Dc superconducting quantum interference device amplifier for gravitationalwave detectors with a true noise temperature of 16 mu K

We report on the noise characterization of a two-stage dc superconducting q uantum interference device (SQUID) amplifier developed for resonant gravita tional wave detectors. The back action noise is estimated by coupling the S QUID to an electrical resonator at 1.6 kHz with Q=1.1x10(6). From measureme nts of back action and additive SQUID noise, performed in the temperature r ange 1.5-4.2 K, an upper limit is set on the noise temperature T-n of the d evice at the resonator frequency. The best value obtained at 1.5 K is T(n)l ess than or equal to 16 muK and corresponds to 200 resonator quanta. The th ermal component of the noise temperature is found in reasonable agreement w ith the predicted value. (C) 2001 American Institute of Physics.