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 Internatio nal Temperature Scale (ITS-90), The system is composed of a thin-film resis tive-SQUID (R-SQUID) and a de 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 electr ical constraints demand that the readout amplifier to the R-SQUID have a lo w system noise. We used a de SQUID series array in a direct readout Bur-loc ked loop scheme to achieve a system bandwidth in excess of 1 MHz with a whi te noise level less than 1 mu Phi(0)/ root Hz.