Quasiparticle mixing close to the gap frequency in aluminum tunnel junctions

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