Atomic‐scale impurity spins, also called color centers, in an otherwise spin‐free diamond host lattice have proven to be versatile tools for applications in solid‐state‐based quantum technologies ranging from quantum information processing (QIP) to quantum‐enhanced sensing and metrology. Due to its wide band gap, diamond can host hundreds of different color centers. However, their suitability for QIP or sensing applications has only been tested for a handful of these, with the nitrogen vacancy (NV) strongly dominating this field of research. Due to its limited optical properties, the success of the NV for QIP applications however strongly depends on the development of efficient photonic interfaces. In the past years the negatively charged silicon vacancy (SiV−) center received significant attention due to its highly favourable spectral properties such as narrow zero phonon line transitions and weak phonon sidebands. Here, the recent work investigating the SiV center's orbital and electron spin coherence properties is reviewed as well as techniques to coherently control its quantum state using microwave as well as optical fields. Also, potential future experimental directions to improve the SiV's coherence time scale and to develop it into a valuable tool for QIP applications are outlined.
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.
One of the most common impurities in synthetic diamond is single substitutional nitrogen, which is incorporated in the diamond lattice substituting a carbon atom. If the nitrogen is adjacent to a vacancy in the diamond lattice, it forms the nitrogen-vacancy (NV) colour centre (CC). The negatively charged state of this CC, NV-, is particularly well studied since its quantum properties are suitable for applications such as quantum information processing, single-photon sources and optical magnetometry. NV CCs in the neutral state (NV0) are less widely studied. This CC exhibits broadband luminescence at slightly shorter wavelengths than NV-, and hence is also potentially of interest for tuneable and ultrafast visible laser applications. In this report, we present a detailed study of the laser-related spectroscopic properties of a diamond containing NV0 and NV-CCs.
The physical properties of diamond crystals, such as color or electrical conductivity, can be controlled via impurities. In particular, when doped with nitrogen, optically active nitrogen-vacancy centers (NV), can be induced. The center is an outstanding quantum spin system that enables, under ambient conditions, optical initialization, readout, and coherent microwave control with applications in sensing and quantum information. Under optical and radio frequency excitation, the Zeeman splitting of the degenerate states allows the quantitative measurement of external magnetic fields with high sensitivity. This study provides a pedagogical introduction to the properties of the NV centers as well as a step-by-step process to develop and test a simple magnetic quantum sensor based on color centers with significant potential for the development of highly compact multisensor systems.
The development of reliable devices to generate single photons is crucial for applications in quantum cryptography, as well as for fundamental quantum optics experiments. Due to their extremely high photostability at room temperature, optically active defects in solids, the so called color centers, are among the most promising candidates. Single NV (nitrogen-vacancy) centers in diamond have been demonstrated to be able to generate single photons and have already shown advantages compared to attenuated laser pulses in a quantum cryptography experiment. The nitrogen-vacancy center (NV center) in diamond consists of a substitutional nitrogen atom and an adjacent vacancy. This complex exhibits an optical transition with a fluorescence lifetime of 11.6 ns. The room temperature spectrum of the NV center consists of the zero-phonon line at 637 nm and broad phononic sidebands with a FWHM of more than 100 nm. Due to the existence of a shelving level the efficiency of the NV center as a single photon source is limited. Additionally, the high diffraction index of bulk diamond restricts the efficiency to collect the fluorescence light.