Coupled tunnel junction experiments at the gap frequency

By using a capacitively coupled set of Josephson tunnel junctions we have performed experiments on the feasibility of using a Josephson junction as a local oscillator at frequencies up to and in excess of the gap sum of the junction. One junction was biased to serve as a local oscillator, whose AC power output could be monitored by observing the constant voltage Josephson steps on the current-voltage characteristic of the second junction. Using this coupling scheme we have coupled large oscillator signal levels (∼ 12.5nW) into the detector junction. For frequencies below the gap sum frequency our measurements are consistent with the Werthamer theory. For bias voltages above the gap sum we observe an anomalously fast roll off of the detected power from the oscillator junction.