ABSTRACT
Josephson voltage standards are nowadays well established as an important application of the Josephson effects for voltage measurements at ultimate precision. This chapter describes the development of Josephson voltage standards from conventional DC to modern AC standards focused on refractory Josephson junctions based on the Nb/Al-Al2O3 technology and its enhancements for Josephson voltage standards. The development of conventional DC Josephson voltage standards for output voltages up to 10 V was performed over a period of about 25 years. AC Josephson voltage standards are therefore based on overdamped Josephson junctions showing a nonhysteretic current-voltage characteristic, which remains singlevalued under microwave irradiation. The demands for overdamped Josephson junctions used in AC voltage standards partly differ from that for underdamped junctions used in conventional standards. Pulse-driven and binary-divided series arrays containing up to 300,000 junctions have been demonstrated for output voltages up to 1 V and 10 V or even 20 V, respectively.
