Lasers work by stimulated emission of atomic transitions. Stimulated emission produces two photons which, because the particle number is well-defined, projects the field into a Fock state. However, it is a known fact that lasers emit light in a coherent state. How does the field evolve from a particle-state to a superposition of particle-states? Omitting normalization:
$$ | n \rangle \rightarrow \sum_{n=0}^{\infty}\frac{\alpha^n }{\sqrt{n!}}| n \rangle $$
I guess one way of looking at it is that the field shifts according to $\Delta n \Delta \phi \geq 1$ from certain particle number to certain phase but it feels like a superficial answer to me. What I want to understand is the mechanism that allows this to happen. Is it the reflection with the mirror? Is it the imposed boundaries of the resonating cavity? Pumping method?
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