  We formulate macroscopic quantum electrodynamics for the most general linear,
absorbing media. In particular, Onsager reciprocity is not assumed to hold. For
media with a non-local response, the field quantisation is based on the
conductivity tensor and the Green tensor for the electromagnetic field. For a
local medium response, 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 non-reciprocal responses are allowed for.

Quantum Physics