RF Applications of Superconducting Tunneling Devices

In recent years a lot of interest has been focused on the Josephson junction as a microwave device in the millimeter and submillimeter range. The primary reason for this interest is that it has been verified experimentally that the Josephson junction makes the receiver with the lowest noise temperature in a large part of the microwave spectrum. Already now radio astronomy receivers with a Josephson junction as the sensitive element have been tested sucessfully and total receiver noise temperatures as low as 150 K have been realized in the 40-120 GHz range. Although the interest is now focused largely on the so-called SIS mixer,the Josephson junction being a highly nonlinear device may be used also as a parametric amplifier. For both mixers and parametric amplifiers, modes of operation exist, in which the pump (or local oscillator) is inherent in the junction; alternatively, another Josephson junction may be used as a microwave generator. That unique feature becomes attractive in the higher end of the microwave spectrum where sources are typically bulky and expensive. The development of practically usable Josephson microwave elements is still being pursued actively around the world. A noticeable trend now is that this research is increasingly being transferred from the physics laboratories to the microwave community; the progress that has taken place has also benefitted much from large research projects aimed at developping a new generation of computers based on Josephson junctions. Several good recent reviews of the field existsl12'3'4'5'6 and may serve as an introduction for those who wish more information.