Manipulation of quantum effects for computing is an important step in the creation of faster and more powerful computational tools. The nitrogen vacancy centre (NVC) in diamond, when in the negative charge state, has shown significant promise towards being used as a qubit, such as optical polarization and readout, long coherence times (T∗ 2 and T2), and a long relaxation time T1. Such times are desired for producing good quantum memory, utilising the spin state of the sites. Cryogenic conditions are required for facilitating the optical entanglement of two NVC, and can serve to significantly extend these times. These sites have been created using a laser writing technique, capable of producing a large number of NVC with good coherence characteristics at arbitrary locations within diamond samples. This thesis reports on methods to reliably create NVC with good coherence characteristics for quantum computing applications by altering the annealing regime used. This annealing regime has been improved by the design and construction of a new vacuum furnace with better temperature control and thermometry. Steps towards the ideal combination of annealing conditions for a range of desired NV concentrations and characteristics have been undertaken, showing good conversion of laser written sites to NVC. NVC have been systematically tracked using a confocal fluorescence microscope apparatus throughout multiple annealing steps, with the collection of coherence data including T1, T2 and T∗ 2 on the same NV centre at multiple annealing temperatures, for the first time. This data allows for tracking of the influence of annealing temperature on the coherence properties of sites without the caveat of comparing data from either different sites, or different diamonds altogether. Such centres show coherence times above 800 μs, with production rates at and above 40% for single emitters. NVC are limited by their weak emission into the zero phonon line. These issues can be alleviated by increasing the collection efficiency of photons, which requires thin diamonds capable of being used with cavity systems. Steps towards producing such diamonds by a thin-region design have been undertaken and show promising results. Further efforts include the use of nanostructures on the diamond surface, which has been implemented and detailed. Magnetic control of NVC within diamond, especially at cryogenic temperatures, introduces some operational complexity to potential quantum operations. By implementing the use of diamond cut at the (111) crystal plane, this alignment is likely to become more straightforward, enabling future work with diamond at this crystal plane. Diamonds have been produced, with NVC laser written in, for the first time.
Manipulation of quantum effects for computing is an important step in the creation of faster and more powerful computational tools. The nitrogen vacancy centre (NVC) in diamond, when in the negative charge state, has shown significant promise towards being used as a qubit, such as optical polarization and readout, long coherence times (T∗ 2 and T2), and a long relaxation time T1. Such times are desired for producing good quantum memory, utilising the spin state of the sites. Cryogenic conditions are required for facilitating the optical entanglement of two NVC, and can serve to significantly extend these times. These sites have been created using a laser writing technique, capable of producing a large number of NVC with good coherence characteristics at arbitrary locations within diamond samples. This thesis reports on methods to reliably create NVC with good coherence characteristics for quantum computing applications by altering the annealing regime used. This annealing regime has been improved by the design and construction of a new vacuum furnace with better temperature control and thermometry. Steps towards the ideal combination of annealing conditions for a range of desired NV concentrations and characteristics have been undertaken, showing good conversion of laser written sites to NVC. NVC have been systematically tracked using a confocal fluorescence microscope apparatus throughout multiple annealing steps, with the collection of coherence data including T1, T2 and T∗ 2 on the same NV centre at multiple annealing temperatures, for the first time. This data allows for tracking of the influence of annealing temperature on the coherence properties of sites without the caveat of comparing data from either different sites, or different diamonds altogether. Such centres show coherence times above 800 μs, with production rates at and above 40% for single emitters. NVC are limited by their weak emission into the zero phonon line. These issues can be alleviated by increasing the collection efficiency of photons, which requires thin diamonds capable of being used with cavity systems. Steps towards producing such diamonds by a thin-region design have been undertaken and show promising results. Further efforts include the use of nanostructures on the diamond surface, which has been implemented and detailed. Magnetic control of NVC within diamond, especially at cryogenic temperatures, introduces some operational complexity to potential quantum operations. By implementing the use of diamond cut at the (111) crystal plane, this alignment is likely to become more straightforward, enabling future work with diamond at this crystal plane. Diamonds have been produced, with NVC laser written in, for the first time.
Nitrogen vacancy centres in diamond have shown strong potential for use as qubits in quantum computing. However, nitrogen vacancy centres have a low percentage of overall emission emitted through the Zero Phonon Line (ZPL). In addition, the fabrication of NV centres with controlled positioning is limited. This thesis presents the ZPL linewidth characterisation of NV centres fabricated using laser writing and laser diffusion. Only one out of the six NV centres characterised showed a <100 MHz average single scan linewidth, which suggests that the laser diffusion process needs to improved. The low ZPL emission can be enhanced by cavity coupling to Fabry-Pérot microcavities. Fabry-Pérot microcavities are susceptible to vibrations, which will decouple the cavity from the NV centre and reduce the photon enhancement. A cavity stabilisation system based on cavity fringe stabilisation is implemented, and the results of the stability are presented. The Fabry-Pérot microcavity can be stabilised with 0.13 nm using fringe stabilisation. High collection efficiency of the emission is also desired for maximising the entanglement rate and is reduced by optical aberrations. For NV centres in bulk diamond, using Solid Immersion Lenses (SILs) removes the spherical aberration that is introduced when collecting NV emission through a planar interface. This thesis presents the results of modelling the aberrations for NV centres cavity coupled to a Fabry-Pérot microcavity and for hemispherical SILs on diamond. The wavefronts of the aberrations introduced by the planar mirror of the microcavity and the aberrations introduced by the positioning error of the SIL are modelled numerically. A deformable mirror is implemented into a confocal microscope to monitor the aberrations of the photonic devices and the results are compared with the numerical model. For Fabry-Pérot microcavities, the intensity loss caused by the aberrations introduced by the planar mirror was predicted to be less than 10 \% due to the thin mirror substrate and low numerical aperture of the cavity mode. This prediction was consistent with the experimental results where an effective 3 \% increase in light collection was observed with aberration correction. For the SILs, the model predicts that the collection efficiency is more sensitive to the lateral positioning of the SIL relative to the NV centre than the axial positioning error.
The fluorescence of the nitrogen-vacancy (NV) centre in artificial diamond has been widely studied for applications such as quantum communication protocols, quantum computation, fluorescence-based biolabelling, and optical sensing. Optical sensing by itself encompasses a wide range of possible applications: from magnetic and electric field sensing to temperature sensing and enabling, for example, the detection of paramagnetic ions and the identification of molecules by nuclear magnetic resonance. Compared to many other systems capable of optical sensing, the NV centre works at room temperature, and its spin-dependent fluorescence emission is the key property to achieve a high sensitivity to the surrounding environment. Additionally, the NV centre emission is highly photostable, has a relatively short lifetime (~ 12ns) and spans a broad wavelength range, from the red to the near-infrared region of the electromagnetic spectrum (600 - 800nm). While the common denominator of all mentioned applications is the NV centre, the properties of the artificial diamond surrounding the NV centre have a significant impact on its sensing performance. At present, the processing of artificial diamond has not been fully optimised, and research and development towards the realisation of the `best¿ diamond sample for optical sensing is far from over. Specifically, the role of other defects in diamond and their effects on the photo-physical properties of the NV centres are not entirely understood. In this thesis, I will discuss the potential benefits of a novel processing technique to create ensembles of NV centres, and the importance that surrounding nitrogen defects has on the achievable magnetic field sensitivity. Additionally, I will show both theoretical and experimental investigations of the NV centres photo-physical processes, with focus on magnetic field sensing. I will initially explore a protocol for microwave-free magnetometry. This protocol predicts a theoretical magnetic field sensitivity <10µT/√Hz in the range 0.3 - 30mT and I will demonstrate it experimentally with a maximum sensitivity of 3.6mT/Hz. Following, I will focus on exploring techniques to increase the magnetic field sensitivity by improving the material properties of the diamond samples. I will demonstrate that the simultaneous electron irradiation and annealing improves the creation efficiency of an ensemble of NV centres by up to 117%. Finally, I will investigate how substitutional nitrogen defects in the surrounding lattice affects the charge state of the NV centre, its emission intensity, its lifetime and its magnetic field sensitivity. I will study a sample characterised by multiple regions across its volume, with nitrogen concentration changing between 10ppm and 100ppm in each region. There si an optimal region with ~ 40ppm of substitutional nitrogen which demonstrates the highest fluorescence emission, mainly from the negatively-charged NV centre, and the highest magnetic field sensitivity of 1.6 ± 0.2µT/√Hz. However, both the regions with lower and higher substitutional nitrogen concentration than the optimal region show lower emission intensity and worse magnetic field sensitivity. Following is a summary of the content in each chapter: Chapter 1 will present an introduction to the diamond material and the classification of artificial diamond samples. It will introduce the NV centre, its fluorescence properties and the process to create NV centres in diamond. The chapter will also introduce the concepts around optical magnetometry and the working principle that allows the NV centre to sense magnetic fields as well as other external quantities, such as temperature or pressure. Finally, it will include a brief overview of the defects that are found in artificial diamond or are created during the sample processing. Chapter 2 will describe the working principle of the instruments utilised in this work. I will list the details of each instrument and describe the methods used for the analysis of the data. Chapter 3 will introduce the background of modelling the fluorescence emission from NV centres. I will discuss the implementation of a computational model based on the Lindblad master equation to simulate the photo-physical dynamics of the NV centre. I will demonstrate the successful simulation of the saturation emission, fluorescence lifetime and optically detected magnetic resonance using the theoretical model. Finally, I will explore the theoretically predicted sensitivity of microwave-free magnetometry, based on the NV centre emission intensity change under an external magnetic field. Chapter 4 will demonstrate experimentally the microwave-free magnetometry technique. In relation to the study of microwave-free magnetometry, this chapter will explore an unexpected increase in emission intensity at high magnetic fields > 100mT. Combining the experimental observations and the theoretical simulation developed in the previous chapter, I will identify and discuss a phenomenological decrease in the NV centre transition rate from the triplet state to the singlet state. The chapter will conclude comparing the experimental and theoretical sensitivities associated with the microwave-free magnetometry technique. Chapter 5 will show the characterisation of a diamond sample at each step of the process to create NV centres. I will study the electron irradiation followed by separate annealing on a high-pressure high-temperature diamond sample. I will then characterise the sample employing visible and infrared absorption spectroscopy, as well as measure the fluorescence spectra and lifetime of the defects identified at each step. Chapter 6 will examine an alternative method to create NV centres in diamond. I will compare diamond samples irradiated and annealed separately with samples simultaneously irradiated and annealed (referred to as 'annealing in situ'). To compare the two processes, I will estimate the concentration of nitrogen defects and NV centres and calculate their N-to-NV conversion efficiency. Chapter 7 will discuss the effects of substitutional nitrogen on the properties of the NV centres in diamond. For this discussion, I will use a single-crystal diamond that shows the presence of 4 growth sectors. Each growth sector is characterised by a different nitrogen concentration, as I will demonstrate at the beginning of the chapter. I will focus on 4 main properties affected by the concentration of nitrogen in the different sectors: the balance between the charge states of the NV centre, the overall fluorescence intensity from each sector, the fluorescence lifetime and its relation to non-radiative processes, and the magnetic field sensitivity obtained with the optically detected magnetic resonance technique. Chapter 8 will gather the results and conclusions of the entire body of work. It will also discuss future avenues of research in the topic of diamond processing and characterisations, to improve the existing optical sensing applications.
Quantum computation and information processing is an exciting multi-disciplinary field, whose objective is to harness the power of superposition and entanglement of quantum states to establish a new regime of computation for Mankind. This work demonstrates our efforts, in both materials and device engineering, towards building an efficient spin-photon interface for a promising solid-state spin qubit, namely the nitrogen-vacancy (NV) centre in diamond. Improvement in optical coherence of NV centre is realised via the novel laser-writing fabrication technique and quantified using high-resolution spectroscopy. A new metric concerning the charge-state stability of the NV centre is introduced to better translate the measured NV quality to entanglement generation efficiency. To address the low branching ratio of NV emission into the zero-phonon line (ZPL) transition, a hybrid open-cavity device incorporating low-dimensional micro-structured NV-containing diamond membrane is fabricated and characterised. In addition to the more commonly quoted emission enhancement factor, the collection efficiency of the cavity device is also quantified via numerical simulations. Combining all observed improvements in key metrics under the framework of double-heralding entanglement schemes, a two order of magnitude increase in entanglement generate rate can be expected. The established simulation methodology is widely applicable in the design and analysis of similar devices with an emitter-in-cavity setup.
This thesis presents research on improving spin coherence times of bulk and single NV centres in CVD diamond via material engineering, alongside electrical readout of the NV electronic ground state. A chemical-mechanical polishing (CMP) technique is investigated to remove sub-surface damage on (001) surfaces caused by scaife polishing. 13C-limited coherence times of shallow implanted NV centres were compared across samples prepared with ICP-RIE or CMP, with CMP samples showing an improvement of 340µs. The effect of co-doping with phosphorus and sulphur on charge stability, creation yield, spin coherence times, and AC sensitivity of shallow NV centres was studied. 10 CVD diamond samples were characterised using above-bandgap imaging, photoluminescent, and Raman spectroscopy. Coherence times in four samples were measured with a home-built confocal microscope. A sample implanted with 110keV32S ions at 1011cm-2 showed the best shot noise-limit to the AC sensitivity. Preferentially-oriented NV centres were produced on pseudo-(111) surfaces. (001) diamonds were nickel-etched to produce (111) trenches overgrown with nitrogen-containing diamond. Preferential orientation was evidenced via optically detected magnetic resonance under an external ⟨111⟩ magnetic field. High-density nitrogen overgrowth in trenches gave short T2 times (<2µs), while lower-density samples showed mean spin coherence times of 160µs, closer to the unetched surface. Finally, a home-built device for electrical spin-state readout is described, including a confocal microscope, metallic contacts, and a readout PCB. Photoelectrically detected magnetic resonance contrast of 63% (compared to 5% optically) is demonstrated , with initial RF device development.
The nitrogen-vacancy center (NVC) in diamond is an exciting candidate system for building a quantum computer, because of its long coherence time at the relatively high temperature of 5 K. Scaling up to a useful quantum computer requires having an array of many individual NVCs in one diamond; such arrays can be created, but suffer from much shorter coherence times than in natural NVCs. This study uses laser writing to create arrays of single NVCs with ``natural'' spin coherence times, at the right depth for use with optical cavities, enabling arrays of 200,000 coherent NVCs per diamond.
The aim of this proposal is to demonstrate the key elements needed to construct a logical qubit in diamond by exploiting the remarkable quantum properties of the nitrogen-vacancy (NV) optical centre. Specifically, the focus of the work was to address the materials and fabrication issues. We have demonstrated (i) that individual NV qubits can be fabricated by ion implantation with long coherence times (>350 microseconds at room temperature), (ii) electrical control of the optical output of each individual centre is achievable via the Stark shift, (iii) coherent population trapping of ensembles and single NV centres allowing for all-optical control of qubit operations and (iv) that waveguides, cavities and photonic band-gap structures can be fabricated in single crystal diamond. We have thus demonstrated a nanofabrication tool-kit for diamond which is sufficiently robust and mature to justify future investment in the design and implementation of a scalable quantum computing architecture for diamond.