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 gravitational-wave antenna. The transducer uses a two-stage SQUID amplifier and has a noise temperature of 1.1 mK, of which 0.70 mK is due to back-action noise from the superconducting quantum interference device (SQUID) chip. The total noise includes thermal noise from the transducer mass, which has a measured Q of 2.60× 106. The noise temperature exceeds the expected value of 3.5 μK 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. Typeset using REVTEX a)Present address: Department of Physics, Syracuse University, Syracuse, New York 13244-1130 b)Present address: Department of Applied Physics, Yale University, PO Box 208284, New Haven, Connecticut 06520-8284