  We demonstrate and verify quantum detector tomography of a superconducting
nanowire single-photon detector (SNSPD) in a multiplexing scheme which permits
measurement of up to 71000 photons per input pulse. We reconstruct the positive
operator valued measure (POVM) of this device in the low photon-number regime,
and use the extracted parameters to show the POVMs spanning the whole dynamic
range of the device. We verify this by finding the mean photon number of a
bright state. Our work shows that a reliable quantum description of large-scale
SNSPD devices is possible, and should be applicable to other multiplexing
configurations.
