Realization of a Monolithic, Planar SLUG Amplifier for the Quantum Electronics Toolbox

The Superconducting Low-inductance Undulatory Galvanometer (SLUG) amplifier is an ultra-low noise rf amplifier that is simple to operate and can be designed for a wide range of microwave frequencies. A variant of the dc SQUID rf amplifier, the SLUG differs in that the rf signal is directly injected in the SQUID loop, allowing high frequency operation. Here, we realize a planar SLUG amplifier constructed with nanobridge weak-link junctions that is fabricated from a single layer of niobium. Impedance models for the SLUG gain element are presented in conjunction with a discussion of appropriate impedance matching techniques. Experimental characterization of a proof-of-concept SLUG amplifier designed with a single-stub tuning circuit show modulation of the SLUG operation frequency in line with biasing of the SLUG, a well-matched input network and forward gain of 8 dB.