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Dissertation
Generation and Detection of Microwave Radiation with MoGe/Au Josephson Junction Arrays
Authors: --- ---
Year: 2024 Publisher: Leuven KU Leuven. Faculteit Wetenschappen

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Abstract

The discovery of superconductivity over 110 years ago has impacted both fundamental and applied research. Its unique properties of dissipationless current flow and perfect diamagnetism could not be explained by classical mechanics, instead requiring a quantum mechanical description. This was ultimately achieved by Bardeen, Cooper, and Schrieffer by developing the BCS theory. The theory explains that electrons are subjected to an attractive interaction, which results them to be in bound states called Cooper pairs. Being a boson, the Cooper pair form a condensate and populate the superconducting ground state in the superconductor. When two superconducting materials are connected by an insulator or a weak link, a Josephson junction is formed. This junction is central to the practical applications of modern day superconducting devices, due to the Josephson Effect. The Josephson Effect is manifested by two different mechanisms. (i) The DC Josephson effect is the Cooper pair tunnelling through the junction from one side of the superconducting material to the other, giving rise to dissipationless supercurrent, which depends linearly on the gauge invariant phase difference between the wavefunctions of the superconducting materials. (ii) The AC Josephson effect is the oscillation of the supercurrent when the junction is under a DC bias. Consequently, the Josephson junction can be thought of as a DC-to-AC converter, functioning as a radiation source. Conversely, the Josephson junction can be thought of as an AC-to-DC converter by displaying Shapiro steps in the V(I) curve when it is under AC bias, functioning as a radiation detector. This is also known as the inverse AC Josephson Effect. In the context of the Third Quantum Revolution, the Josephson junction has great potential to be implemented for qubit technology. Qubit manipulation and readout typically operate in the frequency range between 4-8 GHz. As a result, connections using expensive Radio Frequency (RF) cables from room temperature to cryogenic temperature are required. Moreover, the presence of large components inside the cryostat takes up space sacrifices cooling power. The Josephson junction can be a solution to these problems as an on-chip emitter and detector of radiation. A single junction has a low impedance, which prevents an easy integration with electronics. Fortunately, using a 2D array of Josephson junctions provides a better control on the impedance of the resulting device; additionally, it provides an increased power and sharper line-width, features that scale proportionally to the number of junctions in the array. This thesis studies the 2D Superconductor/Normal/Superconductor (SNS) Josephson junction array. The array consists of Molybdenum Germanium (MoGe) superconducting islands deposited on a Gold on Titanium Ti/Au thin film. The work in the thesis built on previous research done in the group on radiating superconducting arrays and aimed to (i) increase the output power and decrease line-width of the emitted radiation, (ii) investigate the reproducibility of emission by measuring two Josephson junction arrays in one sample, and (iii) perform an on-chip radiation and detection test on the same sample. These different goals were explored. Additionally, numerical simulations simulating the Josephson junction array under different biases and magnetic field were developed and conducted.

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