Frequency conversion in a hydrogen-filled hollow-core fiber: power scaling, background, and bandwidth.

Large-area quantum networks based on optical fibers allow photons at near-infrared wavelengths to travel with minimal loss. Quantum frequency conversion is a method to alter the wavelength of a single photon while maintaining its quantum state. Most commonly, nonlinear crystals are employed for this conversion process, where near-unity conversion efficiency at high fidelity has been demonstrated. Still, the crystal-based conversion process is plagued by strong background noise, very limited spectral bandwidth, and inhomogeneous temperature profiles at strong pump fields. In the previous work, we have demonstrated frequency conversion in hydrogen-filled hollow-core fibers and claimed that this conversion process does not compromise performance at strong pump fields, is essentially free of background noise, and is intrinsically broadband. Here, we demonstrate that these three claims are justified: we demonstrate the quadratic scaling with pump field intensity, quantify the background level, and present coarse tuning over a range of 10 nm.