Noise measurements on a tunnel junction dc SQUID for a gravitational radiation detector at ultralow temperature

We present noise measurements made at temperatures down to 200 mK on a tunnel junction d.c. SQUID, to be used as amplifier for the gravitational-wave experiment Nautilus of the Rome group. We find that the flux noise decreases with temperature, as expected, with a best measured value of ?n?7?10-7?0/Hz---? at 0.2 K. The presence of an excess noise term independent of temperature is discussed.