Hynes, Michael; (2025) Synthetic spin-orbit coupling in one-dimensional nanostructures with micromagnet arrays. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Micromagnet arrays offer a promising platform for generating synthetic spin-orbit coupling in one-dimensional systems through spatially varying magnetic fields. While individual micromagnets enable spin-electric coupling in spin qubits, arrays have been predicted to provide controlled coupling throughout one-dimensional devices, particularly important for suspended carbon nanotube systems. For topological qubits, these arrays can generate synthetic Rashba spin-orbit coupling with strengths comparable to those found in high-SOI III-V semiconductors, potentially expanding the range of viable materials and increasing the topological gap in existing platforms. In this thesis, I investigated micromagnet arrays in two distinct quantum computing platforms: InAs/Al nanowires for topological qubits and suspended carbon nanotubes for spin qubits. I demonstrated novel atomic force microscopy techniques for precise nanomanipulation of InAs nanowires and controllable switching of micromagnets, maximising the coupling between nanowires and micromagnet arrays. Through transport measurements and modelling of hybrid InAs/Al nanowire devices, I identified Andreev bound states in the system and studied their behaviour when coupled to micromagnet arrays in different configurations. The antiparallel and parallel configurations, generating locally rotating and linear magnetic fields respectively, resulted in a 40µeV shift in the Andreev bound state energy, which remained insensitive to external magnetic fields. I developed and validated a transport model that successfully describes these observations. Finally, I explored the integration of micromagnet arrays with suspended carbon nanotube devices through numerical simulations, showing that carefully designed arrays can achieve MHz-scale Rabi frequencies, yielding qubit quality factors around 10 with state-of-the-art carbon nanotube lifetimes. Given that the topological phase transition requires Zeeman energy to exceed the superconducting gap, future implementations could benefit from improved ferromagnetic materials for the micromagnets. These results establish micromagnet arrays as a versatile platform for engineering synthetic spin-orbit coupling in nanoscale quantum systems, offering promising new directions for both conventional and topological quantum computation.

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