Yuta Masuyama

The negatively charged nitrogen vacancy (NV ⁻ ) center in diamond is a promising point defect for highly sensitive quantum sensing. The formation of high-density NV ⁻ centers is essential for improving sensitivity. We performed room-temperature electron beam irradiation (EBI) and annealing on nitrogen-doped high-pressure high-temperature diamond crystals, aiming to convert all substitutional nitrogen into NV structures by increasing the EBI fluence. While the N _s ⁰ to NV ^0/− conversion process dominated at low EBI fluences, in a high-fluence region, NV ^0/− center and N _s ^0/+ concentrations decreased with increasing EBI fluence, indicating the formation of unknown nitrogen-related defects such as the H3 center, which is a nitrogen and vacancy aggregation defect. Although the H3 centers were observed at high EBI fluence, their annealing temperature of 1375 ± 25 °C was lower than the typically reported temperatures over 1600 °C. We attribute this low-temperature formation to vacancy-mediated nitrogen diffusion and aggregation.

Solid-state quantum sensors offer the promising prospect of high-sensitivity sensing across various environments, including at room temperature and atmospheric pressure. Highly sensitive solid-state quantum sensors require long ensemble dephasing times (T2*) in mm-scale sample areas. This requirement demands a highly uniform bias magnetic field across all qubits, which has traditionally required large magnetic field sources such as Helmholtz coils or a pair of magnets. Meanwhile, to expand the range of applications of quantum sensors, portable sensors are required, but it is difficult to carry these large magnets. Balancing the miniaturization of the quantum sensor with high sensitivity is a significant challenge. In this study, we achieve a long ensemble dephasing time for nitrogen-vacancy (NV) centers in a large sample volume using a single, compact, and optimized magnet assembly. By using a magnet design derived from an inverse problem-based analysis, we reduced the magnet assembly volume to 0.23% of the Helmholtz coil volume. We achieve an ensemble dephasing time of 5.32 μs for the entire NV center ensemble across the sample thickness, comparable to that obtained with a standard Helmholtz coil setup. This dephasing is limited by strain-induced dephasing across the diamond sample thickness, with a dephasing rate of 188 kHz, which represents the state-of-the-art diamond quality for bulk quantum sensors. Our work paves the way for the miniaturization of high-sensitivity quantum sensors, enabling new sensor designs and the construction of compact, highly sensitive quantum sensor arrays.

Control of residual stress in diamond is crucial to improve the spin dephasing time (T2*) of nitrogen-vacancy centers for quantum applications. When the surface of a substrate used for homoepitaxial growth is crystallographically tilted at a slight angle away from a major crystal plane [e.g., (001)], the atomic structure of the surface at the homoepitaxial growth front changes, which in turn alters the homoepitaxial growth mode. As a result, the incorporated stress in the homoepitaxial diamond layer is also likely to vary. In this study, we investigated the effect of the substrate inclination-direction in the (001) plane, the so-called inclination-direction, on the residual stresses. The 400 μm-thick nitrogen-doped (001) diamond films were grown on the substrates with different inclination-directions, a [110] or a [100]. Mapping of the stress tensor revealed that the inclination-direction alters the nature of the residual stress: the [110] inclination-direction led to a dominant gradient in the axial stress (σxx + σyy + σzz) along the [001] direction, whereas the [100] inclination-direction led in shear stress (σxy, σyz, and σzx). Although the nature of the residual stress changed with the inclination-direction, the mean values of the measured T2* were comparable for both samples (1.25 μs for [110] and 1.26 μs for [100]). This result was consistent with the fact that the dephasing rate due to the stress in the CVD diamond film (0.25 MHz for [110] and 0.26 MHz for [100]) was comparable.

Color centers in wide band-gap semiconductors, which have superior quantum properties even at room temperature and atmospheric pressure, have been actively applied to quantum sensing devices. Characterizing the quantum properties of the color centers in the semiconductor materials and ensuring that these properties are uniform over a wide area are key issues for developing quantum sensing devices based on color centers. In this article, we have developed an optics design protocol optimized for evaluating the quantum properties of color centers and have used this design approach to develop a new microscopy system called columnar excitation fluorescence microscope (CEFM). The essence of this system is to maximize the amount of fluorescence detection of polarized color centers, which is achieved by large-volume and uniform laser excitation along the sample thickness with sufficient laser power density. This laser excitation technique prevents undesirable transitions to undesirable charge states and undesirable light, such as unpolarized color center fluorescence, while significantly increasing the color center fluorescence. This feature enables fast measurements with a high signal-to-noise ratio, making it possible to evaluate the spatial distribution of quantum properties across an entire mm-size sample without using a darkroom, which is difficult with typical confocal microscope systems.

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’.

Color centers in wide band-gap semiconductors, which have superior quantum properties even at room temperature and atmospheric pressure, have been actively applied to quantum sensing devices. Characterization of the quantum properties of the color centers in the semiconductor materials and ensuring that these properties are uniform over a wide area are key issues for developing quantum sensing devices based on color center. In this article, we will describe the principle and performance of a newly developed confocal microscope system with a long Rayleigh length (LRCFM). This system can characterize a wider area faster than the confocal microscope systems commonly used for color center evaluation.

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%.