  We present the experimental demonstration of a superconducting photon number
resolving detector. It is based on the series connection of N superconducting
nanowires, each connected in parallel to an integrated resistor. The device
provides a single voltage readout, proportional to the number of photons
absorbed in distinct nanowires. Clearly separated output levels corresponding
to the detection of n=1-4 photons are observed in a 4-element detector
fabricated from an NbN film on GaAs substrate, with a single-photon system
quantum efficiency of 2.6% at the wavelength of 1300nm. The series-nanowire
structure is promising in view of its scalability to large photon numbers and
high efficiencies.
