4 PHOTONS SENSITIVITY HETERODYNE-DETECTION OF SUBMILLIMETER RADIATION WITH SUPERCONDUCTING TUNNEL-JUNCTIONS

An ultra low noise SIS receiver has been prepared for radio astronomy. The minimum double sideband receiver noise temperature is about 30 K which corresponds to 2 (h) over bar omega/k or an equivalent number of 4 photons of noise, The minimum measured double sideband mixer noise temperature is about 10 K which corresponds to 0.6 (h) over bar omega/k or approximately to 1 photon of noise. The fixed tuned SIS mixer operates in the 290-370 GHz frequency range. The submicron niobium junctions were fabricated using photoresist lines for junction definition. The receiver has been used at the 30 m IRAM radio telescope in Spain in winter 1994. Telescope system noise temperature of 500 K single sideband has been achieved when the sky opacity was about 0.17.