Abstract: The main goal of this thesis is to initiate an alternative path towards building coherent quantum annealers based on superconducting flux qubits. An uncoupled flux qubit device is designed to benchmark qubit coherence that will be used as the building block for future iterations of coherent quantum annealers. The Hamiltonian design of the flux qubits is focused on reducing the persistent current to reduce flux noise susceptibility. Coherent control of flux qubits is achieved in the form of coherent Rabi oscillations, which are repeated for many flux operation points to understand the noise mechanisms. Decay times of 40us are shown, which are among the best results for flux qubits. However, the qubit coherence is low, leading to coherence times shorter than 20ns, probably because of flux noise. The coherence results of the flux qubits measured represent an important benchmark in the development of a coherent quantum annealer and provide very valuable information on how to improve the flux qubits for future iterations of quantum annealing processors.

In parallel to the development of qubit experiments, intrinsic properties of thin-film aluminum has been studied, being the most common material for building superconducting qubit circuits. The magnetic penetration depth of thin film superconducting aluminum has been studied as a function of the film thickness. This calibration, not previously performed in the literature, is especially relevant for the design of flux-sensitive devices, such as flux qubits. This study allows for a better design determination of the inductance in superconducting circuits. Moreover, it points to the possibility of a change in superconductivity type in aluminum in the typical range of thicknesses used in superconducting circuit experiments.

In this thesis, many techniques and methods have been established, and the results obtained represent the first coherent control of superconducting qubits in the QCT group, and, in general, in Southern Europe.  

Thesis director: Dr. Pol Forn Díaz

Tutor: Dr. Joan Soto Riera