Ultrasensitive hot-electron kinetic-inductance detectors operating well below the superconducting transition

While most experimental studies of kinetic-inductance sensors have been limited so far by the temperature range near the superconducting transition, kinetic-inductance detectors can be very sensitive at temperatures well below the transition, where the number of equilibrium quasiparticles is exponentially small. In this regime, a shift of the quasiparticle chemical potential under radiation results in the change of the kinetic inductance. We modeled the noise characteristics of the kinetic-inductance detectors made from disordered superconducting Nb, NbC, and MoRe films. Low-phonon transparency of the interface between the superconductor and the substrate causes substantial retrapping of phonons providing high quantum efficiency and the operating time of ∼1 ms at ≈1 K. Due to the small number of quasiparticles, the noise equivalent power of the detector determined by the quasiparticle generation-recombination noise can be as small as ∼10^-19W/ Hz at He₄ temperatures.