  Single photon sources are necessary for optical quantum technology. The
nonclassicality of emitted single photons manifests itself in diverse
experiments and applications. The applications already require different
nonclassical aspects of single photons, however, a suitable hierarchy of
criteria is missing. We propose variable experimental tests, based on
adjustable linear optical networks and single photon detectors, giving a
hierarchy of such nonclassicality criteria. The hierarchy goes beyond Hanbury -
Brown - Twiss test of photon antibunching and allows us to compare faithfully
quality and performance of single-photon sources via their nonclassical
properties.
