Negatively charged nitrogen vacancy (NV−) centres in diamond crystals are promising colour centres for high-sensitivity quantum sensors. A long dephasing time (T2* > 10 μs) is essential for achieving increased sensitivity and higher uniformity of T2* in millimetre-scale diamond is strongly desired for femto-tesla weak magnetic field detection. High uniformity of T2* for NV− centres is achieved herein. The median value of T2*, <T2*>, in the 12C-enriched high-pressure, high-temperature (HPHT) grown diamond with a nitrogen concentration of 1.3 ± 0.4 ppm is 4.5 μs. The variance of T2* is only 10% over a millimetre-scale region (1.1 × 1.1 mm2) within the 0.4 mm thick {111} growth sector. <T2*> is ~2/3 times the value limited by the dipole-dipole interaction from the electron-spin bath of nitrogen impurities, suggesting that the residual strain gradient in the HPHT diamond crystal partially limits T2*. Reducing the strain gradient in diamond crystals provide a pathway to achievement of high sensitivity magnetometry using NV quantum sensing. Negatively charged nitrogen vacancy centres in diamond crystals are promising for achieving high-sensitivity quantum sensors. Here, a long dephasing time with high uniformity is achieved in a millimetre-scale area along the {111} plane using 12C enriched high-pressure high-temperature diamond crystals.
Negatively charged nitrogen vacancy (NV−) centres in diamond crystals are promising colour centres for high-sensitivity quantum sensors. A long dephasing time (T2* > 10 μs) is essential for achieving increased sensitivity and higher uniformity of T2* in millimetre-scale diamond is strongly desired for femto-tesla weak magnetic field detection. High uniformity of T2* for NV− centres is achieved herein. The median value of T2*, <T2*>, in the 12C-enriched high-pressure, high-temperature (HPHT) grown diamond with a nitrogen concentration of 1.3 ± 0.4 ppm is 4.5 μs. The variance of T2* is only 10% over a millimetre-scale region (1.1 × 1.1 mm2) within the 0.4 mm thick {111} growth sector. <T2*> is ~2/3 times the value limited by the dipole-dipole interaction from the electron-spin bath of nitrogen impurities, suggesting that the residual strain gradient in the HPHT diamond crystal partially limits T2*. Reducing the strain gradient in diamond crystals provide a pathway to achievement of high sensitivity magnetometry using NV quantum sensing. Negatively charged nitrogen vacancy centres in diamond crystals are promising for achieving high-sensitivity quantum sensors. Here, a long dephasing time with high uniformity is achieved in a millimetre-scale area along the {111} plane using 12C enriched high-pressure high-temperature diamond crystals.
Negatively charged nitrogen-vacancy (NV − ) centres formed in diamond crystals are point defects that have potential applications in various quantum devices such as highly sensitive magnetic sensors. To improve the sensitivity of magnetic sensors using NV − centres, it is essential to precisely control the nitrogen concentration in the crystals. In this paper, we demonstrated that nitrogen concentration in diamond can be controlled with high precision for the following two representative growth methods. One is the high-pressure/high-temperature (HPHT) synthesis method and the other is the chemical vapour deposition (CVD) method. The nitrogen concentration of HPHT-grown diamond decreased semi-logarithmically with increasing contents of titanium or aluminium as nitrogen getter materials. The nitrogen concentration of CVD-grown diamond increased linearly with increasing the flow rate ratio of nitrogen to carbon. NV − centres were formed by controlling the total fluence of electron beams so that approximately 20% of the nitrogen became NV − centres. The coherence time of electron spin of NV − centres obtained by the Hahn-echo pulse sequence T 2 of these diamond crystals was inversely proportional to the nitrogen concentration. A comparison of T 2 of the NV − centres for HPHT-synthesized and CVD-grown diamonds showed no significant difference between them. This article is part of the Theo Murphy meeting issue ‘Diamond for quantum applications’.
Ensembles of nitrogen-vacancy (NV) centers in diamond are versatile quantum sensors with broad applications in the physical and life sciences. The concentration of neutral substitutional nitrogen ( [ N s 0 ]) strongly influences NV electronic spin coherence times, sensitivity, and optimal sensing strategies. Diamonds with [ N s 0 ] ∼ 1–10 ppm are a focus of recent material engineering efforts, with higher concentrations being favorable for continuous-wave optically detected magnetic resonance (CW-ODMR) and lower concentrations expected to benefit pulsed magnetometry techniques through extended NV spin coherence times and improved sensing duty cycles. In this work, we synthesize and characterize low- [ N s 0 ] ( ∼ 0.8 ppm), NV-enriched diamond material, engineered through low-strain chemical vapor deposition (CVD) growth on high-quality substrates, 12 C isotopic purification, and controlled electron irradiation and annealing. Our results demonstrate good strain homogeneity in diamonds grown on CVD substrates and spin-bath-limited NV dephasing times. By measuring NV spin and charge properties across a wide range of optical NV excitation intensity, we provide direct comparisons of photon-shot-noise-limited magnetic field sensitivity between the current low- [ N s 0 ] and previously studied higher- [ N s 0 ] ( ∼ 14 ppm) NV-diamond sensors. We show that low- [ N s 0 ] diamond can outperform higher- [ N s 0 ] diamond at moderate and low optical NV excitation intensity. Our results provide practical benchmarks and guidance for selecting NV-diamond sensors tailored to specific experimental constraints and sensing requirements.
The sensitivity of magnetic and electric field sensors based on nitrogen-vacancy (NV) center in diamond strongly depends on the available concentration of NV and their coherence properties. Achieving high coherence times simultaneously with high concentration is a challenging experimental task. Here, we demonstrate that by using a temperature gradient method of high-pressure, high-temperature growing technique, one can achieve nearly maximally possible dephasing times, limited only by carbon nuclear spins at low nitrogen concentrations or nitrogen electron spin at high nitrogen concentrations. Hahn-echo T 2 coherence times were also investigated and found to demonstrate reasonable values. Thus, the high-pressure, high-temperature technique is a strong contender to the popular chemical vapor deposition method in the development of high-sensitivity, diamond-based sensors.
To improve the sensitivity of magnetic sensors using negatively charged nitrogen-vacancy (NV−) centers, it is essential to improve the quality of diamond crystals and precisely control the nitrogen concentration in the crystals. We have utilized high-pressure, high-temperature synthesis [1] and chemical vapor deposition [2] for the growth of nitrogen-doped diamond. The concentration of substitutional nitrogen in the neutral charge state [NS0] were evaluated by electron paramagnetic resonance (EPR) measurements. As shown in Fig. 1, [NS0] were precisely controlled by the weight ratio of nitrogen getter material in the metal solvent for the HPHT method (Fig. 1(a)) and by the carbon to nitrogen gas ratio for the CVD method (Fig. 1(b)). Then, the diamond crystals were irradiated with electron beams followed by vacuum annealing so that 10% of the nitrogen at the substitutional site became NV− centers, and the negatively charged NV− centers were successfully formed preferentially [3]. T2 of these diamond crystals was inversely proportional to the nitrogen concentration. The product of T2 and nitrogen concentration of the obtained HPHT diamond crystal is slightly higher than the maximum value reported so far [4], suggesting that the crystal is expected to have high magnetic sensitivity.
Abstract With their optical addressability of individual spins and long coherence time, nitrogen-vacancy (NV) centers in diamond are often called “atom-like solid spin-defects”. As observed with trapped atomic ions, quantum interference mediated by indistinguishable photons was demonstrated between remote NV centers. In high sensitivity DC magnetometry at room temperature, NV ensembles are potentially rivaling with alkali-atom vapor cells. However, local strain induces center-to-center variation of both optical and spin transitions of NV centers. Therefore, advanced engineering of diamond growth toward crystalline perfection is demanded. Here, we report on the synthesis of high-quality HPHT (high-pressure, high-temperature) crystals, demonstrating a small inhomogeneous broadening of the spin transitions, of T 2 * = 1.28 μs, approaching the limit for crystals with natural 13 C abundance, that we determine as T 2 * = 1.48 μs. The contribution from strain and local charges to the inhomogeneous broadening is lowered to ~17 kHz full width at half maximum for NV ensemble within a > 10 mm 3 volume. Looking at optical transitions in low nitrogen crystals, we examine the variation of zero-phonon-line optical transition frequencies at low temperatures, showing a strain contribution below 2 GHz for a large fraction of single NV centers.
Abstract Single negatively charged nitrogen vacancy (NV−) centers in diamond have emerged as promising platforms for quantum information science, where long coherence times are essential for advancing quantum technologies. However, traditional fabrication methods often introduce lattice damage during the irradiation process used to create vacancies, significantly impairing the spin coherence properties of NV− centers. In this study, we systematically investigate high-pressure/high-temperature (HPHT) annealing as a non-destructive approach to generate high-coherence single NV− centers and elucidate the underlying mechanisms. Our results reveal that the distortion of the lattice field significantly influences the preferential formation of NV− centers at the interface of the 〈111〉-growth sector during diamond crystallization. Moreover, HPHT annealing effectively mobilizes nitrogen and vacancies, significantly promoting the formation of single NV− centers. This process also mitigates the influence of the distortion of the lattice field, resulting in a marked enhancement of coherence properties, with T 2 times reaching 578 μs. Notably, a naturally formed single NV− center in a low-stress region achieves an exceptional T 2 time of 721 μs, representing one of the highest values reported for diamonds with natural 13C abundance. Building on these insights, we have successfully achieved controlled, large-scale fabrication of high-coherence single NV− centers, providing essential guidance for future efforts to further enhance their coherence and establishing a robust foundation for their application in quantum technologies.