Josephson Junctions: Fabrication and Applications for the Axion Dark Matter eXperiment

The observation of axions could revolutionize the world of physics. Through microwave frequency cavity readout of the photons associated with these axions, the ADMX project at WashU utilizes multiple different forms of the Josephson junction (JJ), a superconductive circuit element. The physics behind the JJ are essential to understanding its operation for resonant cavity readout in addition to parametric amplification. Parametric amplifiers produced using JJs can approach the signal-to-noise ratio set by quantum mechanics, and prove essential for the amplification chain used by the ADMX experiment for axionic detection. The limits of these amplifiers are set by the noise tuning source used for calibration. This tuning device can be used to solve for the intrinsic gain and added noise present within the amplifier, which defines the effectiveness of the amplifier. Multiple models for a tuning device, based upon different models of noise, will be analyzed. Also discussed is the fabrication behind the tunnel junction-based noise source, in addition to the Dolan bridge procedure and corresponding physics used for design of experiment and parameter manipulation.