Efficiency limits for linear optical processing of single photons and   single-rail qubits
  We analyze the problem of increasing the efficiency of single-photon sources or single-rail photonic qubits via linear optical processing and destructive conditional measurements. In contrast to previous work we allow for the use of coherent states and do not limit to photon-counting measurements. We conjecture that it is not possible to increase the efficiency, prove this conjecture for several important special cases, and provide extensive numerical results for the general case. 
Dominic W. Berry, A. I. Lvovsky, Barry C. Sanders