  The theory of gauge-fixed Maxwell equations in linear isotropic dielectrics
is developed using a generalisation of the standard $R_\xi$ gauge-fixing term.
In static space-times, the theory can be quantised using the Gupta-Bleuler
method, which is worked out explicitly for optical fibres either in flat
space-time or at a constant gravitational potential. This yields a consistent
first-principles description of gravitational fibre-optic interferometry at the
single-photon level within the framework of quantum field theory in curved
space-times.
