Macroscopic quantum electrodynamics in nonlocal and nonreciprocal media

We formulate macroscopic quantum electrodynamics in the most general linear, absorbing media. In particular, Onsager reciprocity is not assumed to hold. The field quantization is based on the source-quantity representation of the electromagnetic field in terms of the dyadic Green's tensor. For media with a nonlocal response, a description in terms of a complex conductivity tensor is employed. As an alternative description, we introduce the permittivity, permeability and magnetoelectric susceptibilities to obtain an explicitly duality-invariant scheme. We find that duality invariance only holds as a continuous symmetry when nonreciprocal responses are allowed for.