extending the spectral range and scalability of SNSPDs to large areas. High temperature superconductors have been investigated for SNSPDs with some encouraging recent reports. SNSPDs have been created from magnesium diboride with some single photon sensitivity in the visible and near infrared. There is considerable interest and effort in scaling up SNSPDs to large multipixel arrays and cameras. A kilopixel SNSPD array has recently been reported. A key challenge is readout, which can be addressed via multiplexing or digital readout using superconducting single flux quantum logic. Applications Many of the initial application demonstrations of SNSPDs have been in the area of quantum information, such as quantum key distribution and optical quantum computing. Other current and emerging applications include imaging of infrared photoemission for defect analysis in CMOS circuitry, single photon emitter characterization, LIDAR, on-chip quantum optics, optical neuromorphic computing, fibre optic temperature sensing, optical time domain reflectometry, readout for ion trap qubits, quantum plasmonics, single electron detection, single α and β particle detection, singlet oxygen luminescence detection, deep space optical communication, dark matter searches and exoplanet detection. A number of companies worldwide are successfully commercializing complete single-photon detection systems based on superconducting nanowires, including Single Quantum, Photon Spot, Scontel, Quantum Opus, ID Quantique, PhoTec and Pixel Photonics. Wider adoption of SNSPD technology is closely linked to advances in cryocoolers for 4 K and below, and SNSPDs have recently been demonstrated in miniaturized systems. References Particle detectors Photodetectors Radiometry Sensors Superconducting detectors Quantum optics Superconductivity Optoelectronics Photonics Optical metrology Engineering