The DC-SQUID amplifier for the NAUTILUS gravitational wave detector at CERN: preliminary measurements

NAUTILUS, a third generation Cryogenic Gravitational Radiation Detector, now being tested at CERN by the Rome group, uses a dc SQUID as low noise amplifier. For such a system we have developed a superconducting transformer to realize the noise matching between the SQUID and the gravitational detector, and an OFHC holder for the SQUID to be cooled at T ? 100 mK. Some preliminary measurements has been made on the SQUID connected to a small dilution refrigerator obtaining a flux noise of ?n=7?10-7?o/Hz--? at 0.2 K. Further measurements show that this noise is not due to the SQUID but to a noise source associated to the SQUID holder.