Low noise Nb-SIS mixers at far above the gap frequency

There are great interests in developing Nb SIS mixers because of the extremely low noise temperatures and because of the need of low local oscillator (LO) power. Several groups have demonstrated experimentally that Nb SIS mixers with integrated tuning elements can perform near the quantum noise limit up to the Nb gap frequency (~700 GHz). Beyond this frequency, a major loss will take place in the tuning elements, which are Nb-SiO₂-Nb micro-striplines, because the incoming photons can break the Cooper-pairs, thus increasing the surface resistance of the Nb strips. Applying highly conductive Al instead of Nb for a stripline can reduce the loss since the former has a lower surface resistance at higher frequencies. In this work, we report the fabrication of waveguide Nb SIS mixers with Al striplines and heterodyne measurement results around 1 THz