Josephson Junctions

Josephson junctions (JJs) driven by a single-frequency microwave source have been established as quantum-accurate dc voltage standards [1]. Similar JJ devices are biased by patterns of fast pulses to create precise time-varying voltages [2], used for ac voltage standards and pseudo-random voltage noise references integrated into noise thermometry systems. Voltage standards typically use niobium-based JJs, but superconductors with higher-transition temperature, T_c, can be an attractive alternative. These high-T_c JJ circuits can have critical currents near 1 milliamp at temperatures between 60 K and 80 K and thus can use compact low-power cryocoolers.

Investigation of high-temperature JJs in quantum voltage devices at NIST has focused primarily on grain-boundary junctions in YBa₂Cu₃O_7-x. By driving these high-T_c junctions with pulse codes clocked at microwave frequencies we have demonstrated synthesis of programmable quantum-based voltage waveforms in a miniaturized cryocooler system. We’ll also discuss relevant uncertainties, sensitivity of the output to bias currents, and applicability of this waveform-synthesis system to Johnson-noise thermometry.

[1] C.A. Hamilton, Rev. Sci. Inst., 71, 3611, 2000.
[2] S.P. Benz, and C.A. Hamilton, Appl. Phys. Lett., 68, 3171, 1996.