12.1 INTRODUCTION

Single photons are generated in two generals ways. First, a standard light source emitting a stream of photons well-described by a Poisson-like process, can be attenuated so that the average photon number emitted is one. An attenuated semiconductor light-emitting diode (LED) is a good example. A source can also be made by pumping a nonlinear crystal with a laser to produce down-converted photon pairs. Again, in the low photon pair emission regime a single photon source, called a heralded source [1], can be made where the detection of one photon of the pair ensures the other is present in the channel. A second type of source is a regulated light source which insures that the phonon number emitted by the source is either 0 or 1 in a desired time bandwidth. The regulated source is the subject of this chapter. Regulated single photon sources are called nonclassical because they cannot be accurately described by a classical electromagnetic field. These sources are based on the finite spontaneous emission decay rate associated with a single nondegenerate, atomic-like transition. The finite decay rate and nondegeneracy of the fermonic initial state guarantees that only one fermion will decay to a lower energy state producing only one photon at the transition energy. Single atoms are the obvious first choice for such a source; indeed, the first sources were demonstrated in the late...