Detection of single x-ray photons by an annular superconducting tunnel junction

We present an experiment that detects photons by use of an annular Nb-based superconducting tunnel junction (STJ). In one magnetic field configuration , we stably trapped a single magnetic fluxon in the STJ barrier during a tr ansition to the superconducting state. This is an innovative configuration which avoids the use of an externally applied field during detector operati on. It offers potential benefits for STJs used in imaging arrays. In this c onfiguration, and also in the conventional one with an externally applied p arallel magnetic field, we observed current pulses produced by single 6 keV x-ray photons. The pulses were the same for the two configurations. (C) 20 01 American Institute of Physics.