Whaites, Oliver;
(2024)
Dynamical Decoupling Based Nuclear Spin Control: Hyperpolarisation and Adiabatic Methods.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
Electronic qubit coherence times can be significantly increased by applying dynamical decoupling (DD). Here, dephasing due to a spin bath environment is averaged out via a series of microwave pulses. However, coherence protection dramatically fails when the pulse spacing is resonant with individual nuclear spin precession frequencies, appearing in the coherence trace as characteristic dips. This discovery has formed the basis of quantum sensing and nanoscale MRI. Intrinsic electron-nuclear spin entanglement introduced by DD protocols at resonant pulse spacing has paved the way for methods of nuclear spin control, such as dynamic nuclear polarisation (DNP) - the mapping of the central qubit state onto a nuclear spin. Commonly, studies of these protocols for hyperpolarisation assume entanglement of a central qubit with independent nuclear spins, often invalid for realistic clusters. We investigate central-spin-mediated nuclear spin-pair effects of DNP protocols, demonstrating the emergence of polarisation blocking dark states and the resonance expulsion of weakly-coupled nuclear spins, we term polarisation or spin blockade. Additionally, the off-resonance effects of DNP are often neglected. Nuclear resonance conditions may be difficult to meet owing to inhomogeneous/drifting magnetic fields, nuclear detuning or polarisation blockades. We show even after theoretically infinite repetitions of off-resonant DNP, nuclear hyperpolarisation may fail to saturate at maximal fidelity, an essential requirement for quantum computing. Lastly, Floquet spectroscopy has been shown to associate characteristic coherence dips with avoided crossings in the underlying Floquet phase spectrum. With analogy to adiabatic sweeps of avoided crossings in the eigenenergy spectrum, we propose similar adiabatic sweeps over avoided crossings in the Floquet spectrum. Standard adiabatic methods struggle to outpace electronic coherence times, whereas adiabatic sweeps over the Floquet spectrum simultaneously decouple the central qubit from the environment - combining smooth adiabatic transitions with long coherence times and setting a new paradigm of nuclear spin control.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Dynamical Decoupling Based Nuclear Spin Control: Hyperpolarisation and Adiabatic Methods |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2024. 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-pp.ucl.ac.uk/id/eprint/10186212 |
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