  Recently, we have developed a unified approach to QED in arbitrary linearly
responding media in equilibrium--media that give rise to absorption [Phys. Rev.
A \textbf{75}, (2007) 053813]. In the present paper we show that, under
appropriate conditions, the theory can be quite naturally generalized to
amplifying media the effect of which is described within the framework of
linear response theory. We discuss the limits of validity of the generalized
theory and make contact with earlier quantization schemes suggested for the
case of linearly and locally responding amplifying dielectric-type media. To
illustrate the theory, we present the electromagnetic-field correlation
functions that determine the Casimir force in the presence of amplifying media.

Quantum Physics