Tailoring Single and Multiphoton Probabilities of a Single Photon   On-Demand Source
  As typically implemented, single photon sources cannot be made to produce single photons with high probability, while simultaneously suppressing the probability of yielding two or more photons. Because of this, single photon sources cannot really produce single photons on demand. We describe a multiplexed system that allows the probabilities of producing one and more photons to be adjusted independently, enabling a much better approximation of a source of single photons on demand. 
A. L. Migdall, D. Branning, S. Castelletto and M. Ware