Coherent quantum frequency bridge: phase preserving, nearly-noiseless parametric frequency converter

We characterize an efficient and nearly-noiseless parametric frequency upconverter. The ultra- low noise regime is reached by the wide spectral separation between the input and pump frequencies and the low pump frequency relative to the input photons. The background of only approximate to 100 photons per hour is demonstrated. We demonstrate phase preservation in a frequency upconversion process at the single-photon level. We summarize our efforts to measure this ultra-low noise level, and discuss both single-photon avalanche photodiode measurements and a photon-counting transition edge sensor (TES) measurements. To reach the required accuracy, we supplemented our TES with a dark count reduction algorithm. The preservation of the coherence was demonstrated by simultaneously upconverting the input of each arm of a Mach-Zehnder interferometer through high interference fringe contrast. We observe fringe visibilities of >= 0.97 with faint coherent input.