Abstract

A two-stage SQUID (Superconducting QUantum Interference Device) amplifier based on a modified commercial sensor is operated strongly coupled to an electrical resonator which simulates, as regards resonance frequency (≅1 kHz) and quality factor (≅106), the load of one of the mechanical modes of a resonant cryogenic gravitational wave detector. The behavior of the system is stable and, in the temperature range 1.5–4.2 K, the noise of the resonator is in agreement with the equipartition theorem. Thanks to the very high quality factor it is possible to evaluate the back action noise of the SQUID amplifier which is found to be thermal and in agreement with the theory. The open input broadband noise is also thermal and in agreement with the theory down to 300 mK and levels off to a constant value which corresponds to 35 h at lower temperatures. The noise temperature of the SQUID amplifier, obtained from the broadband and back action noise measurements, is 15 μK at 1.5 K. On the basis of these results the operation of a resonant gravitational wave detector with an amplifier with an energy sensitivity better than 100 h seems achievable.

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