Measured flatness of microwave-induced steps at Josephson tunnel junctions with near-zero-current bias

In voltage standards that use series-connected hysteretic Josephson tunnel junctions with zero-current bias, the working point on a microwave induced step is not the middle of the step. Towards the ends of a step, theories predict that noise will produce appreciable deviations from the well-defined DC voltage. Measurements of these deviations on the zeroth and first step are presented. The voltage across the Josephson junction was compared with the voltage drop produced across a cryogenic resistor by passing a SQUID-stabilized current through it. The voltage difference, down to the picovolt region, was detected by a second SQUID. Experimental results are compared with theoretically calculated values. With the equivalent noise temperature, external as well as intrinsic noise can be taken into account when designing a voltage standard with zero-current-biased Josephson junctions.