We present an experimental method to characterize multi-photon detectors with a small overall detection efficiency. We do this by separating the nonlinear action of the multiphoton detection event from linear losses in the detector. Such a characterization is a necessary step for quantum information protocols with single and multiphoton detectors and can provide quantitative information to understand the underlying physics of a given detector. This characterization is applied to a superconducting multiphoton nanodetector, consisting of an NbN nanowire with a bowtie-shaped subwavelength constriction. Depending on the bias current, this detector has regimes with single and multiphoton sensitivity. We present the first full experimental characterization of such a detector.
Renema, J. J., Frucci, G., Zhou, Z., Mattioli, F., Gaggero, A., Leoni, R., Dood, de, M. J. A., Fiore, A., & Exter, van, M. P. (2012). Modified detector tomography technique applied to a superconducting multiphoton nanodetector. Optics Express, 20(3), 2806-2813. https://doi.org/10.1364/OE.20.002806