We give a complete framework for the Gupta-Bleuler quantization of
the free electromagnetic field on globally hyperbolic space-times. We describe oneparticle
structures that give rise to states satisfying the microlocal spectrum condition.
The field algebra in the so-called Gupta-Bleuler representations satisfies
the time-slice axiom, and the corresponding vacuum states satisfy the microlocal
spectrum condition. We also give an explicit construction of ground states on ultrastatic
space-times. Unlike previous constructions, our method does not require
a spectral gap or the absence of zero modes. The only requirement, the absence of
zero-resonance states, is shown to be stable under compact perturbations of topology
and metric. Usual deformation arguments based on the time-slice axiom then lead
to a construction of Gupta-Bleuler representations on a large class of globally hyperbolic
space-times. As usual, the field algebra is represented on an indefinite inner
product space, in which the physical states form a positive semi-definite subspace.
Gauge transformations are incorporated in such a way that the field can be coupled
perturbatively to a Dirac field. Our approach does not require any topological
restrictions on the underlying space-time.