ABSTRACT
This chapter describes devices designed to emit single-photons at controlled time intervals, as needed for various quantum information applications. Before describing how such a device might be constructed, let us dene exactly what we mean by a source of singlephotons. Such a light source is quite different from a classical source such as a laser or a light emitting diode, where the number of photons per pulse varies randomly, usually following a Poisson distribution. However, a realistic single-photon source will never be perfect, sometimes emitting zero or multiple photons, and thus the distinction between a classical photon source and a realistic single-photon source is subtle. We therefore need quantitative measures to distinguish between light sources based on photon statistics. From the viewpoint of system applications, other characteristics are also important. The most important parameters describing a single-photon source are:
the two or multi-photon probability per pulse, p p n g nm = ≥( ) ≤ ( )2 1 2 02 2( / ) , where 〈n〉 is the average photon number per pulse. A photon source with 〈n〉 = 1 and with, g0
2 1( ) = , g02 1( ) < , or g02 0( ) = is called a Poisson, sub-Poisson or ideal single-photon source, respectively. Since a realistic single-photon source has 0 10
2< <g( ) , it is a sub-Poisson photon source operating in the regime of n
< 1. When a single-photon
source is used in quantum key distribution systems, the robustness against a specic eavesdropping attack, called the photon number splitting attack, requires a small value of g0
2( ). 2. An external quantum efciency ηtotal, which denes an overall out-coupling ef-
ciency of internally generated single-photons into a desired single waveguide mode.
