  Single Photon Detectors are integral to quantum optics and quantum
information. Superconducting Nanowire based detectors exhibit new levels of
performance, but have no accepted quantum optical model that is valid for
multiple input photons. By performing Detector Tomography, we improve the
recently proposed model [M.K. Akhlaghi and A.H. Majedi, IEEE Trans. Appl.
Supercond. 19, 361 (2009)] and also investigate the manner in which these
detectors respond nonlinearly to light, a valuable feature for some
applications. We develop a device independent model for Single Photon Detectors
that incorporates this nonlinearity.
