Photon counting from visible to X-ray wavelengths with NbAl superconducting tunnel junctions

Niobium based superconducting tunnel junctions with 120 nm thick aluminium trapping layers have been used in photon counting experiments at visible and X-ray wavelengths. Due to the efficient trapping of quasiparticles in the Al layers and a highly transmissive tunnel barrier, signals as high as 2.3 × 108 electrons of detected charge at an X-ray photon energy of 5.9 keV are observed. The high signal levels together with the low noise levels of these devices have allowed the detection of signals from single photons in the UV/optical wavelength range 220–500 nm at an operating temperature of 370 mK. The typical resolution is 100 nm (FWHM) at λ = 300 nm, which is dominated by the electronic noise.