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Less than one photon, what does it mean?

  1. Mar 10, 2014 #1

    What does less than one photon mean? 0.5 or 1/2 photon what does it mean?
  2. jcsd
  3. Mar 10, 2014 #2

    Simon Bridge

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    Nothing without the context - where have you seen this?
  4. Mar 10, 2014 #3


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    There is nothing preventing you from having less than one photon on average. This is e.g. always the case for quantum key distribution experiments that use attentuated lasers as sources ; the distibution is Poissionian but by attenuating the source enough you can create a situation where you get pulses with either 0 or 1 photon most of the time (but you will of course also get som pulses with 2, 3,4 photons etc). However, if you do this you will find that each pulse has -on average- less than one photon in it.

    It is also true for situation where "number of photons" is a measure of the energy/h*f; I frequently do experiment where we measure devices which have less than one photon in them, and when we plot our data we have <n> on the x-axis.
  5. Mar 10, 2014 #4

    Simon Bridge

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    It's like the average family having 2.6 children ... what does 0.6 of a child mean?

    ... you can also have the situation where someone says "less than one photon is inside the cavity" and then they mean the wavelength of the photon is larger than the length of the cavity.

    Then again it could be something else entirely.
    i.e. 2.6 kids could mean two moderately fat ones or one obese one.
  6. Mar 10, 2014 #5
  7. Mar 11, 2014 #6


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    Ironically, the question "n photons, what does it mean?" is just as non-intuitive, because if we have an eigenstate of n, for n large, we get a zero electromagnetic field (in the limit). We can only get the classical limit of nonzero electromagnetic field by having a state that is not an eigenstate of number of photons.
  8. Mar 23, 2014 #7

    thanks i understand now
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