Characterizing the temporal response function of single-photon detectors (SPDs) is essential for quantum communication protocols and time-resolved measurements. Typically, this characterization is obtained from the arrival time statistics of photons from a pulsed laser. In this work, we present an alternative approach using time-correlated photon pairs generated in spontaneous parametric down-conversion (SPDC). We demonstrate a continuous wavelength-tunability from 526 nm to 661 nm for one photon of the pair, and 1050 nm to 1760 nm for the other photon a range comparable to existing pulsed-laser systems. With this source, we characterized single-photon avalanche detectors sensitive to the two distinct wavelength bands, one based on Silicon, the other based on Indium Gallium Arsenide.
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