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 4 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 T2 ranging from 45.5 to 549 μs 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.

Negatively charged nitrogen-vacancy (NV) centers in diamond are promising magnetic field quantum sensors. Laser threshold magnetometry theory predicts improved NV center ensemble sensitivity via increased signal strength and magnetic field contrast. Here, we experimentally demonstrate laser threshold magnetometry. We use a macroscopic high-finesse laser cavity containing a highly NV-doped and low absorbing diamond gain medium that is pumped at 532 nm and resonantly seeded at 710 nm. This enables a 64% signal power amplification by stimulated emission. We test the magnetic field dependency of the amplification and thus demonstrate magnetic field–dependent stimulated emission from an NV center ensemble. This emission shows an ultrahigh contrast of 33% and a maximum output power in the milliwatt regime. The coherent readout of NV centers pave the way for novel cavity and laser applications of quantum defects and diamond NV magnetic field sensors with substantially improved sensitivity for the health, research, and mining sectors.

Single substitutional nitrogen atoms [Formula: see text] are the prerequisite to create nitrogen-vacancy (NV) centers in diamonds. They not only serve as the electron donors to create the desired NV− center and provide charge stability against photo-ionisation but also are the main source of decoherence. Therefore, precise and quick determination of [Formula: see text] concentration is a key advantage to a multitude of NV-related research in terms of material improvement as well as applications. Here, we present a method to determine the [Formula: see text] concentration based on absorption spectroscopy in the UV-Visible range and fitting the 270 nm absorption band. UV-Visible spectroscopy has experimental simplicity and widespread availability that bear advantages over established methods. It allows a rapid determination of [Formula: see text] densities, even for large numbers of samples. Our method shows further advantages in determining low concentrations as well as the ability to measure locally, which is highly relevant for diamonds with largely varying [Formula: see text] concentrations in a single crystal. A cross-check with electron paramagnetic resonance shows high reliability of our method and yields the absorption cross section of the 270 nm absorption band [Formula: see text] cm−1 ppm−1 (in common logarithm) or [Formula: see text] cm−1 ppm−1 (in natural logarithm), which serves as a reference to determine [Formula: see text] concentrations and makes our method applicable for others without the need for a known [Formula: see text]-reference sample and calibration. We provide a rapid, practical, and replicable pathway that is independent of the machine used and can be widely implemented as a standard characterization method for the determination of [Formula: see text] concentrations.

The nitrogen-vacancy colour centre in diamond is emerging as one of the most important solid-state quantum systems. It has applications to fields including high-precision sensing, quantum computing, single photon communication, metrology, nanoscale magnetic imaging and biosensing. For all of these applications, a high quantum yield of emitted photons is desirable. However, diamond samples engineered to have high densities of nitrogen-vacancy centres show levels of brightness varying significantly within single batches, or even within the same sample. Here we show that nearby nitrogen impurities quench emission of nitrogen-vacancy centres via non-radiative transitions, resulting in a reduced fluorescence quantum yield. We monitored the emission properties of nitrogen-vacancy centre ensembles from synthetic diamond samples with different concentrations of nitrogen impurities. All samples were irradiated with high energy electrons to create high densities of nitrogen-vacancy centres relative to the concentration of nitrogen impurities. While at low nitrogen densities of 1.81 ppm we measured a lifetime of 13.9 ns, we observed a strong reduction in lifetime with increasing nitrogen density. We measure a lifetime as low as 4.4 ns at a nitrogen density of 380 ppm. The change in lifetime matches a reduction in relative fluorescence quantum yield from 77.4 % to 32 % with an increase in nitrogen density from 88 ppm to 380 ppm, respectively. These results will inform the conditions required to optimise the properties of diamond crystals devices based on the fluorescence of nitrogen-vacancy centres. Furthermore, this work provides insights into the origin of inhomogeneities observed in high-density nitrogen-vacancy ensembles within diamonds and nanodiamonds.

Herein, the in situ generation of nitrogen‐vacancy (NV) centers in diamond during chemical vapor deposition (CVD) is investigated depending on the electric‐field strength at the sample position. The alteration of the electric‐field strength is induced by changing the resonance conditions within the resonator cavity while keeping the growth input variables constant. The electric‐field strength distribution is obtained by simulation results. During the growth experiments, optical‐emission spectroscopy data is collected, which shows the impact of the electric‐field strength on the radical concentrations within the plasma and the gas temperature. Through the reduction of the electric‐field strength, the synthesis of thick, high‐quality, nitrogen‐doped diamond without the formation of a polycrystalline rim around the sample edges and the twinning‐induced growth of polycrystalline grains was accomplished. Therefore, a reduced internal and more homogeneous stress distribution is achieved. Furthermore, significant influences on the in situ NV doping are discovered. In addition to a considerable gain of the in situ NV generation, also a major enhancement of the in situ incorporation efficiency of NV centers in comparison to centers up to almost 3% is observed. Depending on the application, this makes posttreatment processes for additional NV generation dispensable.