Quasiparticle mixing close to the gap frequency in aluminum tunnel junctions

The conversion efficiency of quasi-particle mixers was tested at frequencies close to the superconducting gap frequency. Arrays of aluminum SIS and SIN tunnel junctions at frequencies around 75 GHz and temperatures down to 0.3 K were investigated. This frequency corresponds to about 80 percent of the gap frequency. The experimental conversion loss was large due to severe impedance mismatches. However, a good agreement (within 1 dB) to the quantum theory of mixing was obtained, indicating that nonequilibrium effects do not play a significant role for these low current density junctions.