The Microstrip SQUID Amplifier: Upgrading the Axion Dark Matter Experiment (ADMX)

The axion detector, now at the University of Washington, Seattle. requires a very low noise amplifier in the 1-GHz frequency range. In the first generation detector, the cavity was cooled to 1.5 K and used a HEMT (High Electron Mobility Transistor) amplifier with a noise temperature T$_N$ of 1.7 K. Thus, the system noise temperature T$_S$ was 3.2 K. To achieve significantly lower noise temperatures, we developed the Microstrip SQUID Amplifier (MSA) in which the input coil forms a microstrip with the SQUID washer. When the length of the coil corresponds to a half-wavelength of the signal, the gain is typically 20 dB. We measured the gain and noise of an MSA at 0.62 GHz, and achieved a minimum noise temperature T$_N = 48 \pm 5$ mK for a bath temperature of 50 mK and at a frequency slightly below resonance, as predicted. The quantum limit is 30 mK. Since the time for the axion detector to scan a given frequency range scales as T$_S^2$, replacing the HEMT with a SQUID and cooling the cavity to 50 mK potentially reduces the scan time by three orders of magnitude. In a proof-of-principle run, the system was operated at 1.7 K with an MSA readout, and performed as predicted. A total of 88,370, 80-second data sets were acquired, corresponding to 82 days of data acquisition.