  For photon-counting applications at ultraviolet wavelengths, there are
currently no detectors that combine high efficiency (> 50%), sub-nanosecond
timing resolution, and sub-Hz dark count rates. Superconducting nanowire
single-photon detectors (SNSPDs) have seen success over the past decade for
photon-counting applications in the near-infrared, but little work has been
done to optimize SNSPDs for wavelengths below 400 nm. Here, we describe the
design, fabrication, and characterization of UV SNSPDs operating at wavelengths
between 250 and 370 nm. The detectors have active areas up to 56 ${\mu}$m in
diameter, 70 - 80% efficiency, timing resolution down to 60 ps FWHM, blindness
to visible and infrared photons, and dark count rates of ~ 0.25 counts/hr for a
56 ${\mu}$m diameter pixel. By using the amorphous superconductor MoSi, these
UV SNSPDs are also able to operate at temperatures up to 4.2 K. These
performance metrics make UV SNSPDs ideal for applications in trapped-ion
quantum information processing, lidar studies of the upper atmosphere, UV
fluorescent-lifetime imaging microscopy, and photon-starved UV astronomy.
