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I How can laser photons have the same precise energy?

  1. Sep 14, 2018 #1

    referframe

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    The photons generated by a conventional quantum laser are all in the same quantum state. Doesn't that mean that they all have the same exact energy?

    Yet, because of energy-time uncertainty, the exact energy of any particle can never be measured. Also, the Copenhagen Interpretation says that it is meaningless to talk about qualities of quantum objects that can never be observed.

    Comments?

    Thanks in advance.
     
  2. jcsd
  3. Sep 14, 2018 #2

    Charles Link

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    Photons are bosons, so this allows more than one photon in the same state. In all lasers, there is still some bandwidth, and you will not find all of the photons in the same boson state.
     
  4. Sep 14, 2018 #3
    Yes. No.

    In an idealized treatment, the output of the laser is the coherent state which does not correpond to photons with the same energy.
     
  5. Sep 14, 2018 #4

    Vanadium 50

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    Do you mean is the energy distribution narrower than the same line when its not lasing? No. Why should it be?
     
  6. Sep 14, 2018 #5

    PeterDonis

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    No, because the state that all of the photons generated by the laser are in is not an eigenstate of energy.
     
  7. Sep 15, 2018 #6

    referframe

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    Oh, okay. I just recently learned about the existence of these "Coherent States". Very interesting to me that eigenstates of non-Hermitian operators should play such an important role. Thanks.
     
  8. Sep 15, 2018 #7

    Paul Colby

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  9. Sep 15, 2018 #8

    PeterDonis

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    This is a separate issue from the issue raised in posts #3 and #5. Line width means that the photons emitted by any real laser are not perfectly in phase. But the laser photons being in a coherent state means that, even for an idealized laser that emits all its photons exactly in phase, the photons are not in an eigenstate of energy and so cannot be said to all have the same energy. All that can be said is that they all have exactly the same phase (in the idealized case).
     
  10. Sep 15, 2018 #9

    Charles Link

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    Spectral line width indicates the laser is composed of photons of different frequencies.
     
  11. Sep 15, 2018 #10

    Paul Colby

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    It may be observed, just never without some error or limit to measurement. All devices and all measurements will have noise sources.
     
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