Realizations and comparisons of Josephson array voltage standards

This paper reviews implementations of reference standards of voltage based on arrays of Josephson junctions and efforts to compare standards of different laboratories. Estimates of total uncertainty in laboratory realizations range from a few parts in 10⁷ to a few parts in 10¹⁰. The higher values of uncertainty are mainly due to limitations of secondary standards. These limitations are removed when standards are compared using a transportable Josephson array voltage standard, as demonstrated by the comparisons carried out by the BIPM. The comparison results show that the Josephson voltage standards of nine national metrology laboratories agree to better than 1 part in 10⁹