  We demonstrate a single-photon source based on a quantum dot in a micropost
microcavity that exhibits a large Purcell factor together with a small
multi-photon probability. For a quantum dot on resonance with the cavity, the
spontaneous emission rate is increased by a factor of five, while the
probability to emit two or more photons in the same pulse is reduced to 2%
compared to a Poisson-distributed source of the same intensity. In addition to
the small multi-photon probability, such a strong Purcell effect is important
in a single-photon source for improving the photon outcoupling efficiency and
the single-photon generation rate, and for bringing the emitted photon pulses
closer to the Fourier transform limit.
