Recent advancements and perspectives of INRiM’s time-dependent Josephson voltage standards

Quantum voltage standards based on ac Josephson effect have been used in metrology since a few years after the discovery of the physical phenomenon. The role of quantum standards is now crucial following the SI redefinition in 2019: electrical units are today defined in terms of the fundamental constants e (elementary charge) and h (Planck’s constant). The extremely low uncertainty in dc measurements, that can be lower than 1 nV/V at 10 V, is stimulating research to extend application to ac and signals arbitrarily changing with time. Approaching the dc accuracy is challenging, however. The two main technologies used for the generation of non-steady voltage signals are programmable and pulsed Josephson junction arrays. In this work we discuss the main advancements obtained at the Italian Istituto Nazionale di Ricerca Metrologica (INRiM) with both technologies, and the most recent developments taking advantage of He-free device cooling techniques.