Photon-number resolving detector based on a series array of   superconducting nanowires
  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. 
Saeedeh Jahanmirinejad, Giulia Frucci, Francesco Mattioli, Dondu   Sahin, Alessandro Gaggero, Roberto Leoni and Andrea Fiore
physics.ins-det physics.optics quant-ph