  Since their first demonstration in 2001, superconducting-nanowire
single-photon detectors have witnessed two decades of great developments.
SNSPDs are the detector of choice in most modern quantum optics experiments and
are slowly finding their way into other photon starved fields of optics. Until
now, however, in nearly all experiments SNSPDs were used as binary detectors,
meaning they can only distinguish between 0 and more than 1 photons and photon
number information is lost. Recent research works have demonstrated proof of
principle photon number resolving (PNR) SNSPDs counting 2 to 5 photons. The
photon-number-resolving capability is highly demanded in various quantum-optics
experiments, including HOM interference, photonic quantum computing, quantum
communication, and non Gaussian quantum state preparation. In particular, PNR
detectors at the wavelength range of 850 to 950 nm are of great interest due to
the availability of high quality semiconductor quantum dots and
high-performance Cesium-based quantum memories. In this paper, we demonstrate
NbTiN based SNSPDs with over 94 percent system detection efficiency, sub 11 ps
timing jitter for one photon, and sub 7 ps for two photon. More importantly,
our detectors resolve up to 7 photons using conventional cryogenic electric
readout circuitry. Through theoretical analysis, we show that the current PNR
performance of our detectors can still be further improved by improving the
signal to noise ratio and bandwidth of our readout circuitry. Our results are
promising for the future of optical quantum computing and quantum
communication.
