Single-photon-counting hotspot detector with integrated RSFQ readout electronics

Absorption of an infrared photon in an ultrathin film (such as 10-nm NbN) c reates a localized nonequilibrium hotspot on the submicron length scale and sub-ns time scale. If a strip similar to 1 mu m wide is biased in the midd le of the superconducting transition, this hotspot will lead to a resistanc e pulse with amplitude proportional to the energy of the incident photon. T his resistance pulse, in turn, can be converted to a current pulse and indu ctively coupled to a SQUID amplifier with a digitized output, operating at 4 K or above. A preliminary design analysis indicates that this data can be processed on-chip, using ultrafast RSFQ digital circuits, to obtain a sens itive infrared detector for wavelengths up to 10 mu m and beyond, with band width of 1 GWz, that counts individual photons and measures their energy wi th 25 meV resolution. This proposed device combines the speed of a hot-elec tron bolometer with the single-photon-counting ability of a transition-edge microcalorimeter, to obtain an infrared detector with sensitivity, speed, and spectral selectivity that are unmatched by any alternative technology.