Optically active point defects in crystals have gained widespread attention as photonic systems that can find use in quantum information technologies. However challenges remain in the placing of individual defects at desired locations, an essential element of device fabrication. Here we report the controlled generation of single nitrogen-vacancy (NV) centres in diamond using laser writing. The use of aberration correction in the writing optics allows precise positioning of vacancies within the diamond crystal, and subsequent annealing produces single NV centres with up to 45% success probability, within about 200 nm of the desired position. Selected NV centres fabricated by this method display stable, coherent optical transitions at cryogenic temperatures, a pre-requisite for the creation of distributed quantum networks of solid-state qubits. The results illustrate the potential of laser writing as a new tool for defect engineering in quantum technologies.Comment: 21 pages including Supplementary informatio

The zero-phonon line (ZPL) at 1.68 eV has been attributed to the negatively charged silicon split-vacancy center in diamond, (Si-V) − , and has been extensively characterized in the literature. Computational studies have predicted the existence of the neutral charge state of the center, (Si-V) 0 , and it has been experimentally observed using electron paramagnetic resonance (EPR). However, the optical spectrum associated with (Si-V) 0 has not yet been conclusively identified. In this paper the 1.31 eV band visible in luminescence and absorption is attributed to (Si-V) 0 using an approach which combines optical absorption and EPR measurements. The intensities of both 1.68 eV and 1.31 eV bands are found to increase in deliberately Si-doped chemical vapor deposition (CVD) grown diamond, and also after electron irradiation and annealing, suggesting the involvement of both Si and a vacancy in the centers. The 1.31 eV ZPL is unambiguously associated to Si by its shift to a lower energy when the dominant Si isotope is changed from 28 Si to 29 Si. Charge transfer between (Si-V) − and (Si-V) 0 induced via ultraviolet photoexcitation or heating in the dark allows calibration factors relating the integrated absorption coefficient of their respective ZPLs to the defect concentration to be determined. Preferential orientation of (Si-V) 0 centers in CVD diamond grown on {110}-oriented diamond substrates is observed by EPR. The (Si-V) 0 centers are shown to grow predominantly into CVD diamond as complete units, rather than by the migration of mobile vacancies to substitutional Si (Si S ) atoms. Corrections for the preferential alignment of trigonal centers for quantitative analysis of optical spectra are proposed and the effect is used to reveal that the 1.31 eV ZPL arises from a transition between the 3 A 2g ground state and 3 A 1u excited state of (Si-V) 0 . A simple rate equation model explains the production of (Si-V) 0 upon irradiation and annealing of Si-doped CVD diamond. In as-grown Si-doped diamond the (Si-V) defects only account for a fraction of the total silicon present; the majority being incorporated as Si S . The data show that both Si S and (Si-V) are effective traps for mobile vacancies.

Nitrogen is ubiquitous in both natural and laboratory-grown diamond, but the number and nature of the nitrogen-containing defects can have a profound effect on the diamond material and its properties. An ever-growing fraction of the supply of diamond appearing on the world market is now lab-grown. Here, we survey recent progress in two complementary diamond synthesis methodshigh pressure high temperature (HPHT) growth and chemical vapour deposition (CVD), how each is allowing ever more precise control of nitrogen incorporation in the resulting diamond, and how the diamond produced by either method can be further processed (e.g. by implantation and/or annealing) to achieve a particular outcome or property. The burgeoning availability of diamond samples grown under well-defined conditions has also enabled huge advances in the characterization and understanding of nitrogen-containing defects in diamondalone, and in association with vacancies, hydrogen and transition metal atoms. Amongst these, the negatively charged nitrogen-vacancy (NV −) defect in diamond is attracting particular current interest on account of the many new and exciting opportunities it offers for, e.g., quantum technologies, nanoscale magnetometry and biosensing. 2 Laboratory Based Synthesis of Diamond and Nitrogen-Containing Diamond 2.1 High Pressure High Temperature (HPHT) Methods. Nature was the inspiration for the HPHT method, by which diamond growth was demonstrated by Swedish company ASEA in 1953 (though not reported at that time) and subsequently by US company General Electric in 1955. 14 , 15 Most present-day HPHT synthesis exploits the temperature-gradient growth (TGG) method developed later in that decade. 16 However, it took many further years before the design and control of HPHT reactors yielded diamonds of