On-chip heralded single photon sources

Time correlated photon pairs are used to produce heralded single photon states for quantum integrated circuits. These states are generated by photon sources, which are called heralded single photon sources. They are based on the detection of one photon which heralds the presence of the other. In this way, pure single photon states can be probabilistically generated and, subsequently, manipulated in complex photonic circuits. Heralded single photon sources are the topic of this review. The authors detail the main parameters and the experiments involved in their characterization, with a focus on their use in integrated photonic circuits. Different geometries and technological platforms are compared, and an assessment of their performances is performed. The final result is that nearly perfect single photon sources are possible and feasible in an integrated platform. A few open issues are also underlined.