A major milestone has been reached in electrical metrology with the successful completion of the first key comparison of Josephson voltage standards for AC voltage, carried out between the Bureau International des Poids et Mesures (BIPM) and the Physikalisch-Technische Bundesanstalt (PTB).

The comparison, completed in 2025, covered AC voltage amplitudes of 0.75 V and 7 V across a frequency range from 62.5 Hz to 1.25 kHz, demonstrating agreement between the two systems at the level of a few tens of nanovolts per volt.

This comparison represents an important step in extending the international measurement infrastructure for electrical quantities, supporting traceability and equivalence for AC voltage at the highest level. It strengthens confidence that AC voltage measurements realized in different countries are consistent, comparable and anchored to the same quantum-defined reference.

Photo caption: Stéphane Solve and Régis Chayramy (BIPM) preparing and operating the measurement setup used for the key comparison of Josephson standards for AC voltage.

 

How the comparison was carried out

The comparison was carried out using a stable AC voltage source as a transfer standard, which was alternatively measured by the PTB and BIPM AC quantum voltage standards. The AC quantum voltage standards generated a stepwise-approximated sinewave of the same frequency and the same nominal root-mean-square value as the transfer standard. The small voltage differences between the transfer standard and the AC quantum voltmeters were measured with a synchronized sampler. The measured voltage differences together with the predictable quantized voltages allowed us to determine the root-mean-square value of the signal from the transfer standard.

The results: Across both voltage amplitudes and nearly all frequencies, the two systems showed excellent agreement, with relative differences at the level of a few tens of nanovolts per volt, confirming the robustness and maturity of quantum-based AC voltage measurements.

This confirms that Josephson-based techniques can deliver highly reliable and reproducible AC voltage measurements, suitable for key comparison purposes.

The full comparison results have been published in Metrologia and can be accessed here.

Background

Since the early 1990s, the BIPM has coordinated an on-site, on-going key comparisons programme of Josephson voltage standards for DC voltage, supporting global equivalence in electrical measurements through comparisons such as BIPM.EM-K10. As the international use of quantum measurement technology has expanded into AC applications, the need for equivalent comparison capabilities has grown.

In response, the Consultative Committee for Electricity and Magnetism (CCEM) tasked the BIPM with extending the key comparison BIPM.EM-K10 to AC voltage. Over the past seven years, the BIPM — with significant technical collaboration from PTB, NIST and KRISS — has carried out pilot studies to develop suitable measurement methods, assessed AC signal generators, and established a robust comparison protocol. This protocol was formally approved by the CCEM in 2023.

Photo caption: From left to right: Luis Palafox (PTB), Stéphane Solve and Régis Chayramy (BIPM) and Ralf Behr (PTB) in the voltage laboratory at the PTB (Braunschweig) during the bilateral AC Josephson voltage key comparison.