Lewis, Dylan;
(2025)
Minimally controlled quantum information processing.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
Finding practically useful computational problems that a noisy intermediate-scale quantum (NISQ) device can solve faster than a classical computer is a key aim of quantum computing research. Noise severely limits the depth of quantum circuits and is the primary impediment to realising a quantum advantage on a NISQ device. One route that inadvertently introduces noise is by controlling the quantum system, with lasers or gate voltages for example. This thesis explores approaches that could lead to practical quantum advantages by reducing the noise through minimising the time-dependent control. Instead, we propose using the natural system dynamics. Two experimental platforms are considered: linear arrays of trapped ions, and semiconductor quantum dots. We show that the spatial search problem can be solved in optimal time, quadratically faster than is possible classically, in ion traps by utilising longrange interactions. An extension of spatial search is explored by considering the problem of finding multiple marked sites with multiple excitations: a quantum QR-code reader. Inspired by how spin-spin interactions are generated in ion traps, a scheme is proposed for quantum gates between quantum dots using the phonon modes of a one-dimensional electron nanowire. Furthermore, we find that the coherent transfer of electrons in quantum dots with minimal control allows energetically-efficient information transfer. The final part of this thesis directly addresses the problem of designing multi-qubit quantum gates with minimal control – i.e. with restricted time-independent Hamiltonians. Our solution is a geodesic algorithm that utilises the differential geometry of the Lie group structure of quantum gates. While the topics covered in this thesis are varied, the underlying principle is that of minimal control, which could help in the search for practical quantum advantages for NISQ devices.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Minimally controlled quantum information processing |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2025. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10205229 |
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