27 ℏ SQUID amplifier operating with high-Q resonant input load

Abstract

We have extended to ultracryogenic temperatures the complete noise characterization of a low-noise two-stage superconducting quantum interference device (SQUID) amplifier developed for resonant gravitational wave detectors. The additive current noise is evaluated from open input measurements. To evaluate the back action voltage noise, the SQUID is strongly coupled to a high-Q macroscopic electrical resonator operating at 11.7 kHz. From these measurements, we estimate a minimum noise temperature of 15μK, corresponding to 27 times the quantum-limited noise temperature. Implications of this result for the sensitivity of resonant gravitational wave detectors are briefly discussed.

Strongly coupled, low noise dc-SQUID amplifiers

“…5.12 of about ≈ 0.3 K. This suggests that a coupled energy resolution of about 50h is possible. This is close to the optimum performance achieved for the original design [156].…”

“…The minimum equivalent input noise temperature is not needed for MiniGRAIL. The achievable values in the kHz frequency regime are in the order of µK [156], which is several orders of magnitude below the intended operation temperature of 20 mK. The more interesting properties for MiniGRAIL are the bandwidth and the stability.…”

“…Sensors based on this layout were investigated in detail, see for example references [27,156,164]. A coupled energy resolution of ε VV = 52h was reached for a sensor with an input inductance L IN of 1.6 µH [156]. For this design, also the minimum reachable noise temperature, represented by ε 0 , was determined.…”

“…According to the discussion in section 2.5.3, see page 37, this decreases problems with stability of the SQUID operated in FLL with a capacitive input circuit. Sensors based on this layout were investigated in detail, see for example references [27,156,164]. A coupled energy resolution of ε VV = 52h was reached for a sensor with an input inductance L IN of 1.6 µH [156].…”

“…For this design, also the minimum reachable noise temperature, represented by ε 0 , was determined. By coupling the sensor to a high quality factor capacitive input circuit, see section 2.5.3 on page 34, and measuring the back-action noise of the SQUID, ε 0 was estimated to 27h [156].…”