Two-stage superconducting-quantum-interference-device amplifier in a high-Q gravitational wave transducer

We report on the total noise from an inductive motion transducer for a grav itational-wave antenna. The transducer uses a two-stage superconducting qua ntum interference device (SQUID) amplifier and has a noise temperature of 1 .1 mK, of which 0.70 mK is due to back action noise from the SQUID chip. Th e total noise includes thermal noise from the transducer mass, which has a measured Q of 2.60x10(6). The noise temperature exceeds the expected value of 3.5 mu K by a factor of 200, primarily due to voltage noise at the input of the SQUID. Noise from flux trapped on the chip is found to be the most likely cause. (C) 2000 American Institute of Physics. [S0003-6951(00)00211- 4].