  We derive expressions for the quantum electromagnetic field in a dispersive
and dissipative dielectric medium, treating the medium as a continuum. We
compare the Langevin approach with the Fano diagonalization procedure for the
coupled system composed of the electromagnetic field, the dielectric medium,
and the reservoir. In particular, we show that the quantized electric and
magnetic fields obtained in both methods have exactly the same form. More
generally, in thermal equilibrium, correlation functions computed in both
methods are identical.
