Niobium nitride devices for SIS mixers

The superconductor-insulator-superconductor (SIS) tunnel junction is an attractive candidate for millimeter wave mixers offering conversion gain and quantum limited noise.
¹
SIS mixers have been operated at 36 GHz with a single sideband noise temperature of 3.7K which is within a factor of 2 of quantum limited performance.
²
These results were obtained with devices fabricated from lead alloy technology. While this technology is very useful for the demonstration of quantum limited noise an all refractory junction technology would be desirable as it would have the advantages of being mechanically rugged, cycle well between room temperature and 4K be resistant to radiation damage and have enhanced high frequency response.