Linear-optical processing cannot increase photon efficiency.

@article{Berry2010LinearopticalPC,
  title={Linear-optical processing cannot increase photon efficiency.},
  author={Dominic W. Berry and A. I. Lvovsky},
  journal={Physical review letters},
  year={2010},
  volume={105 20},
  pages={
          203601
        }
}
We answer the question whether linear-optical processing of the states produced by one or multiple imperfect single-photon sources can improve the single-photon fidelity. This processing can include arbitrary interferometers, coherent states, feedforward, and conditioning on results of detections. We show that without introducing multiphoton components, the single-photon fraction in any of the single-mode states resulting from such processing cannot be made to exceed the efficiency of the best… 

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