Optical-to-electrical conversion, which is the basis of the operation of optical detectors, can be linear or nonlinear. When high sensitivities are needed, single-photon detectors are used, which operate in a strongly nonlinear mode, their response being independent of the number of detected photons. However, photon-number-resolving detectors are needed, particularly in quantum optics, where n-photon states are routinely produced. In quantum communication and quantum information processing, the photon-numberresolving functionality is key to many protocols, such as the implementation of quantum repeaters 1 and linear-optics quantum computing 2 . A linear detector with single-photon sensitivity can also be used for measuring a temporal waveform at extremely low light levels, such as in longdistance optical communications, fluorescence spectroscopy and optical time-domain reflectometry. We demonstrate here a photon-number-resolving detector based on parallel superconducting nanowires and capable of counting up to four photons at telecommunication wavelengths, with an ultralow dark count rate and high counting frequency.Among the approaches proposed so far for photon-numberresolving (PNR) detection (Table 1) are detectors based on charge integration or field-effect transistors 3-5 , which are, however, affected by long integration times, leading to bandwidths of ,1 MHz. Transition edge sensors 6 operate at 100 mK and show long response times (several microseconds). Approaches based on photomultipliers 7 and avalanche photodiodes, such as the visiblelight photon counter 8,9 , two-dimensional arrays of avalanche photodiodes 10,11 and time-multiplexed detectors 12,13 are not sensitive or are plagued by high dark count rates (DKs) and long dead times in the telecommunication spectral windows. Arrays of single-photon detectors (SPDs) also involve complex readout schemes 11 or separate contacts, amplification and discrimination
14. The parallel nanowire detector (PND) presented here significantly outperforms these approaches in terms of simplicity, sensitivity, speed and multiplication noise.The basic structure of the PND comprises the parallel connection of N superconducting nanowires, each connected in series to a resistor R 0 (Fig.