School of Physics - Theses
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ItemPrinciples and applications of quantum decoherence in biological, chemical, and condensed matter systems( 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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ItemPhysics of low-dimensional nanostructures( 2012)Nanoscale constructs are offering access to the quantum mechanical regime due to their constrained size. The unusual, and often counterintuitive, behaviours of such constructs are of considerable interest to those developing new devices across several fields, including (but not limited to) quantum computing, communications, in vivo applications such as the bionic eye and bio-sensors, standard electronics and computing, and magnetometry. The physics of zero-, one-, and two-dimensional nanostructures comprised of various dopants or arrays of dopants in either diamond or silicon are presented and discussed. In particular, the zero-dimensional Xe-related defects in diamond are considered theoretically, via density functional theory, lattice dynamics, and thermodynamics. Xe defects have also been characterised experimentally via the probe-enhanced Ra- man spectroscopy (PERS) technique. In silicon, a one-dimensional nanowire consisting of P donors is studied with density functional theory. This wire is monatomically thin in one direction, and two donors wide in the other, with the donors spaced at the currently realisable sheet density of 25%. The two-dimensional case of infinite monatomically thin sheets of P donors is considered, both with effective mass theory and density functional theory (which is again undertaken for the most common experimental sheet density, 25%). The effective mass theory model has been applied to several sheet densities, agrees well with literature calculations of sheets with in-plane disorder, is far more rapid in execution, and offers an analytic scaling theory to describe the dependence of several key results on the sheet density. The density functional theory approach is then extended to the quasi-two dimensional case of bilayers of monatomically thin P sheets, in order to address the approach to minimal two-dimensional confinement.
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ItemThe theory of the nitrogen-vacancy colour centre in diamond( 2012)The nitrogen-vacancy (NV) colour centre in diamond is a model system for many quantum technologies including, metrology, information processing and communications. The NV centre is also highly suitable for employment in various nanotechnology applications, such as biological and sub-diffraction limit imaging, and in tests of fundamental physics, such as cavity quantum electrodynamics and the quantum entanglement of mesoscopic systems. The remarkable properties of the centre are however, not currently fully understood, with several unresolved issues limiting the performance of the centre in its many important applications. As the unresolved issues are interrelated and concern different aspects of the centre's properties, they may only be resolved by the development of a single self-consistent theory of the NV centre. The aim of this work has been to develop such a theory. The theory has been developed using a combination of the molecular model of deep level defects in semiconductors, group theoretical methods and ab initio calculations. The highly structured nature of the theory will enable its future use in the systematic identification of other colour centres that possess properties that exceed those of the NV centre.
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ItemNanoscale quantum sensing using nitrogen-vacancy centres in diamond( 2012)Devices that detect and spatially image magnetic fields are important in many areas of study including chemistry, electronics, materials science and biology. By extending the boundaries of what is currently achievable we may begin to explore areas previously inaccessible to science, such as wide-field imaging of neuron signaling or structural determination of single molecules. Here we present experimental progress towards the development of a nanoscale magnetic sensor operating under ambient conditions using the nitrogen-vacancy (NV) centre in diamond. This thesis describes the construction of a lab-built, confocal microscope capable of detecting single NV centres, with additional microwave control for coherent manipulation of the NV spin. The feasibility of using NV-diamond for real-time detection of the action potential generated by a neuron, and with high spatial resolution is experimentally demonstrated. The quantum coherence of single NV spins is monitored in various chemical solutions, and detection of nanoscale magnetic environments external to the diamond are demonstrated. This work sets the experimental foundation for using manufactured single quantum systems as sensitive probes of external chemical environments. In addition, the first measurements of single quantum coherent spins inside living cells are performed with NV centres in nanodiamonds. These studies on NV centres inside living cells demonstrate their promise as magnetic sensors for biology. Furthermore, alternative quantum sensing technologies emerge including rotational tracking of nanodiamonds and enhanced identification of fluorescent nanoparticles.
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ItemSurface engineering for quantum information processing in NV diamond( 2011)Due to an extensive list of extreme and often complementary materials properties, diamond has become a leading candidate in a range of advanced mechanical, thermal, optical and electronics applications. These include devices designed to operate under extraordinarily harsh conditions such as the low-earth orbit environment, and applications with highly demanding performance requirements such as high power electronics. Perhaps the most demanding of these areas in which diamond shows great promise is the pursuit of scalable quantum information processing (QIP) devices, with the ultimate goal being the production and integration of a large number of interacting and controllable quantum bits (Qubits), within a single device. Although the production of this so called quantum computer is theoretically compatible with diamond’s ideal intrinsic properties, experimental realization of this type of device will require absolute control over material characteristics, both in terms of the bulk material used and following the various processing steps required for device construction. Most currently successful processing techniques can be seen to leave some form of crystal damage either near the surface or within the bulk of the material and due to the extreme sensitivity of any form or qubit this residual damage can be very hard to measure, and even harder to prevent or subsequently remove. Specifically for optical QIP devices, the Nitrogen- Vacancy (NV) centre in diamond holds enormous potential in a variety of qubit architectures, most of which require coupling of this defect to a photonic cavity or waveguide system. The biggest roadblock to realization of this lofty goal however is an apparent lack of NV centres found close enough to any diamond sample surfaces, without significant degradation in their spectral qualities. In this thesis we have primarily addressed the issue of near-surface NV by conducting a set of experiments aimed at understanding whether NV centres can be feasibly created close to diamond surfaces (<100nm), whilst maintaining their desirable stable electronic/spectral properties. To this end we have completed a number of interrelated studies on single crystal and nanocrystalline diamond samples, focusing on clarifying the effect of near-surface diamond characteristics, both intrinsic and affected by fabrication procedures, on the quantum and electronic properties of near-surface diamond colour centres. We show that high crystalline quality diamond can be grown using chemical vapour deposition techniques, including the controllable growth of nanocrystalline diamond particles, and that this material is a compatible host for commensurably high spectral quality NV centres. We also show that novel colour centres can be deliberately grown using these techniques, with advantageous quantum optical properties, possibly rivaling those of the NV centre itself. Utilizing optical spectroscopy measurements, and following a range of thermal annealing steps, we show that near-surface NV centres produced by ion-implantation exhibit significant inhomogeneous and homogeneous spectral instability, making them unsuitable for optical QIP applications and that post-growth fabrication techniques, such as mechanical polishing, have also been shown to produce significant near-surface crystal damage, which is likely to further degrade the performance of near-surface NV centres. We utilize advanced surface analysis techniques, such as synchrotron based x-ray absorption, to show that nanodiamonds (<30nm) are subject to enhanced thermal and chemical reactivity, which can be significantly alleviated with the application of surface hydrogen termination schemes. Conversely, the diffusion of hydrogen into CVD grown diamond materials has been shown to passivate near-surface NV centres and mitigation of this effect is likely to be the only significant remaining hurdle in the production of near-surface NV qubits for optical QIP devices. We have seen that hydrogen plasmas do provide a useful surface processing technique, capable of removing crystal damage, which is otherwise resistant to standard wet-chemical etching, and that nitrogen dosed hydrogen plasmas form a new and advantageous plasma-based planarization technique for diamond surfaces, which may be capable of replacing the ubiquitous mechanical polishing methods. Further, we have seen that pure nitrogen plasmas have the potential to form a thermally stable surface termination barrier for diamond, offering a possible alternative to hydrogen surface passivation. Finally, we have directly measured the effect of surface termination on nearsurface unoccupied electronic states, confirming that NV centres within 100nm of the surface have a positive future in optical QIP devices.
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ItemFrom geometric phases to intracellular sensing: new applications of the diamond nitrogen-vacancy centre( 2010)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.
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ItemNovel single photon emitters based on color centers in diamond( 2010)Exploitation of emerging quantum technologies requires efficient fabrication of key building blocks. Single photon sources are one of these fundamental constituents that are presently pushing the bounds of existing materials and fabrication techniques. Color centers in diamond are very attractive in this respect since they are the only photostable solid-state single photon emitters operating at room temperature known to date.