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In an exact quantum-mechanical framework, we show that expectation values ofthe second-quantized electro-magnetic fields in the Coulomb gauge, and in thepresence of classical sources, automatically lead to causal and retardedelectro-magnetic field strengths. The classical $\hbar$-independent Maxwell'sequations naturally emerge from this fundamental quantum-mechanical approach interms of expectation values of quantum fields, and are therefore alsoconsistent with the special theory of relativity. The fundamental differencebetween interference phenomena due to the linear nature of the classicalMaxwell theory as, e.g., in classical optics, and interference effects ofquantum states is clarified. The framework outlined also provides for a simpleapproach to, e.g., spontaneous photon emission and/or absorption processes aswell as to the classical Vavilov-Cherenkov radiation. The inherent andnecessary quantum fluctuations, limiting a precise space-time knowledge ofexpectation values of the quantum fields considered, are, finally, recalled.

Causality in Quantum Field Theory with Classical Sources - Quantum Electrodynamics