Superconductive devices for millimeter wave detection mixing and amplification

Single particle (quasiparticle) tunneling through an insulating barrier between two superconductors or between a superconductor and a normal conductor is used to make very low noise detectors and mixers for millimeter wavelengths. The nonlinearity of the I vs. V curve obtained from tunneling between two superconductors can be so strong that classical theory breaks down and photon assisted tunneling theory must be used to understand device performance. Quantum theory predicts that a quasiparticle tunnel junction can be operated as a microwave photon detector with quantum efficiency close to unity or as a heterodyne mixer with conversion gain and with mixer noise temperature comparable with the quantum noise limit. Both of these predictions are experimentally realized using superconductor insulator superconductor junctions.