Single photon state generation from a continuous-wave non-degenerate   optical parametric oscillator
  We present a theoretical treatment of conditional preparation of one-photon states from a continuous-wave non-degenerate optical parametric oscillator. We obtain an analytical expression for the output state Wigner function, and we maximize the one-photon state fidelity by varying the temporal mode function of the output state. We show that a higher production rate of high fidelity Fock states is obtained if we condition the outcome on dark intervals around trigger photo detection events. 
Anne E. B. Nielsen and Klaus M{\o}lmer