What is the regime under which we may consider quantum opticsquantum optics description of light a good approximation of a more correct theory such as QED?
By quantum optics I mean describing the electromagnetic field as a collection of harmonic oscillators, and its interaction with particles through the Hamiltonian $ H = \frac{(p-A)^2}{2m} + V_{coul} + H_{free} $$$ H = \frac{(p-A)^2}{2m} + V_{coul} + H_{free} $$ where $H_{free}$ is a sum of harmonic oscillators.
