  Single-photon detectors and nanoscale superconducting devices are two major
candidates for realizing quantum technologies. Superconducting-nanowire
single-photon detectors (SNSPDs) comprise these two solid-state and optic
aspects enabling high-rate (1.3 GBit s-1) quantum key distribution over long
distances (>400 km), long-range (>1200 km) quantum communication as well as
space communication (239,000 miles). The attractiveness of SNSPDs stems from
competitive performance in the four single-photon relevant characteristics at
wavelengths ranges from UV to the mid IR: high detection efficiency, low
false-signal rate, low uncertainty in photon time arrival and fast reset time.
However, to-date, these characteristics cannot be optimized simultaneously. In
this review, we present the mechanisms that govern these four characteristics
and demonstrate how they are affected by material properties and device design
as well as by the operating conditions, allowing aware optimization of SNSPDs.
Based on the evolution in the existing literature and state-of-the-art, we
propose how to choose or design the material and device for optimizing SNSPD
performance, while we also highlight possible future opportunities in the SNSPD
technology.
