Towards a Near Quantum-Limited Planar SLUG Amplifier at Microwave Frequencies

The Superconducting Low-inductance Undulatory Galvanometer (SLUG) microwave amplifier is distinct from a conventional SQUID amplifier in that the signal to be amplified is directly injected into the SQUID loop, allowing high frequency operation. Here, we discuss a planar SLUG amplifier constructed with nanobridge weak-links fabricated from a single niobium layer. DC characterisation of the SLUG gain element is presented for two nanobridge fabrication techniques. Measurements show that junctions fabricated from Ne focused-ion-beam, and from 100 keV electron-beam lithography, have significantly different DC characteristics with critical currents on the order of 100
$\mu$
A and 1 mA, respectively. Both variants exhibit maximum forward transfer functions greater than 1.5 mV/
$\Phi_{0}$
, which is larger than those reported in Nb SIS-junction SLUGs. Theoretical modelling has shown that an amplifier containing a planar, nanobridge SLUG as a gain element is expected to exhibit gain exceeding 15 dB with large instantaneous bandwidth, and can be designed to operate well in excess of 20 GHz.