  Owing to the analogy with the ordinary photons in the visible range of the
electromagnetic spectrum, the Glauber theory is generalized to address the
quantum coherence of the gauge field fluctuations parametrically amplified
during an inflationary stage of expansion. The first and second degrees of
quantum coherence of relic photons are then computed beyond the effective
horizon defined by the evolution of the susceptibility. In the zero-delay limit
the Hanbury Brown-Twiss correlations exhibit a super-Poissonian statistics
which is however different from the conventional results of the single-mode
approximation customarily employed, in quantum optics, to classify the
coherence properties of visible light. While in the case of large-scale
curvature perturbations the degrees of quantum coherence coincide with the
naive expectation of the single-mode approximation, the net degree of
second-order coherence computed for the relic photons diminishes thanks to the
effect of the polarizations. We suggest that the Hanbury Brown-twiss
correlations are probably the only tool to assess the quantum or classical
origin of the large-scale magnetic fluctuations and of the corresponding
curvature perturbations.
