Improved energy resolution of a superconducting single-photon detector

Abstract

We demonstrated successful operation of an NbN single photon detector in the temperature range from 6 K to 1.2 K using a ³He sorption refrigerator combined with a pulse-tube mechanical cooler. The detector was read out either by microwave amplifiers or by a broadband SQUID-amplifier that limited the maximum counting rate to 107 counts per second. This counting rate was only one third of the maximum rate provided by the detector. Besides an increase in the quantum efficiency in the visible and near-infrared spectral range with the decrease of the operation temperature, we found a more than twofold improvement in the energy resolution as compared to earlier demonstrated 1 eV at 6.5 K. The noise equivalent power estimated at 4.2 K for visible light was better than 10^-18 W Hz^1/2. We verified that the lowest achieved dark count rate was still caused by the harsh electrical conditions in the mechanical cooler.