A novel cryogenic voltage standard

The most recent and improved Josephson standards are obtained with cryogenic voltage comparators, i.e., by transferring the critical elements of the comparison circuit between the Josephson voltage and the reference voltage to the low temperature of the junction. A cryogenic voltage comparator not of the usual resistive types is presented, which is based on the conversion of the ratio between the two voltages to be compared into an equal ratio between two dc currents, measured with a superconducting dc ratio standard and related SQUID detector. This voltage comparator uses the irradiated Josephson junction as an ideal voltage generator and requires no calibration of resistance ratios. It looks very promising for realizing a cryogenic voltage standard that is simple in both construction and operation and with an uncertainty at 10⁻⁹ level. The preliminary implementations of the apparatus and the first results are presented.