1:30000 cryogenic current comparator with optimum SQUID readout

We have developed a 1:30000 winding ratio cryogenic current comparator (CCC) for measurement of small currents, e.g., generated by a single electron transport device. A dedicated low-noise SQUID is used for the readout, which allows reaching a sensitivity close to ideal, that is without loss of sensitivity in the flux transformer coupling the CCC and the SQUID. The CCC-SQUID equivalent current input noise using a 1:30 000 winding ratio is 2.1 fA/Hz/sup 1/2/, measured down to 0.1 Hz. Room temperature impedance measurements on CCCs appear to be a useful method for the determination of the screening by shields surrounding the CCC as well as for the detection of internal CCC resonances.