School of Physics - Theses

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    Principles and applications of quantum decoherence in biological, chemical, and condensed matter systems
    Hall, Liam Terres ( 2013)
    This thesis focuses on the use of the Nitrogen-Vacancy (NV) defect centre in diamond as a single spin sensor of nanoscale magnetic fields. The NV system has attracted considerable interest in recent years due to its unique combination of sensitivity, nanoscale resolution, room temperature operation, and stable fluorescence, together with the inherent biocompatibility of diamond; making it ideal for measuring coherent quantum processes in biological, chemical and condensed matter systems. Existing NV-based sensing techniques, however, are ultimately limited by sources of magnetic noise that act to destroy the very resource required for their operation: the quantum phase coherence between NV spin levels. We address this problem by showing that this noise is a rich source of information about the dynamics of the environment we wish to measure. We develop protocols by which to extract dynamical environmental parameters from decoherence measurements of the NV spin, and a detailed experimental verification is conducted using diamond nanocrystals immersed in a MnCl2 electrolyte. We then detail how sensitivities can be improved by employing sophisticated dynamic decoupling techniques to remove the decoherence effects of the intrinsic noise, whilst preserving that of the target sample. To characterise the effects of pulse errors, we describe the full coherent evolution of the NV spin under pulse-based microwave control, including microwave driven and free precession intervals. This analysis explains the origin of many experimental artifacts overlooked in the literature, and is applied to three experimentally relevant cases, demonstrating remarkable agreement between theoretical and experimental results. We then analyse and discuss two important future applications of decoherence sensing to biological imaging. The first involves using a single NV centre in close proximity to an ion channel in a cell membrane to monitor its switch-on/switch-off activity. This technique is expected to have wide ranging implications for nanoscale biology and drug discovery. The second involves using an array of NV centres to image neuronal network dynamics. This technique is expected to yield significant insight into the way information is processed in the brain. In both cases, we find the temporal resolution to be of millisecond timescales, effectively allowing for real time imaging of these systems with micrometre spatial resolution. We analyse cases in which environmental frequencies are sufficiently high to result in a mutual exchange of energy with the NV spin, and discuss how this may be used to reconstruct the corresponding frequency spectrum. This analysis is then applied to two ground-breaking experiments, showing remarkable agreement. Protocols for in-situ monitoring of mobile nanodiamonds in biological systems are developed. In addition to obtaining information about the local magnetic environment, these protocols allow for the determination of both the position and orientation of the nanocrystal, yielding information about the mechanical forces to which it is subjected. These techniques are applied in analysing a set of experiments in which diamond nanocrystals are taken up endosomally by human cervical cancer cells. Finally, we focus our attention on understanding the microscopic dynamics of the spin bath and its effect on the NV spin. Many existing analytic approaches are based on simplified phenomenological models in which it is difficult to capture the complex physics associated with this system. Conversely, numerical approaches reproduce this complex behaviour, but are limited in the amount of theoretical insight they can provide. Using a systematic approach based on the spatial statistics of the spin bath constituents, we develop a purely analytic theory for the NV central spin decoherence problem that reproduces the experimental and numerical results found in the literature, whilst correcting the limitations and inaccuracies associated with existing analytical approaches.
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    From geometric phases to intracellular sensing: new applications of the diamond nitrogen-vacancy centre
    This thesis consists of two parts, each of which proposes a new application of the diamond nitrogen-vacancy (NV) centre. We first consider the NV centre as a device to detect geometric phases. We show that the Aharonov-Casher phase and Berry’s phase may be produced in the NV centre’s spin sublevels and observed using existing experimental techniques. We give the background theory to geometric phases, then show how these phases apply to the NV system. Finally, we outline a number of realistic experiments to detect these phases. The second part considers the behaviour of an NV centre within a diamond nanocrystal which rotates, in a Brownian sense, in a fluid. Our aim is to understand the effect of rotational motion on the initialisation, evolution and readout of an NV centre, motivated by the idea of using colloidal nanodiamonds for biological imaging. We first develop a model to describe the quantum evolution of a rotationally diffusing nanocrystal. The model uses theory developed in NV magnetometry and also the geometric phase theory developed in the first part of this thesis. We then explore the consequences of this model for nanoscale sensing. We show that the tumbling NV system may be used as a sensitive magnetometer with nanoscale resolution and also as a probe of its own rotational motion.