  We demonstrate quantum detector tomography of a commercial 2x2 array of
superconducting nanowire single photon detectors. We show that
detector-specific figures of merit including efficiency, dark-count and
cross-talk probabilities can be directly extracted, without recourse to the
underlying detector physics. These figures of merit are directly identified
from just four elements of the reconstructed positive operator valued measure
(POVM) of the device. We show that the values for efficiency and dark-count
probability extracted by detector tomography show excellent agreement with
independent measurements of these quantities, and we provide an intuitive
operational definition for cross-talk probability. Finally, we show that
parameters required for the reconstruction must be carefully chosen to avoid
oversmoothing the data.
