Current proposals for the design of quantum computer architectures include combining different quantum systems with designated tasks to build a device that can efficiently store, process, and transfer quantum information. Electron spins in solid-state quantum systems are a viable platform for storing information in these multi-quantum frameworks. While extensive research has been performed to couple solid-state systems to photons and microwaves, an alternative line of research focuses on coupling these systems to phonons, or mechanical motion. The use of phonons in solid-state devices opens up a new approach to interface different quantum systems. This dissertation presents experimental progress in developing and controlling a spin-mechanical system, specifically the interaction between the electron spin of a nitrogen vacancy (NV) center in diamond and mechanical vibrations on the surface of the diamond, and discusses theoretical methods for limiting decoherence in the system. To investigate the strain properties of the NV center, we couple acoustic waves to the NV spin via an optical excitation. We transfer population between the spin ground states by driving phonon-assisted optical transitions and demonstrate the formation of a non-radiative state, which can be used to adiabatically transfer population between two states, through the same mechanism. To mitigate the effects of the nuclear spin bath on the NV center, we study and show preliminary results on the semiclassical dressed states, or quantum states of the NV interacting with optical fields. The dressed states can be insensitive to magnetic fluctuations, thus preserving the quantum state of the system. Finally, we consider a transitionless quantum driving technique that decouples the NV center from a radiative state, preventing decoherence through spontaneous emission. These developments are essential in advancing our understanding of phonon-based interfaces between quantum systems. This dissertation includes previously published and unpublished co-authored material.

The nitrogen-vacancy (NV) centre in diamond has proven to be an excellent tool to probe electro-magnetic fields and temperature. It has a number of unique features: High sensitivity and resolution, long coherence and lifetimes, the ability to operate from cryogenic temperatures to hundreds of Kelvin, chemical inertness and addressability via optics and microwaves. Recent progress includes the detection of NMR and spectroscopy of single proteins on a diamond surface and in-vivo temperature measurements. However, while the NV centre in bulk diamond has received a lot of attention, the nitrogen-vacancy in nanodiamond has not been investigated extensively due its widely seen inferior properties. It is only very recently that problems with the stability of photoluminescence and short coherence times have been overcome. The NV centre in nanodiamond is thus increasingly seen as an interesting tool for research requiring nanoscale sensors, e.g. in cells. The findings of this thesis facilitate applications of the NV centre in nanodiamond and demonstrate its high potential for future research. Most notably, the nuclear host spin, which is intrinsic to the point defect, can be used to enhance sensitivity and resolution of measurements. In addition, the sensitivity can be improved by time-tagging the emission from the NV centre. Furthermore, the graphite layer covering nanodiamonds can be removed by annealing. This does not have negative effects on the spin properties of the hosted NV centres but enables functionalisation of the surface and therefore advanced in-vivo measurements. Finally, the capabilities of the NV centre in nanodiamond in investigating the formation of magnetic domains are demonstrated at low temperatures. These results enable and motivate the use of the NV centre in nanodiamond for future research, most especially in biological systems.

Diamond is a wide band gap material with many optically active defect centers. Among all, the most interesting negatively charged nitrogen vacancy (NV-) defect center in diamond has been investigated for almost two decades, often in relation to applications in quantum computing and quantum sensing. Nitrogen vacancy centers are formed by a substitutional nitrogen atom next to a vacancy trapped at an adjacent lattice position. Usually, these centers are prepared in synthetic diamond, where single substitutional nitrogen impurities are in the ideal case homogenously dispersed. To obtain bright luminescence from a sample, additional vacancies are created by electron or neutron irradiation and allowing them to diffuse to nitrogen atoms by annealing at temperature above 600 0C. However, already untreated synthetic diamond samples provide a concentration of NV centers well suited for the study of ensembles. Therefore, to investigate ensemble luminescence centers in diamond crystals, the untreated samples are sufficient. The spectral analysis allowed to clearly identify NVs by fluorescence spectroscopy in such samples. Even at room temperature, the zero-phonon line (ZPL) at 638 nm (NV-) is clearly visible and an additional photon contribution results in the characteristic shape with an overall width of about 120 nm and a maximum at ~685 nm. The broad spectral emission is one of the few drawbacks of NV fluorescence. In this thesis, I developed a conventional fluorescence detection technique, with a homebuilt sample stage which can be precisely positioned in x- and y- direction on a sub-micrometer scale. The sample is excited by laser light focused into a spot size of < 500 µm, and the fluorescence sampling is acquired within a sampling distance of 0.25 µm. Taking advantage of this, it is possible to take fluorescence sampling of an ensemble of NVs from the whole of the crystal, or from a desired section applying a fluorescence matrix methodology. Using this technique, a wide variety of CVD and HPHT synthesized diamond samples were investigated giving first-hand experience of omnipresent NV centers in diamond samples containing a nitrogen impurity concentration of less than 1 ppm (or <200 ppm). This study provides a good base for further work aiming at artificially creating near-surface NVs, which is the basis of many applications with the requirement for better sensitivity and strong coupling to the external spins. To ensure that the fluorescence detected is reliable and repeatable, extensive fluorescence measurements were performed within different matrix regions of the sample for several days, and it turned out that the fluorescence emission is identical when the excitation laser is excited at the middle of the sample. The outcome of the experiments evolved in setting a reference sample for other fluorescence measurements. This reference sample was fluorescence measured over several months, and performed identical spectrum characteristic with less than 3-5% difference in absolute fluorescence intensity. In the spectrum, the often mixed Raman line at 573 nm and the NV0 centers were resolved using higher spectrometer grating. A series of annealing studies in HPHT diamond samples was performed at UHV ambience with a base pressure at ~1 x 10-11 mbar on a sample with [N] < 200 ppm. The fluorescence examined on the sample annealed at temperature 500 0C revealed an increased fluorescence intensity, and remained at constant intensity on consecutive annealing cycles at the same temperature under the same conditions. However, at an increased temperature, the fluorescence emission increased, increasing NVs concentration in the crystal. The untreated HPHT diamond crystals varied in fluorescence characteristic feature, but the sample showed the presence of NVs. The differences in spectroscopic features were identified as due to nitrogen content and possibilities of different nitrogen defect complexes present in the crystal, and they were modified when the sample was annealed at temperatures above 500 0C. The most effective defect formation within the crystal takes place at two temperature ranges 650 –750 0C and 800– 850 0C. The calculated activaction energy at 0.22 eV and 1.26 eV are the energy of mobile interstitial atoms and that of substitutional nitrogen atoms respectively. In the process of annealing, the desorbtion of nitrogen atoms from the surface crystal has been identified by a mass spectrometer. The study contributes to the fundemental understanding of anneling effects in diamond crystals, without being bombarded by high energy electron or neutron radiation. For the creation of a high density of NV centers, annealing in UHV could be sufficient, or even controlled NVs in ultra-pure diamond. The CVD diamond crystals with [N] < 1ppm were observed to contain a high density of NVs, and had no significant change when the additional creations of NVs were attempted. Prolonged X-ray radiation followed by annealing of ultra-pure diamond ([N] <5ppb) during the XPS measurements, showed a significant impact in fluorescence intensity at the surface region confirmed by confocal measurements. However, the sensitivity of the fluorescence spectroscopy setup was not enough to observe the ZPL of the NV centers, though significant changes have been observed in the spectra. Finally, the shallow NV- creation with nitrogen ion implantation at energy of 1 keV has been confirmed by an ODMR experiment and confocal imaging.

Among several hundred impurities and defects that can occur in diamond, the nitrogenvacancy (NV) center is one of the most interesting for quantum technologies at room temperature. Several properties make it an excellent platform for many applications, from nanosensor to quantum information processing. This thesis presents the first explorations of NV-based technologies in our laboratory, investigating three sets of studies aimed at learning the know-how of this field of research and developing the necessary infrastructure to explore them in future quantum technologies. The first set of studies focus on magnetometry and how to improve NV-based magnetic sensing. We show that, using engineered ensembles of NV centers in ultrapure synthetic diamonds, one can build a relatively simple apparatus to do magnetic imaging of relatively large areas while determining the full vector field with high spatial resolution and very good sensitivity. We also show that these measurements can be used to reconstruct the current density distribution of nearby sources, opening exciting possibilities to study two-dimensional materials. Another set of studies involve spin coherence of an ensemble of NV centers. For that, we developed a method based on a CCD camera and an imaging protocol that allows implementing pulse sequences like Rabi, Ramsey and Hahn echo, performing Electron Spin Resonance (ESR) spectroscopy over extended areas. Using this method we extract parameters from our sample, including measurements for T1, T2 and T*2. The method was also used to observe Electron Spin Echo Envelope Modulation (ESEEM), due to hyperfine interaction with nearby 15N nucleus, resulting in improved frequency sensitivity. The third set of studies explore how to use femtosecond lasers to produce NV centers in diamond and investigated the nonlinear index of refraction (n2) of the diamond (type IIa) in a broad spectral region, from the infrared (1500 nm) to the ultraviolet (260 nm). / Entre várias centenas de impurezas e defeitos que podem ocorrer no diamante, o centro Nitrogênio-Vacância (NV) é um dos mais interessantes para tecnologias quânticas em temperatura ambiente. Diversas propriedades fazem dele uma excelente plataforma para muitas aplicações, desde nanosensores até o processamento de informações quânticas. Esta tese apresenta a primeira exploração de tecnologias baseadas em NV no nosso laboratório, investigando três conjuntos de estudos com objetivo de aprender o know-how desta área de pesquisa e desenvolver a infra-estrutura necessária para explorá-los em futuras tecnologias quânticas. O primeiro conjunto de estudos enfoca a magnetometria e como melhorar o sensores magnético baseado em centros NV. Mostramos que um aparato relativamente simples pode ser usado para produzir imagens magnéticas do campo vetorial, usando ensembles de centros NV em diamantes sintéticos ultrapuros. Também mostramos que essas medidas podem ser usadas para reconstruir a distribuição de densidade de corrente de fontes próximas, abrindo possibilidades interessantes para o estudo de materiais bidimensionais. Outro conjunto de estudos envolve a coerência de spin de um ensemble de centros NV. Para isso, desenvolvemos um método baseado em uma câmera CCD e um protocolo de imagem que permite implementar sequências de pulsos como Rabi, Ramsey e eco Hahn, realizando espectroscopia de ressonância de spin eletrônico (ESR) sobre áreas estendidas. Usando esse método, extraímos parâmetros de nossa amostra, incluindo medidas para T1, T2 e T*2. O método também foi usado para observar o efeito ESEEM (Envelope de Modulação de Eco de Spin Eletrônico), devido à interação hiperfina com o núcleo próximo de 15N, resultando numa melhor sensibilidade de freqüência. Finalmente, o terceiro conjunto de estudos explorou como usar lasers de femtossegundos para produzir centros NV em diamante e investigou o índice de refração não-linear (n2) do diamante (tipo IIa) em uma ampla região espectral, desde o infravermelho (1500 nm) até o ultravioleta (260 nm).

Exotic aspects of quantum mechanics, such as quantum entanglement, can be exploited to solve computational problems that are impractical to solve with conventional computers. With the realization of robust solid-state qubits, such as Nitrogen Vacancy (NV) centers in diamond, an outstanding challenge is to develop experimental approaches that can control the interactions between individual qubits. This dissertation develops a diamond-based experimental system that exploits acoustic waves or mechanical vibrations to mediate interactions between spin qubits. This spin-mechanical system features three essential elements: robust qubits, high quality-factor diamond nanomechanical resonator, and strong spin- mechanical coupling, thus enabling a new and promising platform for pursuing solid- state quantum computer. For the spin-mechanical system, NV centers are created near the surface of a bulk diamond through nitrogen ion implantation followed by stepwise high temperature annealing. We successfully suppress environmental fluctuations and achieve NV centers with stable and spectrally narrow (< 50 MHz) fluorescence at low temperature, which is crucial for the spin-mechanical system. Diamond nanomechanical resonators with a fundamental frequency near 1 GHz have been successfully fabricated with a diamond-on-insulator approach. The resonators are suspended from a silicon substrate and are supported with long and thin tethers, decoupling the mechanical modes from the surrounding environment. Diamond nanofabrication is still in its infancy. Numerous fabrication problems occurring during etching, mask transfer, and wafer bonding have been painstakingly resolved. Strong spin-mechanical coupling is demonstrated via the strain coupling of the NV excited-states. The spin-mechanical coupling takes place through a 𝚲-type three- level system, where two ground-spin-states couple to an excited-state through a phonon-assisted as well as a direct dipole optical transition. Both coherent population trapping and optically-driven spin transitions have been realized. The coherent population trapping demonstrates the coupling between an acoustic wave and an electron spin coherence through a dark state, thus avoiding the short lifetime of the excited state. The optically-driven spin transitions can enable the quantum control of both spin and mechanical degrees of freedom. This dissertation includes previously published co-authored material.

For cavity quantum electrodynamics systems (cavity-QED) to play a role in quantum information processing applications and in quantum networks, they must be robust and scalable in addition to having a suitable method for the generation, processing and storage of quantum bits. One solution is to develop a composite system that couples a nitrogen vacancy (NV) center in diamond to a whispering gallery mode supported by a fused silica microsphere. Such a system is motivated by the optical and electron-spin properties of the NV center. The NV center is the leading spin-qubit and exhibits atomic like linewidths at cryogenic temperatures and has spin coherence times greater than milliseconds at room temperature. These long coherence times, coupled with nanosecond scale spin readout and manipulation times, allow for millions of quantum operations to be processed. Silica whispering gallery resonators are the only class of microresonators with quality factor high enough to reach the strong coupling regime, which is necessary for some quantum information processing applications. Integrating these two components into a system that could position a diamond nanopillar near the surface of a deformed-double stemmed microsphere system, with nanometer precision, at 10 K was a major achievement of this research. Cavity resonances in deformed microspheres can be excited with a free-space coupling technique which simplifies their integration into cryogenic environments. In these intentionally deformed resonators, an enhanced evanescent field decay length was observed at specific locations along the ray orbit. The double-stem arrangement enables the cavity resonance to be tuned over 450 GHz, with sub-10 MHz resolution, at 10 K. These two features, the enhanced decay length and broad range tuning with high resolution, are indispensible tools for cavity-QED studies with silica microspheres. Diamond nanopillars were fabricated from single crystal diamond with diameters as small as 140 nm in order to maintain a high quality factor. Studies were conducted on NV centers in nanopillars and bulk diamond to determine their suitability for cavity-QED applications. In an attempt to increase the light-matter interaction between NV centers and whispering gallery modes, diamond substrates were optically characterized that were irradiated with nitrogen ions.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第22465号 / 工博第4726号 / 新制||工||1738(附属図書館) / 京都大学大学院工学研究科分子工学専攻 / (主査)教授 関 修平, 教授 水落 憲和, 准教授 菅瀬 謙治, 教授 梶 弘典 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

Quantum information technologies require networks of interacting defect bits. Color centers, especially the nitrogen vacancy (NV-) center in diamond, represent one promising avenue, toward the realisation of such devices. The most successful technique for creating NV- in diamond is ion implantation followed by annealing. Previous experiments have shown that shallow nitrogen implantation (<10 keV) results in NV- centers with a yield of 0.01%–0.1%. We investigate the influence of channeling effects during shallow implantation and statistical diffusion of vacancies using molecular dynamics and Monte Carlo simulation techniques. Energy barriers for the diffusion process were calculated using density functional theory. Our simulations show that 25% of the implanted nitrogens form a NV center, which is in good agreement with our experimental findings.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23388号 / 工博第4880号 / 新制||工||1763(附属図書館) / 京都大学大学院工学研究科材料化学専攻 / (主査)教授 三浦 清貴, 教授 田中 勝久, 教授 藤田 晃司 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

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