You did not mention population inversion
you want to discuss population inversion? fine
first we create two photons from one
then an amplifier must produce millions of photons
adding increasing numbers of atoms to the upper metastable state
and holding them long enough to store energy and allow the production of lorry loads of stimulated photons
the longer lifetime states allow the rate at which atoms are raised into one of these states to exceed the rate at which they leave
atoms can be excited into and held in the upper state leaving an almost empty state below them
atoms can stay in this metastable state without de-exciting while the population is being raised
a population inversion occurs when more atoms are in a higher state of excitation than in the lower state
NOTE: this violates entropy
nor did you touch the idea of stimulated emission
we must remember how a laser works
Light Amplification by Stimulated Emission of Radiation
this is not just a beam of light, it's amplified light
which requires a high energy source (an inductor coil) to drive it
here is my quote again
as you can see I've already stated the process with these terms alone
"Light Amplification by Stimulated Emission of Radiation"
notice the terms "Stimulated Emission of Radiation"
and I've had far more than basic QED (Euler-Lagrange equations) thank you