  In an exact quantum-mechanical framework, we show that expectation values of
the second-quantized electro-magnetic fields in the Coulomb gauge, and in the
presence of classical sources, automatically lead to causal and retarded
electro-magnetic field strengths. The classical $\hbar$-independent Maxwell's
equations naturally emerge from this fundamental quantum-mechanical approach in
terms of expectation values of quantum fields, and are therefore also
consistent with the special theory of relativity. The fundamental difference
between interference phenomena due to the linear nature of the classical
Maxwell theory as, e.g., in classical optics, and interference effects of
quantum states is clarified. The framework outlined also provides for a simple
approach to, e.g., spontaneous photon emission and/or absorption processes as
well as to the classical Vavilov-Cherenkov radiation. The inherent and
necessary quantum fluctuations, limiting a precise space-time knowledge of
expectation values of the quantum fields considered, are, finally, recalled.
