  Most observational techniques in astronomy can be understood as exploiting
the various forms of the first-order correlation function g^(1). As however
demonstrated by the Narrabri Stellar Intensity Interferometer back in the
1960's by Hanbury Brown & Twiss, and which is the first experiment to measure
the second-order correlation function g^(2), light can carry more information
than simply its intensity, spectrum and polarization. Since this experiment,
theoretical and laboratory studies of non-classical properties of light have
become a very active field of research, namely quantum optics. Despite the
variety of results in this field, astrophysics remained focused essentially on
first-order coherence. In this paper, we study the possibility that quantum
properties of light could be observed in cosmic sources. We provide the basic
mathematical ingredients about the first and the second order correlation
functions, applied to the modern context of astronomical observations. The
exploitation of g^(2) is certainly richer than what a modern intensity
interferometer could bring and is particularly interesting for sources of
non-thermal light. We conclude by briefly presenting why microquasars in our
galaxy and their extragalactic parents can represent an excellent first target
in the optical/near-infrared where to observe non-thermal light, and test the
use of g^(2) in astrophysical sources.
