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.