Pullen, Iwan; (2025) Electron Systems for New Generations of Low Dimensional Quantum Devices. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In recent decades, quantum information and quantum computing have become major research areas, with growing contributions from academia and industry. This thesis explores two lesser studied semiconductor systems: ultra-high mobility twodimensional electron gases (2DEGs) in GaAs and high mobility hole gases in doped germanium. Ultra-high mobility GaAs devices have potential uses in fault-tolerant quantum computing, though a lot of current research focuses on improving electron mobility rather than low dimensional device-level performance. In this work, quantum wires are used to evaluate the suitability of the wafers for low-dimensional quantum devices, comparing the results to shallower quantum wells with a slightly lower mobility. Chapter 4 demonstrates conductance quantisation in ultra-high mobility devices, however, the depth of the quantum well appears to complicate the application of voltage. Initial simulations support these findings, though further refinement is required. These devices also provide a platform to observe fractional conductance quantisation, with evidence of fractions occurring at 1/5 and 4/5(e 2/h) in the shallow well, and at 1/5(e 2/h) under magnetic fields in ultra-high mobility devices. In germanium quantum information, most research has focused on undoped systems. However, doped wafers offer fabrication advantages. Chapter 5 shows that doped germanium quantum dots can be tuned to the final hole state, a critical step for qubit formation. Charging energies of 1.68–2.09 meV align with undoped systems, and effective mass calculations yield a value of approximately 0.35m0, consistent with prior studies. Double dot systems demonstrate cross-coupling effects, though current limitations to the dot geometry prevent in depth measurements. New device designs have been fabricated for further exploration. This work highlights the promise of these semiconductor systems for quantum computing applications and lays the groundwork for future advancements.

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