Single Photon Source Using Laser Pulses and Two-Photon Absorption
  We have previously shown that two-photon absorption (TPA) and the quantum Zeno effect can be used to make deterministic quantum logic devices from an otherwise linear optical system. Here we show that this type of quantum Zeno gate can be used with additional two-photon absorbing media and weak laser pulses to make a heralded single photon source. A source of this kind is expected to have a number of practical advantages that make it well suited for large scale quantum information processing applications. 
B.C. Jacobs, T.B. Pittman, and J.D. Franson