Thirteenth International Symposium on Space Terahertz Technology, Harvard University, March 2002. Quantum-limited Mixing in a Transformer-coupled SIS Resonator for the 600 GHz Frequency Band Cheuk-yu Edward Tong, Raymond Blundell Harvard-Smithsonian center for Astrophysics, 60 Garden Street, Cambridge, MA 02138. Krikor G Megerian, Jeffrey A. Stern, Henry G LeDuc Jet Propulsion Lab, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109. Abstract Quantum-limited mixer noise temperature has been achieved in Superconductor-Insulator-Superconductor (SIS) resonant mixer in the 600 — 720 GHz frequency range. Our mixer employs a single full-wave Nb/A1N/Nb tunnel junction resonator, fed by a quarter-wave transformer. The devices have low critical current density (-5 kA/cm 2 ). The mixers were tested in a fixed-tuned waveguide mixer mount. Double-side-band receiver noise temperatures equivalent to a few quanta have been measured for a number of different devices. Using a 0.55 x 25 Inn resonator, a noise temperature of 141 K was recorded at an LO frequency of 700 GHz with the mixer at 4.2 K. The noise temperature dropped to 111 K when the helium bath was pumped down to 2.8 K. High sensitivity has attained over reasonably wide RF bandwidth, —17%. The IF bandwidth of these mixers has also been investigated. I. Introduction The introduction of distributed mixing in Superconductor-Insulator-Superconductor (SIS) junctions is an important development in low-noise receiver technology for sub-millimeter wavelengths [1,2]. The earliest distributed mixers employed fairly long superconducting tunnel junctions, about 2 wavelengths long. The long junction acts as a lossy transmission line in which mixing occurs along the whole length of the line. This class of receiver has demonstrated sensitivities close to the quantum limit. A noise temperature of