Josephson impedance bridges as universal impedance comparators

Josephson impedance bridges have been reported to provide uncertainties close to that from the best conventional bridges when comparing two 10-kΩ resistors. The accuracy of a few parts in 10⁸ has been extended to the comparison of two 100-pF capacitance standards, significantly improving the uncertainty for these measurements at power line frequencies. Preliminary measurements of Josephson quadrature bridges are also presented with uncertainties at the level of 1.6 × 10⁻⁶ (k = 1).