Superconducting nanosensors with mesoscopic number of quasiparticles

Novel approach to detection of low-energy (submillimeter and infrared) photons is based on implementation of the electron heating in superconducting nanostructures with small number of quasiparticles. In a hot-electron sensor, the incoming quanta generate nonequilibrium quasiparticles, which affect either the resistivity (transition-edge sensor) or the inductance (kinetic-inductance sensor operating in the superconducting state). The sensitivity of this sensor is limited by equilibrium fluctuations of the number of quasiparticle excitations, and a small number of quasiparticles is the key issue for high performance. The relaxation time in superconducting structures can be controlled over the range from 10 ps (outdiffusion of quasiparticles) to 0.1 s (phonon cooling). Therefore, hot-electron sensors can be employed as relatively slow ultra-sensitive detectors or fast photon counters, depending on a dominant cooling mechanism. The counter can resolve photons of submillimeter and terahertz ranges with the counting rate of 10¹¹ count/s. Hot-electron nanosensors are expected to deliver the unique performance: the noise equivalent power of 10^-20 W/√Hz and the energy resolution of 10^-21-10^-23 J.