title: Linear-optical processing cannot increase photon efficiency.
abstract: 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 available photon source. If multiphoton components are allowed, the single-photon fidelity cannot be increased beyond 1/2. We propose a natural general definition of the quantum-optical state efficiency, and show that it cannot increase under linear-optical processing.
authors.affiliation: Institute for Quantum Computing, University of Waterloo, Ontario N2L 3G1, Canada.
authors.forename: D W
authors.initials: DW
authors.lastname: Berry
authors.forename: A I
authors.initials: AI
authors.lastname: Lvovsky