by measuring the energy of the photon? Is that a form of entanglement?
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