Generation of Pure-State Single-Photon Wavepackets by Conditional   Preparation Based on Spontaneous Parametric Downconversion
  We study the conditional preparation of single photons based on parametric downconversion, where the detection of one photon from a given pair heralds the existence of a single photon in the conjugate mode. We derive conditions on the modal characteristics of the photon pairs, which ensure that the conditionally prepared single photons are quantum-mechanically pure. We propose specific experimental techniques that yield photon pairs ideally suited for single-photon conditional preparation. 
Alfred B. U'Ren, Christine Silberhorn, Reinhard Erdmann, Konrad   Banaszek, Warren P. Grice, Ian A. Walmsley, and Michael G....