Exciting an electron in a semiconductor from the valence band to the conduction band creates an excited state, a so-called exciton. The spontaneous radiative decay of this exciton results in the emission of a photon. Since a quantum dot has discrete energy levels, it can be achieved that there is never more than one exciton in the quantum dot simultaneously. Therefore, the quantum dot is an emitter of single photons. A key challenge in making a good single-photon source is to make sure that the emission from the quantum dot is collected efficiently. To do that, the quantum dot is placed in an optical cavity. The cavity can, for instance, consist of two DBRs in a micropillar (Fig. 1). The cavity enhances the spontaneous emission in a well-defined optical mode (Purcell effect), facilitating efficient guiding of the emission into an optical fiber. Furthermore, the reduced exciton lifetime Δ Δ t (see Fig. 2) reduces the significance of linewidth broadening due to noise.