Single photon counting, based on single photon sources and
detectors, is a key ingredient for certain applications aiming at
new quantum information technologies. Quantum cryptography, quantum
radiometry, distributed quantum computing, as well as adjacent
technologies such as biomedical and astronomical imaging, and low
power classical communication also rely on single-photon technology.
This paper reviews the present status of single photon sources and
related counting measurement techniques, based on correlated (or
heralded) photons in parametric down-conversion, and their possible
impact on the above mentioned technologies, as well as an assessment
for photon standards in the future.