Mapping of the quantum efficiency of a superconducting single electron detector

Superconducting NbN wires have recently received attention as detectors for visible and infrared photons [1]. We present experiments in which we use a NbN wire for high-efficiency (?40%) detection of single electrons with keV energy. We use the beam of a scanning electron microscope as a focussed, stable, and calibrated electron source. Scanning the beam over the surface of the wire provides a map of the detection efficiency. This map shows features as small as 150 nm, revealing wire inhomogeneities. The intrinsic resolution of this mapping method, superior to optical methods, provides the basis of a characterization tool relevant for photon detectors.