High-efficiency frequency conversion in the single-photon regime

In this Letter we demonstrate frequency conversion in the single-photon regime through Bragg-scattering four-wave mixing with near-unit efficiency in a 750 m long commercially available dispersion-engineered highly nonlinear fiber, where all photons and pump laser frequencies are in the low-loss telecommunications band. We achieve 99.1% ± 4.9% downconversion and 98.0% ± 5.0% upconversion of photons by 12 nm using a weak coherent state with an average input of 0.27 photons per detection gate window.