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

TL;DR

This paper presents a detailed noise analysis of a two-stage SQUID amplifier used in a high-Q gravitational wave transducer, highlighting the dominant noise sources and their impact on sensitivity.

Contribution

It introduces a high-Q gravitational wave transducer with a two-stage SQUID amplifier and provides a comprehensive noise characterization and analysis.

Findings

Total noise temperature of 1.1 mK, dominated by back-action noise.

Measured Q of the transducer mass is 2.60 x 10^6.

Voltage noise at the SQUID input significantly exceeds expectations.

Abstract

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 SQUID chip. The total noise includes thermal noise from the transducer mass, which has a measured Q of 2.60 X 10^6. 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.