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 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.

We investigated charge states of nitrogen vacancy (NV) centers in diamond single crystals. The charge states were evaluated using concentrations of both negatively charged (NV⁻) and neutral-state (NV⁰) NV centers. This study specifically examines diamond crystals containing mainly neutral substitutional nitrogen (N⁰_s), NV⁻ centers, and NV⁰ centers. Results show that the charged states of NV centers evaluated for this study depended on concentrations of both neutral substitutional nitrogen (N⁰_s) and NV^T centers, which is a sum of [NV⁻] and [NV⁰]. The NV centers were fully negatively charged when the ratio of [NV^T] to [N⁰_s] was 1%–20% in our diamond crystals. When [NV^T]/[N⁰_s] is larger than 20%–30%, NV⁰ centers appeared gradually. The concentration ratio between NV⁻ centers and NV⁰ centers was described using an equilibrium equation.

We investigated spin-echo coherence times T₂ of negatively charged nitrogen vacancy center (NV⁻) ensembles in single-crystalline diamond synthesized by either the high-pressure and high-temperature and chemical vapor deposition methods. This study specifically examined the magnetic dipole–dipole interaction (DDI) from the various electronic spin baths, which are the source of T₂ decoherence. Diamond samples with NV⁻ center concentration [NV⁻] comparable to those of neutral substitutional nitrogen concentration [N_s⁰] were used for DDI estimation. Results show that the T₂ of the ensemble NV⁻ center decreased in inverse proportion to the concentration of nitrogen-related paramagnetic defects [N_PM], being the sum of [N_s⁰], [NV⁻], and [NV⁰], which is a neutrally charged state NV center. This inversely proportional relation between T₂ and [N_PM] indicates that the nitrogen-related paramagnetic defects of three kinds are the main decoherence source of the ensemble NV⁻ center in the single-crystalline diamond. We found that the DDI coefficient of NVH⁻ center was significantly smaller than that of N_s⁰, the NV⁰ center, or the NV⁻ center. We ascertained the DDI coefficient of the NV⁻ center $D_{\mathrm{N}{\mathrm{V}}^{-}}$ through experimentation using a linear summation of the decoherence rates of each nitrogen-related paramagnetic defect. The obtained value of 89 μs ppm for $D_{\mathrm{N}{\mathrm{V}}^{-}}$ corresponds well to the value estimated from the relation between DDI coefficient and spin multiplicity.

Electron beam irradiation into type-Ib diamond is known as a good method for the creation of high concentration negatively-charged nitrogen-vacancy (NV⁻) centers by which highly sensitive quantum sensors can be fabricated. In order to understand the creation mechanism of NV⁻ centers, we study the behavior of substitutional isolated nitrogen (P1 centers) and NV⁻ centers in type-Ib diamond, with an initial P1 concentration of 40–80 ppm by electron beam irradiation up to 8.0 × 10¹⁸ electrons/cm². P1 concentration and NV⁻ concentration were measured using electron spin resonance and photoluminescence measurements. P1 center count decreases with increasing irradiation fluence up to 8.0 × 10¹⁸ electrons/cm². The rate of decrease in P1 is slightly lower at irradiation fluence above 4.0 × 10¹⁸ electrons/cm² especially for samples of low initial P1 concentration. Comparing concentration of P1 centers with that of NV⁻ centers, it suggests that a part of P1 centers plays a role in the formation of other defects. The usefulness of electron beam irradiation to type-Ib diamonds was confirmed by the resultant conversion efficiency from P1 to NV⁻ center around 12–19%.

During the excitation–contraction coupling of the heart, sarcomeres are activated via thin filament structural changes (i.e., from the “off” state to the “on” state) in response to a release of Ca²⁺ from the sarcoplasmic reticulum. This process involves chemical reactions that are highly dependent on ambient temperature; for example, catalytic activity of the actomyosin ATPase rises with increasing temperature. Here, we investigate the effects of rapid heating by focused infrared (IR) laser irradiation on the sliding of thin filaments reconstituted with human α-tropomyosin and bovine ventricular troponin in an in vitro motility assay. We perform high-precision analyses measuring temperature by the fluorescence intensity of rhodamine-phalloidin–labeled F-actin coupled with a fluorescent thermosensor sheet containing the temperature-sensitive dye Europium (III) thenoyltrifluoroacetonate trihydrate. This approach enables a shift in temperature from 25°C to ∼46°C within 0.2 s. We find that in the absence of Ca²⁺ and presence of ATP, IR laser irradiation elicits sliding movements of reconstituted thin filaments with a sliding velocity that increases as a function of temperature. The heating-induced acceleration of thin filament sliding likewise occurs in the presence of Ca²⁺ and ATP; however, the temperature dependence is more than twofold less pronounced. These findings could indicate that in the mammalian heart, the on–off equilibrium of the cardiac thin filament state is partially shifted toward the on state in diastole at physiological body temperature, enabling rapid and efficient myocardial dynamics in systole.