  Superconducting nanostrip photon detectors have been used as single photon
detectors, which can discriminate only photons' presence or absence. It has
recently been found that they can discriminate the number of photons by
analyzing the output signal waveform, and they are expected to be used in
various fields, especially in optical quantum information processing. Here, we
improve the photon-number-resolving performance for light with a high-average
photon number by pattern matching of the output signal waveform. Furthermore,
we estimate the positive-operator-valued measure of the detector by a quantum
detector tomography. The result shows that the device has
photon-number-resolving performance up to five photons without any multiplexing
or arraying, indicating that it is useful as a photon-number-resolving
detector.
