Ultralow temperature SQUIDs for primary thermometry

We have designed and constructed a primary thermometry system for operation from 4.2 K to temperatures well below 0.65 K, the bottom of the International Temperature Scale (ITS-90). The system is composed of a thin-film resistive-SQUID (R-SQUID) and a dc SQUID readout amplifier fully integrated on a single chip. We have fabricated our SQUIDs using Ti/Pd/Au shunt resistors, which remain normal conductive down to mK temperatures. Thermal and electrical constraints demand that the readout amplifier to the R-SQUID has a low system noise. We used a dc SQUID series array in a direct readout flux-locked loop scheme to achieve a system bandwidth in excess of 1 MHz with a white noise level less than 1 /spl mu//spl Phi//sub 0///spl radic/Hz.