Φ - Ψ model for electrodynamics in dielectric media: exact quantisation in the Heisenberg representation

We investigate the quantisation in the Heisenberg representation of a model which represents a simplification of the Hopfield model for dielectric media, where the elec- tromagnetic field is replaced by a scalar field φ and the role of the polarisation field is played by a further scalar field ψ. The model, which is quadratic in the fields, is still char- acterised by a non-trivial physical content, as the physical particles correspond to the polaritons of the standard Hop- field model of condensed matter physics. Causality is also taken into account and a discussion of the standard interac- tion representation is also considered.