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We have performed a variety of precision measurements by comparing ac and dc waveforms generated by two independent ac programmable Josephson voltage standard (ACPJVS) systems. The objective of these experiments was to demonstrate the effectiveness of using a sampling digital voltmeter to measure small differences between Josephson waveforms for frequencies up to 3.6 kHz. The low uncertainties that we obtained confirm the feasibility of using this differential sampling method for high accuracy comparisons between ACPJVS waveforms and signals from other sources.

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For decades, the quantum behavior of Josephson junctions has been employed as intrinsic standards for voltage metrology. Conventional dc Josephson voltage standards have been the primary standards for voltage, programmable Josephson voltage standards have been implemented in calibration services and precision measurements, such as the Planck constant, and Josephson arbitrary waveform synthesizers have been employed in ac voltage calibrations and precision measurements of the Boltzmann constant. With the anticipated redefinition of the Système International d'Unités, all types of Josephson voltage standards will become intrinsic standards and equivalent realizations of the unit volt. Here we review the stateof-the art performance, best practices, and current impact of these systems for various applications, with an emphasis on ac voltage metrology. We explain the limitations of each system, especially regarding the many potential systematic errors that affect their accuracy and performance for specific applications.