Impulse response characterization of a quantum frequency converter

We present an experimental method for characterizing the (Green) cross-transfer functions of quantum frequency conversion (QFC) systems, which provide a complete spectral characterization of the underlying parametric interaction. We demonstrate the approach on a kilometer-scale fiber Bragg-scattering four-wave-mixing (BS-FWM) converter, retrieving its transfer functions directly from classical probe measurements. The reconstructed responses reveal clear departures from theory that we attribute primarily to longitudinal dispersion variations in the physical medium. Incorporating such variations in a numerical model yields quantitative agreement with experiment and accurately predicts single-photon conversion efficiencies measured independently in another experiment. The outlined method provides a practical route to characterizing and optimizing complex QFC systems using only classical probes, offering predictive insight directly relevant to applications in the quantum regime.