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Now, we do have equipment to generate single photon at a time, and LASERs are nearly monochromatic.

While typing the question, am realizing that successive photons in case of single photon generation may have slightly different wavelengths, but are its single photons essentially monochromatic? Because we may be able to use the phenomena to make highly coherent lasers.

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  • $\begingroup$ Are you asking if one photon as one wavelength? If so- yes. But that doesn't make the last sentence true. $\endgroup$
    – jhobbie
    Commented Jul 2, 2014 at 13:50

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Any measurement of the photon's energy (i.e. frequency, or free-space wavelength though making a direct identification of particle properties to wave properties is a little sketchy) will return a single value. Every time.

But ... you can't fool Heisenberg and if you have confined the position of the photons---say by insisting that it hit the detector---then

  1. You can not predict exactly what the measurement will return
  2. Measurements of many photons from a single source will show non-zero a width
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  • $\begingroup$ The corresponding uncertainty relation to position-momentum is energy-time. If you measure the wavelength (energy) perfectly, you must have no information on when you measured it. $\endgroup$
    – Ross Millikan
    Commented Jul 2, 2014 at 15:21
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Yes - but not really.

A single photon has a single value for it's related wavelength - so it's perfectly monochromatic, in a formal sense.

But, in a physical sense, the concept of a spectrum (like: monochromatic) basically does not apply to a single photon.

In practice, you do not have these "single photons, with some specific wavelength" to actively work with, as implied in the question - that would mean to ignore Heisenberg.
But you may measure you had one - even with a specific wavelength.

See also
Frequency and wavelength of photons
and
Can a photon exhibit multiple frequencies?
discussing that a photon may be a superposition of multiple frequency states - before measuring, of course.

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  • $\begingroup$ Your penultimate sentence needs some sharpening, I think. As you say, a single photon has a single frequency and wavelength. What you might have physically is a bunch of very closely spaced modes. The photon will live in one of those modes, but we might not know which one. Still, whatever mode is excited it will have a single frequency and wavelength. Photons themselves do not have a spread of frequencies. $\endgroup$
    – garyp
    Commented Jul 2, 2014 at 14:54
  • $\begingroup$ @garyp More innocuous, and correct now I guess? :) $\endgroup$
    – Volker Siegel
    Commented Jul 2, 2014 at 15:14
  • $\begingroup$ @garyp if the photon lives in one of those modes, but we don't know which one, isn't it a mixed state then? A photon in a pure state may be in superposition of multiple modes, but in this case it doesn't live in one of them. $\endgroup$
    – Ruslan
    Commented Jul 2, 2014 at 15:35
  • $\begingroup$ Even if a photon at an instant lies in a single mode, the frequency must have a spread independent of our knowledge, coz otherwise we could say it has a frequency value with infinite accuracy(independent of we can measure it or not) which is not possible. So single wavelength in a single more shouldn't be the case. $\endgroup$
    – Harshfi6
    Commented Jul 2, 2014 at 17:04
  • $\begingroup$ @harshfi6 We may be arguing about language. My point of view comes from what I glean from books (Loudon, and Mandel and Wolf). (I may misrepresent them.) My photon: a single excitation of an energy eigenstate / number eigenstate. It has a single frequency, well-defined occupation number, and completely indeterminate phase. Anything else is a state of the field, but not a photon. Note that the states are degenerate. In free space, $\vec{k}$ can point in any direction, or be a superposition thereof, so with boundaries the field can take on non-space-filling shapes (e.g. cavities). $\endgroup$
    – garyp
    Commented Jul 3, 2014 at 0:25

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