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A 200 ℏ two-stage dc SQUID amplifier for resonant gravitational wave detectors

Starting from commercial chips, a two-stage dc superconducting quantum interference device (SQUID) was developed in order to use it as a low-noise amplifier on the resonant gravitational wave detector AURIGA. The SQUID was coupled to a high-Q electrical resonator, operating in the kilohertz frequency range, which was employed to simulate the real detector. The resonator was successfully stabilized by means of a capacitive damping network. SQUID additive noise and back-action noise were measured as functions of temperature. The best noise temperature of the SQUID amplifier, measured at 1.5 K, was better than 16 μK, and corresponds to a minimum detectable energy of 200 resonator quanta.