B. Korzh

Given the fast progress made in improvement of superconducting nanowire single photon detectors (SNSPD) in the last decade, renewed effort has focused on understanding the fundamental limits of the detection process. A wide range of experiments have been performed across many groups to push beyond the state of the art in individual metrics, such as dark count rate, efficiency, photon number resolution, long wavelength sensitivity and timing resolution. These results have triggered significant improvements in understanding of the detection mechanism, however, gaps between experiment and theory still remain and leaves the door open for further investigations. Timing resolution in particular can be as good as 2.6±0.2 ps for visible wavelengths and 4.3±0.2 ps at 1550 nm. This has an impact on many applications including classical and quantum communication, higher spatial resolution in laser ranging with fewer photons, observation of shorter-lived fluorophores in biomedical applications and fast optical waveform capture. Encouragingly, it is not believed that fundamental limits have been reached yet. The limits of photon energy cut-off are also an ongoing topic of study and single photon response has been observed out to 9.9 μm which is promising for a number of applications, including exoplanet transient spectroscopy.