Silicon has turned from a niche laboratory-scale material into the backbone of modern electronics. Diamond functionalized with color centers could follow a similar path and grow from proof-of-concepts experiments to application-oriented quantum technologies. Quantum sensing, communication, and information processing with color centers has been proven to work in scientific experiments and enhanced sensitivities as well as novel functionalities have been reported. While the first generation of diamond quantum devices relied on color centers formed uncontrolled in natural diamond with a high statistical variation in composition and crystallinity, the situation has changed nowadays. With the emergence of high purity monocrystals and the option for isotopic purification of the carbon matrix, technological progress in materials science on the atomic-scale enabled tailoring the spin properties in diamond. But, since many color centers typically require vacancies to combine with impurities in the carbon lattice via diffusion, the generation of color centers underlies a statistical process providing only little control on the center's final position in the crystal lattice. The negatively charged nitrogen-vacancy (NV
−) center is the most studied optically active defect in diamond and exhibits remarkable spin properties and sensing capabilities, even on the nanoscale and at room temperature. The spin properties of NV
− centers close to the surface (1\,nm - 30\,nm) have been thoroughly studied and the deeper the spins are located in diamond the less decoherence from surface defects is observed. Creating NV
− centers at the surface by nitrogen ion implantation and burying them beneath a thin diamond layer via chemical vapor deposition (CVD) of diamond is reported to enhance the coherence times significantly. By separating the color center generation from controlling the resulting depth by an additional diamond layer, narrow depth profiles can be achieved by reducing the ion energy during implantation. Although the coherence properties improve, many of the implanted NV
− center are reported to vanish in the hydrogen-rich plasma of growth CVD chambers. The mechanism responsible for the instability of NV
− centers towards the plasma remains unclear, though. One hypothesis is the formation of nitrogen-vacancy-hydrogen centers from diffusing hydrogen and implanted NV
− centers but is has not been proven, yet. Here, we demonstrate the fabrication of depth confined NV
− centers by combining low energy (1\,keV - 5\,keV) ion implantation and CVD overgrowth. By increasing the thickness of the overgrown layer (6\,nm - 100\,nm) the influence of surface noise is shown to decrease leading to bulk-like coherence properties of up to $450\,\upmu$s (Hahn-echo) and
T2∗ dephasing times around $20\,\upmu$s. Monitoring the NV
− density as a function of the overgrowth time, we analyze the kinetics of the passivation in case of implanting ion doses of 10
11 15N
+/cm
2 and 10
12 15N
+/cm
2. The passivation turns out to occur faster for lower doses offering insights into sub-surface hydrogen diffusion in diamond. Analyzing the spin bath before and after overgrowth reveals a drastic reduction in paramagnetic spins after the CVD process but no NVH
− is found around the surviving NV
− centers. Checking the chemical composition in overgrown implantation sites with secondary-ion-mass-specrometry (SIMS), however, confirms significant amounts of hydrogen in the
15N-implanted layers. Since the concentration found for hydrogen exceeds the one for nitrogen by far, we suggest that diffusing hydrogen interacts also with vacancies forming probably mostly diamagnetic defects. In the last part of this dissertation, NV
− centers are generated with sub-nanometer precision below a thin
13C-enriched layer for quantum simulation. For fabricating the spin architecture starting from a
12C base layer, we overgrow implanted nitrogen with a
13C layer and bury the spins beneath a 30\,nm-thick
12C capping layer. By measuring the coupling strength between the electron and nuclear spins, we deduce a NV
− placement accuracy reaching below one nanometer with respect to the
13C-layer. Regarding the spin properties of the electron spins, no reduction in
T2 is observed despite of the proximity to the dense
13C-bath. As a consequence, the overgrowth of NV
− centers is demonstrated as a bottom-up approach to produce buried spin architectures with sharp isotopic transitions and efficient
13C-NV
− coupling. Together with the potential for bulk-like coherence properties the synergy of implantation and overgrowth is a powerful tool to fabricate reproducible quantum systems with tailored properties and high depth confinement.
The negatively-charged nitrogen-vacancy (NV) center is the most studied optical center in diamond and is very important for applications in quantum information science. Many proposals for integrating NV centers in quantum and sensing applications rely on their tailored fabrication in ultra pure host material. In this study, we use ion implantation to controllably introduce nitrogen into high purity, low nitrogen chemical vapor deposition diamond samples. The properties of the resulting NV centers are studied as a function of implantation temperature, annealing temperature, and implantation fluence. We compare the implanted NV centers with native NV centers present deep in the bulk of the as-grown samples. The results for implanted NV centers are promising but indicate, at this stage, that the deep native NV centers possess overall superior optical properties. In particular, the implanted NV centers obtained after annealing at 2000 °C under a stabilizing pressure of 8 GPa showed an ensemble linewidth of 0.17 nm compared to 0.61 nm after annealing at 1000 °C. Over the same temperature range, the ensemble NV−/NV0 ratio increased by a factor of ∼5, although this was accompanied by an overall decrease in the NV count.
When the word diamond comes up, the majority of people thinks about engagement rings or gemstones. In fact, it is additionally, one of the most promising host materials for the implementation of advanced technology, exploiting the nature of quantum physics. Such quantum technologies have disruptive potential, ones the quantum effects can be applied in macroscopic, commercial products. The nitrogen-vacancy (NV) centre in diamond is a promising candidate to perform such tasks in the fields of quantum computation, quantum cryptography or metrology. Especially in the field of quantum sensing, the NV sensor impresses with nano-scale spatial resolution paired with high sensitivity. The electronic spin of the atomic-sized point defect can be initialised, manipulated and read-out at ambient conditions and coherence lifetimes are comparably long. Diamond magnetometers based on negatively charged NV centres have shown to be very attractive, and the sensitivity goes down to the single-atom level. In order to exploit such a high sensitivity level for sensing prospects, in this thesis, the NV centres are fabricated well confined in a certain distance to the diamond surface, since the detectable field usually decays with the third power of the distance between sensor and target. The surface depicts a fracture of the spin wise protecting diamond lattice and charge fluctuations affect the stability of NV centres, which are overcome by diamond surface termination. In order to improve the sensitivity further, the sensor density, measurement contrast and interaction time are increased. The nitrogen doping technique, which is applied during a plasma-enhanced chemical vapour deposition (PECVD) diamond growth process is used to fabricate NV centre ensembles aligned along one diamond crystal axis, thereby improving the measurement contrast by a factor up to four. The interaction time limiting spin bath is analysed using the sensor capabilities of the NVs, which leads to enhanced growth strategies and the fabrication of sensitivity optimised NV centre ensembles for magnetometry applications.
Abstract The nitrogen-vacancy (NV) center in diamond is a promising quantum system for magnetometry applications exhibiting optical readout of minute energy shifts in its spin sub-levels. Key material requirements for NV ensembles are a high NV − concentration, a long spin coherence time and a stable charge state. However, these are interdependent and can be difficult to optimize during diamond growth and subsequent NV creation. In this work, we systematically investigate the NV center formation and properties in bulk chemical vapor deposition (CVD) diamond. The nitrogen flow during growth is varied by over four orders of magnitude, resulting in a broad range of single substitutional nitrogen concentrations of 0.2–20 parts per million. For a fixed nitrogen concentration, we optimize electron-irradiation fluences with two different accelerated electron energies, and we study defect formation via optical characterizations. We discuss a general approach to determine the optimal irradiation conditions, for which an enhanced NV concentration and an optimum of NV charge states can both be satisfied. We achieve spin–spin coherence times T 2 ranging from 45.5 to 549 μ s for CVD diamonds containing 168 to 1 parts per billion NV − centers, respectively. This study shows a pathway to engineer properties of NV-doped CVD diamonds for improved sensitivity.
This thesis investigates the theoretical foundations, material properties, and experimental realization of quantum sensors based on the Nitrogen-Vacancy (NV) centers in diamond, a solid-state system that combines quantum coherence with room-temperature operability. The first chapter introduces the principles of quantum sensing, outlining the fundamental definitions, operating protocols, and physical processes underlying the quantum measurement paradigm. Emphasis is placed on the quantum sensing Hamiltonian and the distinction between various classes of quantum sensors. The second chapter focuses on diamond as a host material and on the NV center as a quantum defect. The electronic structure, optical transitions, and spin properties of the NV center are analyzed in detail. Several techniques of NV-based magnetometry are presented, including continuouswave and pulsed optically detected magnetic resonance (ODMR). The discussion also addresses coherence times (T1, T2, and T∗2 ) and methods of diamond synthesis together with NV-center generation through irradiation and annealing. Chapter three develops the physical theory describing the NV center. The Hamiltonian formalism includes spin-spin, hyperfine, and quadrupolar interactions, as well as couplings to external magnetic fields. The chapter further introduces the Bloch sphere representation, magnetic resonance phenomena, and models of lightmatter interaction, ranging from the semiclassical to the fully quantum Rabi framework. A seven-level rate-equation model is proposed to capture the spin dynamics and optical cycling processes of the NV center. The fourth chapter reports the experimental results. After describing materials and methods, several ODMR-based vector magnetometry experiments are presented, highlighting different spectral regimes and the dependence of resonance features on the external magnetic field geometry. Additional studies demonstrate temperature sensing and microwave-free magnetometry techniques for mapping magnetic field distributions. Sensitivity analysis and calibration procedures are also discussed.
This thesis describes a series of experiments on the control of the optical properties of the nitrogen-vacancy (NV) center in diamond, and on control of the electron and nuclear spin states associated with the NV center. The NV center is a fluorescing atomic defect center in diamond, consisting of a substitutional nitrogen atom adjacent to a vacancy in the diamond carbon lattice. The electron spin of the NV center can be initialized and read out optically, and coherently manipulated with high fidelity even at room temperature. This unique combination of properties has attracted much attention to the NV center in recent years for application in quantum information technologies. The experiments in this work focus on exploiting the intrinsic hybrid nature of the NV center. The NV center forms a hybrid spin register, as its electron spin is always coupled to the nuclear spin of its own nitrogen nucleus, and can furthermore be coupled to additional nuclear carbon-13 spins in the diamond. In addition, the electronic spin state can be mapped onto the state of a photon, in principle allowing long range transmission of quantum information and measurement-based entanglement of distant NV center registers. The long-term goal is to achieve a large scale quantum network where the different constituents each perform the task they are most well suited for: the stable nuclear spins store the quantum information, photons transfer quantum information over long distances, and the fast electron spin interfaces between spin and photon states. For measurement-based entanglement of distant NV centers it is of primary importance to maximize the emission and detection of coherent photons emitted by the NV center. Spontaneous emission can be controlled and enhanced by coupling an emitter to a photonic cavity. Chapters 3, 4, and 5 of this thesis describe experiments aimed at maximizing the coherent photon emission rate of an NV center by coupling the NV center to a photonic crystal cavity. One of the main challenges is to position a single NV center into the nanometer-sized cavity mode. Chapter 3 describes the development of a nanopositioning technique, which allows diamond nanocrystals ~50 nm in size containing single NV centers to be placed at arbitrary locations on a chip, with nanometer precision. By using a sharp etched tungsten tip, individual nanocrystals can be picked up, moved and placed under real-time imaging with an electron microscope. We explicitly demonstrate that the unique optical and spin properties of the NV center are conserved by the nanopositioning process. In chapter 4, we apply the nanopositioning technique to couple single NV centers contained in a diamond nanocrystals to gallium phosphide photonic crystal cavities. These cavities resonate in the visible with high quality factors, and are designed to have the lowest energy mode coincide with the zero-phonon line of the NV center. Efficient coupling is evidenced by a strong enhancement of NV center emission at the cavity wavelength. The high quality factor of a photonic crystal cavity is a result of careful design and fabrication. The introduction of a nanocrystal may therefore be expected to have a detrimental effect on the optical properties of a cavity. In chapter 5 we investigate the effect of a nanocrystal on the optical properties of photonic crystal cavities. Our simulations and measurements show that the effect of a nanocrystal is in fact only minor, a promising result for future work on cavity-QED systems in the solid state and with diamond defect centers in particular. Understanding and mitigating decoherence is a key challenge for quantum science and technology. The main source of decoherence for solid-state spin systems is the uncontrolled spin bath environment. Chapter 6 describes experiments that exploit quantum control of the NV center and recently developed dynamical decoupling techniques to study the properties of the surrounding bath of spins belonging to single substitutional nitrogen atoms. The coherence properties of a single NV center are affected by changes in the state of the bath spins, allowing a single NV center to be used as a probe to detect bath spin resonances and study the quantum dynamics of the spin bath. We use quantum control of the spin bath to extend the dephasing time of the NV center, important for the application of the NV center in DC magnetometry. In chapter 7 we describe the development and implementation of decoherence protected quantum gates for the hybrid spin register formed by the NV center electron spin that is coupled to the nuclear spin of its own nitrogen atom. Electron and nuclear spins evolve and decohere at vastly different rates, making it challenging to use such a hybrid system as a fully functional quantum register. We create a universal set of two-qubit quantum gates by integrating dynamical decoupling techniques into the gate operation. We demonstrate the power of our gate design by implementing for the first time Grover's quantum search algorithm on a solid-state spin register.