  We develop a theoretical framework for the analysis of the quantum coherence
of light emitted by two independent single-photon sources in an arbitrary
environment. The theory provides design rules for the control of the degree of
quantum coherence, in terms of classical quantities widely used in
nanophotonics. As an important example, we derive generalized conditions to
generate superradiant and subradiant states of the emitters, and demonstrate
the ability of a structured environment to induce long-range quantum coherence.
These results should have broad applications in quantum nanophotonics, and for
the sensing of fluorescent sources in complex environments.
