We present a study of the microwave characteristics of Josephson junctions based on a superconductor–insulator–normal–insulator–superconductor sandwich, fabricated in Nb/Al/AlOx technology. With the nonhysteretic Shapiro steps and the small parameter spread observed, the junctions are suitable for programmable Josephson voltage standards. Their characteristic voltage Vc≈100 μV enables operation at microwave frequencies up to 100 GHz.

The discovery of the Josephson effects 50 years ago initiated a revolution for electrical voltage metrology. This revolution started with single Josephson junctions delivering a few millivolt at most. Meanwhile, highly integrated series arrays containing more than 10 000 or even 300 000 junctions have been developed and fabricated for output voltages up to 10 V. Josephson voltage standards are nowadays used in many laboratories worldwide for dc applications. After their adoption for the representation of the unit of voltage in 1990, the focus of research shifted towards programmable Josephson series arrays. The increasing interest in highly precise ac voltages resulted in new developments for their metrological applications in the last 15 years. This paper summarizes the principal contributions from PTB to the present state of Josephson voltage standards with particular focus on developments and applications for ac standards in metrology and our proof-of-concept demonstrations. The presentation includes the two most promising versions of ac standards, being the programmable Josephson voltage standard based on binary divided series arrays and the Josephson arbitrary waveform synthesizer based on a pulse-driven series array.

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