Efficient single-photon frequency upconversion at 1.06 μm with ultralow background counts

We demonstrated an efficient single-photon frequency upconversion system for the infrared photons at 1.06 μm with ultralow background counts. By pumping the system at a longer wavelength than that of the single-photon signal, the background counts caused by the pump-induced parametric fluorescence in the nonlinear crystal were reduced markedly. We achieved so far the lowest noise of ∼150/s for a near unity conversion efficiency.