Single photons are important states for quantum information processing and fundamental research. These applications require efficient single-photon generation on demand. In this work asingle-photon device based on single InP and CdSe quantum dots is presented. Intensity correlation measurements were used to verify the single photon emission in pulsed and continuous operation. By using Fourier spectroscopy, the coherence length and spectral bandwidth of the single-photon states was measured. The observation of cross-correlations between several spectral lines revealed the existence of multi-photon cascades and gave information about their decay dynamics. The efficient separation of adjacent photons from a two-level cascade allowed their independent use and an effective doubling of the photon generation rate, similar to multiplexing in classical information processing. For the first time, multiplexing on a single-photon level could be demonstrated in a quantum key distribution experiment.