Fabrication and Characterisation of Thin-Film Superconducting Nanowire Superinductors for Novel Quantum Devices

Abstract

In this master's thesis, we present the design, fabrication and characterisation of superconducting thin-film nanowire superinductances for use in quantum information processing and quantum metrology. In order to characterise the superconducting nanowire, it is placed inside a step-impedance lambda/2 microwave resonator. In a step-impedance resonator, the strong wave impedance mismatch between the feed lines (50 ohm) and the nanowire (ca. 5 kohm) results in the formation of standing waves within the resonator. By measuring the transmission and reflection parameters of the resonator, we can probe the nanowire properties. We have shown that the high kinetic inductance of thin NbN films can be used to fabricate superinductors that exhibit an inductance two to three orders of magnitude larger than an ordinary geometric inductance of the same size. An inductance of 787 nH is demonstrated for a 200nm x 1mm nanowire. This corresponds to a reactive microwave impedance of 25 kohm at 5 GHz, which is higher than the resistance quantum h/(4e^2) = 6.5 kohm.