Observation of nondissipative quasiparticle tunnel currents in superconducting tunnel junctions

We have made the first direct measurement of the quantum susceptance which arises from the nondissipative part of the quasiparticle tunneling in a superconductor-insulator-superconductor (SIS) junction. The resonant frequency of a resonator which contains an SIS junction depends on the dc bias due to the bias dependence of the quantum susceptance. The observed 19% shift of the resonant frequency, form 73 to 87 GHz, is in excellent agreement with the Werthamer-Tucker theory. This quantum susceptance should exist in other devices such as quantum-well structures.