Characterization of the non-classical nature of conditionally prepared   single photons
  A reliable single photon source is a prerequisite for linear optical quantum computation and for secure quantum key distribution. A criterion yielding a conclusive test of the single photon character of a given source, attainable with realistic detectors, is therefore highly desirable. In the context of heralded single photon sources, such a criterion should be sensitive to the effects of higher photon number contributions, and to vacuum introduced through optical losses, which tend to degrade source performance. In this paper we present, theoretically and experimentally, a criterion meeting the above requirements. 
Alfred B. U'Ren, Christine Silberhorn, Jonathan L. Ball, Konrad   Banaszek, Ian A. Walmsley