Micrometer-Wide NbN Strips for Photon-Number-Resolving Detection

We report the development of photon-number-resolving superconducting single-photon detector with micron-scale $\mathrm{Nb}\mathrm{N}$ strips. The detector is designed as serial meander-shaped strips with resistors connected in parallel to each strip. We investigate the performance of the detectors comprising of 5--14 strips with the widths in the range $0.3$--$1\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{m}$ and covering an area as large as $70\ifmmode\times\else\texttimes\fi{}70\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}{\mathrm{m}}^{2}$ making it feasible for coupling to multimode fibers. We demonstrate resolution of up to six photons and analyze the sources of errors in photon-number resolution.