Statistically background-free, phase-preserving parametric up-conversion with faint light.

We demonstrate up-conversion with no statistically significant background photons and a dynamic range of 15 decades. Near-infrared 920 nm photons were converted into the visible at 577 nm using periodically poled lithium niobate waveguides pumped by a 1550 nm laser. In addition to achieving statistically noiseless frequency up-conversion, we report a high degree of phase preservation (with fringe visibilities ≥ 0.97) at the single-photon level using an up-converting Mach-Zehnder interferometer. This background-free process opens a path to single-photon detection with no intrinsic dark count. Combined with a demonstrated photon-number preserving property of an up-converter, this work demonstrates the feasibility of noiseless frequency up-conversion of entangled photon pairs.