Quantum optical coherence can be quantified only by accounting for both the particle-and
wave-nature of light. For an ideal laser beam,–, the coherence can be thought of roughly as
the number of photons emitted consecutively into the beam with the same phase. This
number, C, can be much larger than the number of photons in the laser itself, μ. The limit for
an ideal laser was thought to be of order μ 2 (refs.,). Here, assuming only that a laser
produces a beam with properties close to those of an ideal laser beam and that it has no …

… The central result of this paper is that the true ultimate limit, the Heisenberg limit, for a
laser beam having properties akin to those of the ideal models, is Cideal … Conditions
defining an ideal laser We formally consider the following conditions as defining an ideal
laser assumptions, which are sufficient for deriving the Heisenberg limit to the coherence. …