The Microstrip SQUID Amplifier: Searching for the Axion

Abstract

The axion detector [1] at LLNL 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 the amplifier was a HEMT (High Electron Mobility Transistor) with a noise temperature TN of 1.7 K. Thus, the system noise temperature Ts was 3.2 K. In an attempt to achieve significantly lower noise temperatures, we fabricated amplifiers based on the dc SQUID [2] (Superconducting QUantum Interference Device). The theory [3] for SQUID amplifiers with a resonant input circuit predicts that an optimized device at sufficiently low temperatures should be quantum limited, that is, TN = hf/k at frequency f. To extend the operating frequency to the gigahertz range, we developed the Microstrip SQUID Amplifier (MSA) in which the input coil forms a microstrip with the SQUID washer [4, 5]. 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 [6] of an MSA in which the resistive shunts of the junctions were coupled to cooling fins to reduce hot electron effects [7]. At 0.62 GHz, we achieved a minimum noise temperature TN = 48±5mK for a bath temperature of 50 mK and at a frequency 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 Ts2, replacing the HEMT with a SQUID and cooling the cavity to 50 mK potentially reduces the scan time by three orders of magnitude.