Superconductor-insulator-superconductor quasiparticle junctions as microwave photon detectors

Abstract

The strong nonlinearity of the quasiparticle tunneling current in superconductor‐insulator‐superconductor junctions near the full‐gap voltage 2Δ/e can be used for direct detection of microwave signals. Measurements at 36 GHz yielded a current responsivity of 3500 A/W, which is within a factor of 2 of the quantum‐limited value e/∇ω. The measured N_EP of 2.6±0.8×10⁻¹⁶ W/,Hz^1/2 is the lowest value reported to date and can probably be improved significantly. The experimental results are compared with both the standard classical analysis and photon‐assisted tunneling theory.