  We demonstrate a superconducting photon-number-resolving detector capable of
resolving up to twelve photons at telecommunication wavelengths. It is based on
a series array of twelve superconducting NbN nanowire elements, each connected
in parallel with an integrated resistor. The photon-induced voltage signals
from the twelve elements are summed up into a single readout pulse with a
height proportional to the detected photon number. Thirteen distinct output
levels corresponding to the detection of n=0-12 photons are observed
experimentally. A detailed analysis of the excess noise shows the potential of
scaling to an even larger dynamic range.
