Heralded single-photon generation using imperfect single-photon sources   and a two-photon-absorbing medium
  We propose a setup for a heralded, i.e. announced generation of a pure single-photon state given two imperfect sources whose outputs are represented by mixtures of the single-photon Fock state $\ket{1}$ with the vacuum $\ket{0}$. Our purification scheme uses beam splitters, photodetection and a two-photon-absorbing medium. The admixture of the vacuum is fully eliminated. We discuss two potential realizations of the scheme. 
Thomas Konrad, Artur Scherer, Michael Nock, Juergen Audretsch