Cryogenic current comparators with optimum SQUID readout for current and resistance quantum metrology

This thesis describes the development of several systems based on the Cryogenic Current Comparator with optimum SQUID readout, for current and resistance metrology applications. the CCC-SQUID is at present the most accurate current comparator available. A (type I) CCC consists basically of a superconducting tube containing two sets of windings N1 and N2, carrying the two currents I1 and I2 to be compared. As a result of the Meissner effect, a screening current equal to (I2N2-I1N1) circulates in the tube (overlapped “like a snake swallowing its own tail” to shield the windings from the sensing circuit). The flux associated to the unbalance current is coupled to a SQUID, the most sensitive flux sensor available nowadays. The SQUID is basically a superconducting ring interrupted by one (rf SQUID) or two (dc SQUID) weak links called Josephson junctions. The device works basically as a flux-to-voltage converter.