Realization of quantum operations on photonic qubits by linear optics   and post-selection
  One of the greatest difficulties in the applications of single photon polarization states as qubits is the realization of controlled interactions between two photons. Recently, it has been shown that such interactions can be realized using only beam splitters and high efficiency photon detection by post-selecting a well defined part of the results in the output. We analyze these interactions and discuss schemes for qubit operations based on this mechanism. 
Holger F. Hofmann, Shigeki Takeuchi