High precision measurement of a possible resistive slope of Josephson array voltage steps

A high-precision intercomparison between two Josephson array voltage standards has been used to determine an upper limit to the resistive slope of 70-GHz microwave-induced constant-voltage steps. Over the 20 µa width of a quantized step near 1 V, the variation of step voltage is, in the most sensitive case, less than 7 × 10 -13 V.