Classical vs Quantum Electromagetism

[LarryS]

I'm having trouble relating the classical (Maxwell) model for electromagnetic radiation to the quantum mechanical (Q.E.D.) model. How do the magnetic field wave and electic field wave components of the classical electromagnetic wave relate to the quantum mechanical wave function for a photon?

 

[Meir Achuz]

E and B are found from the QM A^\mu of a photon using the usual classical definitions.
A classical EM wave corresponds to N coherent photons, with N>>>1.