WO2025182852A1 - Diamond spin sensor - Google Patents

Abstract

A diamond spin sensor (100) comprises a diamond including NV ^- centers having an electron spin. Where a lateral relaxation time of the electron spin measured by the Hahn echo method is denoted by T2 μsec, and the concentration of NV ^- centers in the diamond is denoted by Cppm, the product of T2 and C is larger than 65.

Description

Diamond Spin Sensor

This disclosure relates to a diamond spin sensor. This application claims priority to Japanese Application No. 2024-028000, filed February 28, 2024. The entire contents of that application are incorporated herein by reference.

Diamond spin sensors using NV centers (hereafter referred to as NV centers) in diamond are known. When an NV center, consisting of a nitrogen (i.e., N) occupying a substitutional position for a carbon (i.e., C) in diamond and a vacancy (i.e., V: Vacancy) adjacent to the nitrogen, becomes negatively charged, its ground state becomes a triplet state (i.e., spin S = 1). A negatively charged NV center is referred to as an NV ^- center, but for convenience, it will also be abbreviated as an NV center below. When a charged NV center is excited with laser light (i.e., green light) with a wavelength of approximately 530 nm, it emits fluorescence with a wavelength of approximately 635 nm (i.e., red light). The intensity of the fluorescence emission changes depending on the spin state of the NV center, and the spin state of the NV center changes due to magnetic resonance caused by a magnetic field applied to the NV center and microwaves or radio waves, so it can be used as a magnetic sensor.

Detection uses a diamond containing an NV center, which acts as a diamond spin sensor; an optical system that transmits excitation light from a light source and irradiates the NV center; and a transmission line and microwave circuit that transmit microwaves from a power source and irradiate the NV center. Additionally, an optical system is used that collects fluorescence from the NV center and transmits it to a photodetector.

Methods for reducing distortion and impurities in diamond crystals and producing high-purity diamonds are known. For example, Patent Document 1 below discloses that diamonds with low nitrogen content can be produced by adding a getter material such as titanium (i.e., Ti) to the raw material to treat nitrogen, the largest impurity. Patent Document 2 below discloses that crystal distortion can be reduced by cutting a portion of a diamond with few crystal defects to a size of 1 mm or less from a diamond of 3 mm or larger in size to serve as a seed crystal, and using this as a seed substrate for growth.

Japanese Unexamined Patent Publication No. 7-148426 Japanese Patent Application Publication No. 9-165295 International Publication No. 2022/210723 International Publication No. 2022/209512 International Publication No. 2016/013588

A diamond spin sensor according to one aspect of the present disclosure includes a diamond containing an NV ^- center having electron spin, and the product of T2 and C is greater than 65, where the transverse relaxation time of the electron spin measured by the Hahn echo method is T2 μsec and the concentration of the NV ^- center in the diamond is C ppm.

FIG. 1 is a perspective view illustrating a diamond spin sensor according to an embodiment of the present disclosure. FIG. 2 is a schematic diagram showing the crystal planes and orientations of a diamond. FIG. 3 is a block diagram showing the configuration of an apparatus used for measurements using the diamond spin sensor shown in FIG. FIG. 4 is a sequence diagram showing the timing of irradiation of excitation light and electromagnetic waves and the timing of measurement of synchrotron radiation during measurement using the diamond spin sensor shown in FIG. FIG. 5 is a graph showing a schematic relationship between the observed signal intensity (i.e., fluorescence intensity) and the frequency of the electromagnetic wave (i.e., microwave). FIG. 6 is a schematic diagram showing the NV center of a diamond. FIG. 7 is a sequence diagram showing the timing of irradiation of excitation light and electromagnetic waves for measuring the transverse relaxation time T2 of the diamond spin sensor shown in FIG. 1, and the timing of measurement of the emitted light. FIG. 8 is a graph showing the transverse relaxation time T2. FIG. 9 is a schematic diagram showing a diamond synthesis apparatus. FIG. 10 is a schematic diagram showing a method for producing a seed crystal used in diamond synthesis. FIG. 11 is a schematic diagram showing a method for synthesizing diamond using a diamond cut out from the synthetic diamond shown in FIG. 10 as a seed crystal. FIG. 12 is a schematic diagram showing a method for synthesizing diamond using a seed crystal larger than the seed crystal shown in FIG. FIG. 13 is a plan view showing the synthetic diamond shown in FIG. FIG. 14 is a plan view showing a conventional synthetic diamond. FIG. 15 is a table showing the manufacturing conditions of the experimental samples. FIG. 16 is a diagram showing the experimental results in a table format.

[Problem to be solved by the present disclosure]
For sensors using diamond NV centers (also called color centers), the longer the transverse spin relaxation time T2, the higher the sensor sensitivity, which is preferable. In other words, the longer the transverse relaxation time T2, the longer the time that the resonant electromagnetic waves are applied, and therefore the higher the sensor sensitivity. In order to lengthen the transverse relaxation time T2, it is possible to consider reducing the concentration of NV centers in diamond. However, if the NV centers, which are the source of fluorescence, decrease, the emission intensity weakens. As such, there is a trade-off between the transverse relaxation time T2 and the concentration of the NV centers, so it has been difficult to increase the transverse relaxation time T2 without reducing the concentration of the NV centers.

As disclosed in Patent Documents 1 and 2, it is easy to produce high-purity diamond crystals that are almost free of impurities and defects. However, to use diamonds containing NV centers, which require nitrogen and defects (vacancies), as sensors, it is necessary to remove factors that cause fluorescence scattering while leaving trace amounts of strain and impurities. In other words, a method is needed to reduce other impurities and strain while leaving trace amounts of nitrogen impurities and defects (vacancies).

Therefore, the present disclosure aims to provide a diamond spin sensor that can increase the transverse relaxation time without reducing the concentration of NV centers.

[Effects of the present disclosure]
According to the present disclosure, it is possible to provide a diamond spin sensor capable of increasing the transverse relaxation time without reducing the concentration of the NV center.

Description of the embodiments of the present disclosure
The contents of the embodiments of the present disclosure will be listed and explained below. At least some of the embodiments described below may be combined in any combination.

(1) A diamond spin sensor according to a first aspect of the present disclosure includes a diamond containing an NV ^- center having electron spin, and where the transverse relaxation time of the electron spin measured by the Hahn echo method is T2 μsec and the concentration of the NV ^- center in the diamond is C ppm, the product of T2 and C is greater than 65. This allows the transverse relaxation time T2 to be increased without reducing the concentration of the NV ^- center in the diamond.

(2) In the above (1), the product may be greater than 200. This allows the transverse relaxation time T2 to be increased without reducing the concentration of NV ⁻ centers in the diamond.

(3) In the above (1) or (2), the concentration of the NV ^-center may be 0.02 ppm or more and 10 ppm or less, and the average phase difference over the entire surface of the diamond may be 6 nm/mm or less. This allows the transverse relaxation time T2 to be further increased, resulting in a sensor with higher sensitivity.

(4) In (3) above, the average phase difference may be 4 nm/mm or less. This allows the transverse relaxation time T2 to be further increased, resulting in a more sensitive sensor.

(5) In any one of (1) to (4) above, the concentration of the NV ⁻ center may be 0.02 ppm or more and 1.2 ppm or less, thereby further increasing the transverse relaxation time T2 and realizing a sensor with higher sensitivity.

(6) In any one of (1) to (4) above, the concentration of NV ⁻ centers may be 0.02 ppm or more and 10 ppm or less, and the number of dislocation defects in the entire diamond detected by X-ray topography images may be 10 or less. This allows the transverse relaxation time T2 to be further increased, resulting in a sensor with higher sensitivity.

(7) In the above (6), the concentration of NV ⁻ centers may be 0.02 ppm or more and 2 ppm or less, and the number of dislocation defects may be 0. This allows the transverse relaxation time T2 to be further increased, resulting in a sensor with even higher sensitivity.

(8) In any one of the above (1) to (7), the ratio of the concentration of isolated vacancies in the diamond to the concentration of NV ⁻ centers may be 10% or less, thereby further increasing the transverse relaxation time T2 and realizing a sensor with higher sensitivity.

[Details of the embodiment of the present disclosure]
In the following embodiments, the same components are denoted by the same reference numerals, and their names and functions are also the same, so detailed descriptions thereof will not be repeated.

Referring to FIG. 1, the diamond spin sensor 100 according to an embodiment of the present disclosure is a rectangular parallelepiped having a first surface 102 and a first edge 104. The diamond spin sensor 100 may also be a rectangular parallelepiped with equal sides, i.e., a cube. The diamond spin sensor 100 is formed from a single crystal diamond containing an NV center composed of nitrogen (N) and vacancies (V). The first surface 102 is a crystal plane, such as the (001) or (111) plane. When the first surface 102 is a (001) plane, the first edge 104 is formed along the <100> or <010> direction. When the first surface 102 is a (111) plane, the first edge 104 is formed along the <1-10>, <10-1>, or <01-1> direction. Note that the notation "-1" corresponds to the crystal orientation notation of a 1 with a bar (horizontal bar) above it.

Although Figure 1 shows a rectangular parallelepiped diamond spin sensor 100, this is not limiting. The shape of the first surface 102 is not limited to a rectangle and can be any shape. The first surface 102 may be triangular, for example. In that case, the first side 104 corresponds to one side of the triangle. The three-dimensional shape of the diamond spin sensor 100 is not limited to a rectangular parallelepiped, but may also be a pyramid (such as a pyramid or a cone). For example, the diamond spin sensor 100 may be a tetrahedron (e.g., a corner cube with right-angled sides) in which the first surface 102 is triangular (e.g., an equilateral triangle).

With reference to Figure 2, among the crystal planes of diamond, the (001) plane is a plane defined by points A5 to A8 (i.e., a plane passing through these four points). If the first surface 102 is a (001) plane as described above, the diamond spin sensor 100 is realized, for example, as a cube with vertices from points A1 to A8. The <100> direction and <010> direction, which are directions that the first edge 104 can take, are the direction from point A1 to point A2 and the direction from point A1 to point A4, respectively. For example, if the first surface 102 is a plane with vertices from points A5 to A8, the first edge 104 corresponds to the line segment connecting points A5 and A6, or the line segment connecting points A5 and A8.

The (111) plane is a plane defined by points A5, A2, and A4. As described above, the first surface 102 may be the (111) plane. In that case, the <1-10> direction, which is a possible direction of the first side 104, is the direction from point A1 to point B1. That is, the first side 104 corresponds to the line segment connecting points A2 and A4. The <10-1> direction, which is a possible direction of the first side 104, is the direction from point A1 to point B2. That is, the first side 104 corresponds to the line segment connecting points A5 and A2. The <01-1> direction, which is a possible direction of the first side 104, is the direction from point A1 to point B3. That is, the first side 104 corresponds to the line segment connecting points A5 and A4.

Measurements using the diamond spin sensor 100 are performed, for example, by the device shown in Figure 3. The control unit 230 is equipped with a CPU (Central Processing Unit) and a memory unit (neither shown). The processing performed by the control unit 230 is realized by the CPU reading and executing a program pre-stored in the memory unit.

Under the control of the control unit 230, the excitation light generating unit 210 generates excitation light for exciting the NV center of the diamond spin sensor 100. The control unit 230, for example, supplies a voltage to the excitation light generating unit 210 at a predetermined timing to cause the excitation light generating unit 210 to emit light. The excitation light 204 is green light (i.e., wavelength 490 nm to 560 nm). The excitation light 204 is, for example, laser light, and the excitation light generating unit 210 is, for example, a semiconductor laser (e.g., emitted light wavelength 532 nm).

The filter 212 is an element for separating the excitation light 204 incident from the excitation light generator 210 and the light (i.e., fluorescent light) emitted from the diamond spin sensor 100. For example, the filter 212 is a filter that cuts out (i.e., reflects) light with wavelengths below a predetermined wavelength and passes light with wavelengths greater than the predetermined wavelength, or a bandpass filter that passes light with wavelengths within a predetermined wavelength range and cuts out (i.e., reflects) light with wavelengths outside the predetermined wavelength range. Generally, excitation light has a shorter wavelength than fluorescent light, so this configuration is preferable. For example, the filter 212 is a dichroic mirror with this function.

The focusing element 214 focuses the excitation light 204 input from the filter 212. The focusing element 214 is, for example, a spherical lens. The focusing element 214 inputs as much of the excitation light diffused and output from the excitation light generating unit 210 as possible into the end of the optical waveguide 216. The optical waveguide 216 includes a medium for transmitting light and transmits light in both directions. That is, the optical waveguide 216 has a first end and a second end, and transmits the excitation light 204 incident on the first end to the second end located near the diamond spin sensor 100. The optical waveguide 216 also transmits the emitted light (i.e., fluorescence) from the diamond spin sensor 100 incident on the second end to the first end and outputs it. The optical waveguide 216 is, for example, an optical fiber.

The LPF (Long Pass Filter) 218 is a long-pass filter that passes light with wavelengths equal to or greater than a predetermined wavelength and cuts out (e.g., reflects) light with wavelengths smaller than the predetermined wavelength. The fluorescent light 206, which is the emitted light of the diamond spin sensor 100, is red light and passes through the LPF 218. However, the excitation light 204 output from the excitation light generator 210 has a shorter wavelength and does not pass through the LPF 218. This prevents the excitation light 204 emitted from the excitation light generator 210 from being detected by the light detector 220 and becoming noise, which reduces the detection sensitivity of the fluorescent light 206, which is the emitted light of the diamond spin sensor 100. The light detector 220 generates and outputs an electrical signal corresponding to the incident light. The light detector 220 is, for example, a photodiode. The output signal from the light detector 220 is acquired by the control unit 230.

The electromagnetic wave irradiation unit 202 irradiates the diamond spin sensor 100 with electromagnetic waves (e.g., microwaves). The electromagnetic wave irradiation unit 202 is, for example, a coil or microwave resonant circuit formed including an electrical conductor. The electromagnetic waves are supplied from the electromagnetic wave generation unit 232 to the electromagnetic wave irradiation unit 202 via, for example, a coaxial cable. The irradiation of the excitation light and electromagnetic waves to the diamond spin sensor 100 is controlled by the control unit 230, and is performed, for example, at the timing shown in Figure 4.

Referring to Figure 4, the control unit 230 controls the excitation light generating unit 210 to output excitation light for a predetermined period (e.g., time interval t1) at a predetermined timing. The control unit 230 controls the electromagnetic wave generating unit 232 to output electromagnetic waves for a predetermined period (e.g., time interval t2) at a predetermined timing. Any appropriate pulse sequence can be used during time interval t2. This allows the excitation light and electromagnetic waves to be combined in time and space and irradiated onto the diamond. The control unit 230 takes in the output signal of the light detecting unit 220 at a predetermined timing (e.g., time interval t3) and stores it in the memory unit.

NV centers transition from the ground state to an excited state when exposed to green light with a wavelength of 490 nm to 560 nm (e.g., 532 nm laser light), and then return to the ground state by emitting red light with a wavelength of 630 nm to 800 nm (e.g., 635 nm fluorescence). When an NV center captures one electron (i.e., NV ^- ), it forms spin triplet states with magnetic quantum numbers m _s of -1, 0, and +1. In the presence of a magnetic field, the energy levels of the m _s = ±1 states split according to the magnetic field strength (i.e., Zeeman splitting). Microwaves with a frequency of 2.87 GHz are irradiated onto the NV center, causing the m _s = 0 state to transition to the m _s = ±1 state (i.e., electron spin resonance), and then the NV center is excited by irradiating it with green light. As a result, the transitions returning to the ground state include transitions that do not emit light (i.e., fluorescence), and the intensity of the observed emitted light decreases. Therefore, a valley (i.e., a drop in the signal) is observed in the ODMR (Optical Detected Magnetic Resonance) spectrum.

As described above, the control unit 230 controls the excitation light generating unit 210 and the electromagnetic wave generating unit 232 to measure a spectrum such as that shown in Figure 5. The frequency difference Δf, which is the distance between the two observed valleys, depends on the magnetic field strength at the position of the diamond spin sensor 100 (corresponding to Zeeman splitting). The control unit 230 can calculate the magnetic field from the frequency difference Δf. Referring to Figure 6, the magnetic field detected by the NV center is the component in the direction of the axis passing through the N and V of the NV center formed in the diamond (hereinafter referred to as the NV axis). In other words, where φ is the angle between the magnetic field vector B and the NV axis, the diamond spin sensor 100 detects a change in signal intensity (i.e., fluorescence intensity) according to B cosφ.

As explained above, the magnetic field can be calculated from changes in the ODMR spectrum, but it is known that the center frequency of the two resonant frequencies of the NV center has temperature dependence in the range from 120 K to 700 K. Therefore, temperature can be measured by using the diamond spin sensor 100 to measure the change in the center frequency split into Δf.

In the diamond spin sensor 100, the transverse relaxation time of the electron spin of the NV center measured by the Hahn echo method described below is T2 μsec, and the concentration of the NV center in the diamond measured by electron spin resonance or the like (the ratio of the number of NV centers to the number of carbon atoms) is C ppm, and the product α (α = T2 × C) of the transverse relaxation time T2 and the NV ^- center concentration C is greater than 65. This makes it possible to increase the transverse relaxation time T2 without reducing the concentration of the NV ^- center in the diamond spin sensor 100. Therefore, it is possible to suppress the decrease in fluorescence intensity and realize a sensor with higher sensitivity than conventional sensors.

The Hahn echo method, which is used to measure the transverse relaxation time T2, is included in the electron spin echo method (hereinafter referred to as the spin echo method) described below. The spin echo method is a type of ESR (Electron Spin Resonance) measurement. In a typical ESR measurement, microwaves are continuously irradiated while an external magnetic field is applied, and microwave absorption is observed (CW (Continuous Wave)-ESR). In contrast, the spin echo method excites electron spins with microwave pulses and measures their relaxation. The spin echo method can measure the physical quantities of the spin-lattice relaxation time T1 and the spin-spin relaxation time T2. The spin-lattice relaxation time T1 is also called the longitudinal relaxation time T1. The transverse relaxation time T2 mentioned above refers to the spin-spin relaxation time T2.

In spin echo spectroscopy, microwaves are not applied continuously, but rather microwave pulses that rotate the spins by θ° are applied in several increments. The amount of spin rotation (i.e., rotation angle θ) is determined by the microwave intensity and application time. In many cases, the operation is θ1-τ-θ2, where a microwave pulse rotates the spins by θ1°, and after a time τ, another microwave pulse rotates the spins by θ2°. For example, θ1 = 90°, θ2 = 180°.

Consider the behavior of spins in a rotating coordinate system that rotates at the Larmor frequency. The first pulse causes spins aligned along the Z axis (the direction of the magnetic field) to rotate θ1° from the Z axis toward the XY plane. All spins rotate in the same direction around the Z axis, but due to local magnetic field fluctuations, the rotation speed of each spin around the Z axis after a θ1° rotation is slightly different, and over time, the direction of each spin gradually shifts from its position immediately after a θ1° rotation. In other words, a phase delay or advance occurs in the spins. The dispersion of this shift increases in proportion to time τ (i.e., the elapsed time). If a microwave pulse that rotates θ2° is then added, each dispersed spin rotates θ2° in the same direction as when the spins were previously rotated θ1°. After a θ2° rotation, all spins still rotate in the same direction around the Z axis, and similarly, the rotation speed of each spin around the Z axis differs; those that were rotating slowly now rotate slower, and those that were rotating fast now rotate faster. However, the starting position of each spin's rotation around the Z axis (the phase immediately after a θ2° rotation) is opposite to that immediately before the θ2° rotation. In other words, spins that were in a delayed phase due to a slow rotation speed now have an advanced phase, and spins that were in an advanced phase due to a fast rotation speed now have a delayed phase. Therefore, after a θ2° rotation, the phase difference between the spins decreases over time. As a result, the spins align after the passage of time τ. This allows a spin echo (hereafter referred to as an echo) to be detected as a signal due to the spins.

When measuring the relaxation time of NV centers in diamond, microwave pulses are applied in the order of 90°-τ-180°-τ-90° to read the fluorescence intensity due to excited electron spins. The transverse relaxation time T2 is measured, for example, using the spin echo pulse sequence shown in Figure 7. The method of observing signals using such a pulse sequence is called the Hahn echo method. Pulse P1 and pulse P3 are each pulses that rotate the electron spin of the NV center by 90° (π/2) as described above. Pulse P2 is a pulse that rotates the electron spin of the NV center by 180° (π) as described above. Pulses P1, P2, and P3 are applied to the diamond spin sensor 100 with the same time interval τ between them.

As mentioned above, spin echoes are caused by localized fluctuations in the magnetic field. These fluctuations are caused by factors such as inhomogeneity in the pulsed magnetic field, spin-nuclear interactions, and spin-spin dipole interactions. The relaxation time can be determined from the magnitude of decay in echo intensity (fluorescence intensity) as the time interval τ changes. Specifically, by repeatedly measuring echoes using the pulse sequence shown in Figure 7 and plotting the echo intensity (peak value of the echo signal) against time, a graph such as that shown in Figure 8 is obtained. In Figure 8, the solid line schematically represents the measured value (fluorescence intensity). The vertical axis is expressed in arbitrary units (au (arbitrary unit)). The horizontal axis is 2τ (twice the time interval τ). The dashed line is a graph obtained by fitting the measured value graph using an exponential function. The transverse relaxation time T2 is the value of time τ when the value of the exponential function becomes 1/e of its initial value. In other words, the transverse relaxation time T2 represents the time over which the measurement signal is sustained, and the longer the transverse relaxation time T2, the longer the signal can be measured.

The transverse relaxation time T2 mentioned above increases as the number of NV centers decreases. The transverse relaxation time T2 is affected by defects, strain, and impurities (other than nitrogen) in the crystal. Defects, strain, and impurities (other than nitrogen) in the crystal act as disturbances on the transverse relaxation time T2, and it is thought that the transverse relaxation time T2 is inversely proportional to the amount of defects, strain, and impurities (other than nitrogen) (T2 ∝ 1/X). Therefore, the product α (T2 x C) of the transverse relaxation time T2 and the NV center concentration C increases as the number of defects, strain, and impurities (other than nitrogen) in the diamond crystal decreases. If the product α can be increased, a more sensitive sensor can be achieved.

The examples described below show that a diamond spin sensor with a larger product α of the transverse relaxation time T2 (unit: μsec) and the NV center concentration C (unit: ppm) can be realized. That is, the product α of the transverse relaxation time T2 and the NV center concentration C is greater than 65. This allows for a highly sensitive sensor to be realized. The product α may be greater than 120, greater than 160, or greater than 200. The product α may be greater than 250 or greater than 300. The larger the product α, the more sensitive the sensor that can be realized. That is, in a diamond spin sensor, the transverse relaxation time T2 can be increased without reducing the NV center concentration C.

In the diamond spin sensor 100 shown in Figure 1, the NV center concentration may be 0.02 ppm or more and 10 ppm or less. Furthermore, the average phase difference over the entire surface of the diamond spin sensor 100 may be 6 nm/mm or less. As will be described later, the average phase difference represents defects and distortion in the diamond. Having an average phase difference of 6 nm/mm or less reduces defects and distortion in the diamond, and increases the longitudinal relaxation time T2. Such an NV center concentration and average phase difference increases the product α of the relaxation time T2 and the NV center concentration C. Therefore, in the diamond spin sensor, the transverse relaxation time T2 can be increased without reducing the NV center concentration C, resulting in a sensor with higher sensitivity than conventional sensors.

The NV center concentration may be 0.02 ppm or more and 2 ppm or less. The NV center concentration may also be 0.02 ppm or more and 1.2 ppm or less. This allows the transverse relaxation time T2 to be further increased, resulting in a more sensitive sensor. The NV center concentration may also be 0.04 ppm or more and 5 ppm or less. The NV center concentration may also be 0.08 ppm or more and 0.5 ppm or less.

The concentration of NV centers in diamond can be calculated from measurements using, for example, electron spin resonance (CW-ESR). Also, for low concentrations, it can be measured by observing with a fluorescence microscope and counting individual NV centers. For high concentrations, for diamonds containing low concentrations of NV centers, the conversion factor between concentration and fluorescence intensity can be found, and this can be used to calculate the concentration by converting from the fluorescence intensity ratio. Furthermore, the concentration of NV centers can also be calculated from the absorption coefficient at 637 nm in the absorption spectrum measured by ultraviolet-visible absorption spectroscopy.

Explaining the average phase difference. Diamonds inherently have an isotropic crystal structure and an isotropic refractive index (dielectric constant). However, in reality, defects and distortions exist within single-crystal diamonds, resulting in birefringence. When circularly polarized light is irradiated onto a birefringent diamond, a phase difference occurs between the two orthogonal polarized light beams (linearly polarized light), resulting in the output of elliptically polarized light. The optical axis and phase difference can be determined from the orientation of the major and minor axes of the ellipse in the elliptically polarized light output from the diamond, as well as the ratio of the lengths of the major and minor axes. The measured phase difference is the integrated value in the direction in which light passes through the diamond (e.g., the thickness direction of the diamond). Therefore, the phase difference is normalized by the diamond's thickness, i.e., expressed as the phase difference converted to a thickness of 1 mm (unit: nm/mm). The phase difference is measured locally and is distributed two-dimensionally within the measurement surface. Therefore, the phase difference is expressed as the average value within the measurement surface (hereinafter referred to as the average phase difference). Note that "average value" does not mean the phase difference per area, but rather the value obtained by averaging the phase differences obtained by performing multiple local measurements over the surface, i.e., the average frequency distribution of the phase difference over the surface.

The average phase difference over the entire surface of the diamond spin sensor 100 may be 4 nm/mm or less. This allows a larger product α to be achieved, resulting in a more sensitive sensor. The average phase difference over the entire surface of the diamond spin sensor may also be 3 nm/mm or less, 2 nm/mm or less, or 1 nm/mm or less. This allows the transverse relaxation time T2 to be further increased, resulting in a more sensitive sensor.

As mentioned above, when the NV center concentration is between 0.02 ppm and 10 ppm, the number of dislocation defects detected in the entire diamond spin sensor 100 by X-ray topography imaging needs to be 10 or less. A dislocation defect is a defect in which the alignment of one or more crystals in the diamond is misaligned, resulting in a linear boundary with the non-misaligned area. Fewer dislocation defects in the diamond can increase the transverse relaxation time T2, resulting in a more sensitive sensor.

Growth sectors and dislocation defects in diamond can be detected by X-ray diffraction. Specifically, MoKα1 X-rays (characteristic X-rays of molybdenum with a wavelength of λ = 0.71 Å (0.071 nm)) are used as the X-rays, and X-ray topography images are taken with a Lang camera due to diffraction from the (220) plane of a single-crystal diamond. To obtain dislocation defects throughout the entire diamond, X-ray topography images are taken across the entire diamond substrate and linear defects are counted. To more clearly visualize the distribution of defects, it is preferable to process the sample into a thin plate approximately 0.5 mm thick. For example, a single-crystal diamond can be cut into thin plates using a laser processing machine, and the cut surface can be flattened by skiff polishing. Alternatively, a slit can be placed next to the X-ray source to obtain X-ray topography images using diffracted rays from only a limited layer within the sample (tomographic topography: limited projection topography). This allows relatively thick samples to be evaluated for defects without having to be processed into thin plates.

The number of dislocation defects detected throughout the diamond using X-ray topography images may be seven or fewer. The number of dislocation defects detected throughout the diamond using X-ray topography images may be five or fewer, or may be three or fewer.

Furthermore, as mentioned above, when the concentration of NV centers is between 0.02 ppm and 2 ppm, the number of dislocation defects detected in the entire diamond by X-ray topography images needs to be zero (i.e., no dislocation defects are detected). This allows the transverse relaxation time T2 to be further increased, resulting in an even more sensitive sensor.

In the diamond spin sensor 100 shown in FIG. 1 , the ratio of the concentration of isolated vacancies to the concentration of NV ^-centers may be 10% or less. Isolated vacancies refer to vacancies in a state where no nitrogen is present around the vacancies. Isolated vacancies include uncharged vacancies and negatively charged vacancies. Therefore, the concentration ratio can be calculated as the ratio of the sum of the number of uncharged vacancies and the number of negatively charged vacancies to the number of NV ^-centers . This allows for a longer transverse relaxation time T2, resulting in a more sensitive sensor. The vacancy density (concentration) is the sum of the density of neutral isolated vacancies (V ⁰ ) and the density of negatively charged isolated vacancies (V ^- ). The density of each isolated vacancy can be calculated from the integrated absorption (wavelength integral of the absorption coefficient: unit meV×cm ^-1 ) of light at wavelengths of 741 nm and 394 nm, respectively, at liquid nitrogen temperature. The proportionality coefficient k (k=A/β) between the integrated absorption A and the density β (unit: cm ⁻³ ) is 1.2×10 ⁻¹⁶ and 4.8×10 ⁻¹⁶ , respectively. Furthermore, when the vacancy density cannot be calculated using the absorption coefficient in the visible region, it can be calculated by positron annihilation. The relative density value obtained by positron annihilation can be converted into density even in low concentration regions by performing calibration (proportional calculation) using the value in the region where density can be obtained using the absorption coefficient as a reference.

In the diamond spin sensor 100, the ratio of the concentration of isolated vacancies to the concentration of NV ¹ -centers may be 3% or less. Also, in the diamond spin sensor 100, the ratio of the concentration of isolated vacancies to the concentration of NV ¹ -centers may be 1% or less, or may be 0.3% or less. By reducing the ratio of ^the concentration of isolated vacancies to the concentration of NV 1 -centers, an even larger product α can be achieved, resulting in a sensor with even higher sensitivity.

(Manufacturing method of diamond spin sensor)
The following describes a method for manufacturing the diamond spin sensor 100 shown in Fig. 1. Using granular diamond as a seed crystal, a synthetic diamond is produced by a temperature difference method under high pressure, and a portion of the diamond is selected and cut out to be used as a seed crystal in subsequent processes.

Figure 9 shows the configuration of an apparatus for synthesizing diamond by the temperature gradient method under high pressure. In the temperature gradient method, crystals are grown by utilizing the difference in solubility of diamond in a solvent caused by a temperature difference. Referring to Figure 9, a vertical temperature gradient is created within a pressure medium 250 equipped with a graphite heater 252 and an insulating member 254. The insulating member 254 is placed in the high-temperature section, and the seed crystal 300 is placed in the low-temperature section, with the solvent metal 258 placed between them. Single-crystal diamond is grown on the seed crystal 300 by maintaining conditions above the pressure at which diamond becomes thermally stable at a temperature above the temperature at which the solvent metal 258 melts. Diamond powder is preferably used as the carbon source 256. Graphite or pyrolytic carbon may also be used as the carbon source 256. The solvent metal 258 is made of one or more metals selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), and manganese (Mn), or an alloy containing these metals. Pressure is applied by pressure medium 250, which receives external force from a high-pressure generator (not shown), and heating is performed by graphite heater 252, achieving a pressure at which diamond is thermodynamically stable and a temperature condition at which solvent metal 258 is eutectic with carbon. Carbon dissolves into solvent metal 258 from carbon source 256 in the high-temperature section and is transported by diffusion to the low-temperature section below solvent metal 258, where a crystal grows on seed crystal 300, forming synthetic diamond 302.

(1st step)
First, a single crystal diamond with a nitrogen concentration of 30 ppm or less is prepared. In the above-mentioned high-pressure single crystal diamond synthesis method, adding a nitrogen getter to the solvent metal 258 allows for the production of a single crystal diamond with a nitrogen concentration of 30 ppm or less. This allows for the cutting out of a seed crystal with a single growth sector and reduced defects, with 10 or fewer dislocation defects detected by X-ray topography, in the second step described below. The growth sector can be confirmed by a two-dimensional fluorescence image (surface distribution image) of PL (photoluminescence: an emission image caused by ultraviolet irradiation) or CL (cathodoluminescence: an emission image caused by electron beam irradiation). That is, it can be determined by whether the boundary between regions with different fluorescence intensities is linear (see Patent Document 3).

Dislocations are measured, for example, by an etching test (see Patent Document 4). The etching test is performed as follows: A single crystal diamond is immersed in a potassium nitrate (KNO ₃ ) molten solution, which is an etching solution, and heated to 600°C in a platinum crucible for 1 hour (etching). After slow cooling, the single crystal diamond is removed and its surface is observed under an optical microscope at 50x magnification. The number of point-like etch pits is counted within a rectangular measurement area of 1000 μm x 1000 μm to obtain the number of point-like etch pits per mm ^2. Point-like etch pits refer to point-like depressions present on the surface of the single crystal diamond. Point-like etch pits correspond to dislocation defects. Point-like depressions are quadrangular, quadrangular with rounded corners, or approximately circular on the (100) plane of the single crystal diamond, and triangular, triangular with rounded corners, or approximately circular on the (111) plane. The diameter of the dot-like recesses is approximately 1 μm to 50 μm. The number of dot-like etch pits per ^{1 mm2} (dislocation density) is calculated by multiplying the number of dot-like etch pits per 1 ^mm2 by 100. Note that linear etch pits may be observed on the surface of the single crystal diamond after etching, along with dot-like etch pits. Linear etch pits are derived from stacking faults in the single crystal diamond. The number of linear etch pits is not included in the measurement of dislocation defects.

Dislocations can also be detected by X-ray topography (see Patent Document 5). When measuring in transmission mode using synchrotron X-rays, for example, X-rays with a wavelength of 0.71 Å (0.071 nm) are used, and (220) diffraction at a diffraction angle 2θ = 32.9° is used. When measuring in reflection mode, for example, X-rays with a wavelength of 0.96 Å (0.096 nm) can be used, and (113) diffraction at a diffraction angle 2θ = 52.4° can be used. Images can also be taken by changing the X-ray wavelength and the diffraction angle 2θ. Measurements can also be made using a laboratory X-ray diffraction device; for example, (111) diffraction can be observed using a Mo source, and (113) diffraction can be observed using a Cu source. While a CCD (Charge Coupled Device) camera can also be used for measurements, it is preferable to use a nuclear emulsion plate to increase resolution. After developing the nuclear emulsion, dislocations can be identified and quantified by capturing images using an optical microscope.

In the synthesis of single-crystal diamond using the temperature gradient method, for example, the composition of the solvent metal (solvent metal 258) is Fe/Co = 10/90 to 90/10 (mass ratio), and 1.5 to 3 mass% of titanium (Ti) or aluminum (Al) is added to the solvent metal as a nitrogen getter. The temperature gradient is adjusted so that the temperature difference between the carbon source 256 and the seed crystal 300 is 10 to 25°C, and a pressure of 5.0 to 5.5 GPa and a temperature of 1300 to 1350°C are maintained for 80 to 250 hours. As a result, as shown in Figure 10, synthetic diamond 302 is synthesized from the seed crystal 300. If the temperature difference exceeds 25°C, crystal growth becomes disrupted and vicinal growth marks tend to disappear. If the temperature difference is less than 10°C, it takes a long time to grow a crystal of the specified size, which increases production costs. In addition, temperature changes during the holding period are controlled to within 3°C. This further improves crystallinity. If the temperature change is greater than 3°C, growth becomes unstable, resulting in crystal defects, distortion, and the inclusion of inclusions, reducing crystallinity.

(Second process)
A cut diamond 304 (see FIG. 10 ) is cut from the single crystal diamond synthesized in the first step to be used as a seed crystal in the third step described below. The seed surface of the cut diamond 304 used as the seed crystal is preferably, but not limited to, a square or octagon. The size of the seed surface (e.g., the length of the opposite side) is preferably 0.3 mm or more and 3 mm or less. The seed crystal is cut by laser processing into a plate with a thickness of approximately 0.5 mm to 1.0 mm, and the surface of the plate is polished to a surface roughness Ra of 20 nm or less. It is then preferably cut into a plate with a thickness of approximately 0.3 mm × 0.3 mm × 0.3 mm to 3.0 mm × 3.0 mm × 1.0 mm by laser cutting. Larger sizes are preferable because they avoid dislocation defects and make it easier to obtain a single sector. When the cut diamond 304 is a rectangular parallelepiped and the plane used as the seed surface is a (001) crystal plane, one side of the rectangle serving as the seed surface is parallel to the <100> or <010> direction. If the plane to be used as the seed surface is a (111) crystal plane, one side of the rectangle that is the seed surface is parallel to the <1-10> direction, the <10-1> direction, or the <01-1> direction. This allows the cutting margin to be small, and a seed crystal with a seed surface that is less damaged can be obtained.

In single-crystal diamonds synthesized by the temperature gradient method, many dislocation defects exist in the principal plane direction of the seed crystal and in directions with an opening angle of approximately X° from that direction. For example, when the principal plane direction is the <001> direction, many dislocation defects exist in four directions with an opening angle of approximately 35° (i.e., X = 35): the <112> direction, the <-112> direction, the <1-12> direction, and the <-1-12> direction. Furthermore, when the principal plane direction is the <111> direction, many dislocation defects exist in three directions with an opening angle of approximately 19.5° (i.e., X = 19.5): the <112> direction, the <121> direction, and the <211> direction. The above description of the opening angle being in the X° direction uses the expression X° because a slight deviation occurs in the opening angle when the principal plane direction does not coincide with the <001> or <111> direction. When they do not coincide, the opening angle is corrected from 35° or 19.5°. Excluding these opening angles and the area within ±5° of the principal plane direction, a single sector contains a high-quality crystal with few crystal defects. Furthermore, near the boundaries between different sectors, such as the boundary between the {001} sector and the {111} sector, there is a lot of crystal distortion and defects are likely to occur. As the seed substrate becomes larger, the spacing between the opening angles starting from the seed substrate widens, and the boundaries between different sectors are also biased toward the edges, allowing for a larger size of high-quality crystal. Here, the {abc} sector refers to a region grown by stacking on the {abc} plane. For example, the {001} sector is a region grown by stacking {001} planes. Therefore, in this second step, a seed crystal with few defects is obtained by cutting out a cut diamond 304 from a single growth sector of the principal plane growth of the synthetic diamond 302 seed crystal, excluding ±5° of the principal plane growth direction.

Increasing the seed substrate size allows for larger quality crystals. For example, it is possible to cut a seed crystal with reduced defects, with 10 or fewer dislocation defects detected by X-ray topography. Using such a seed crystal results in a single crystal diamond synthesized in subsequent processes with fewer defects and further reduced distortion. Examples of a single growth sector of the primary surface growth of the synthetic diamond 302, excluding ±5° from the primary surface growth direction of the seed crystal, include the {001} sector or the {111} sector. Other crystallographically possible sectors (such as the {113} sector, {115} sector, {110} sector, or {135} sector) may also be used, as long as they are a single growth sector of the primary surface growth excluding ±5° from the primary surface growth direction of the seed crystal. When cutting the cut diamond 304 used as the seed crystal from the synthetic diamond 302, it may be cut from within a single sector, or may be cut to include two or more sectors, as long as it does not include areas with many dislocation defects. That is, the cut diamond 304 may include zero or one sector boundary. In a single crystal diamond, the region of each sector can be identified by an luminescence image obtained by irradiating it with ultraviolet light (ultraviolet-excited luminescence image).

(Third step)
Diamond is synthesized by the temperature difference method as described above using the seed crystal cut in the second step. That is, referring to Figure 11, cut diamond 304 is used as seed crystal 310 to synthesize synthetic diamond 312. This allows for the production of a single crystal diamond with reduced crystal defects and strain.

Specifically, referring to Figure 9, diamond powder is used as the carbon source 256. Iron or cobalt, which has high solubility and affinity for carbon, is used as the solvent metal 258. Depending on the synthesis conditions, trace amounts of nickel or manganese may be incorporated into the diamond. The amount of boron impurity contained in the carbon source 256 and solvent metal 258 is controlled to 1 ppm or less. This allows the boron (B) content in the single crystal diamond to be 0.1 ppm or less based on atomic number. Titanium is added to the solvent metal 258 as a nitrogen getter. The concentration of the added titanium is 1.5 mass% or more and 3 mass% or less. This allows the nitrogen content in the single crystal diamond to be 0.1 ppm or more and 10 ppm or less based on atomic number. Aluminum may be added as a nitrogen getter. In this case, an Fe-Al alloy may be used for the solvent metal 258.

The conditions for the temperature difference method include, for example, adjusting the temperature gradient so that the temperature difference between the carbon source 256 and the seed crystal 300 is 10°C or more and 25°C or less, and maintaining a pressure of 5.0 GPa or more and 5.5 GPa or less and a temperature of 1300°C or more and 1350°C or less for 80 hours or less and 300 hours or less. If the temperature difference exceeds 25°C, crystal growth becomes somewhat disrupted, and vicinal growth often becomes impossible to observe. Furthermore, by controlling the temperature change during maintenance to within 3°C, crystallinity can be further improved.

(4th step)
A cut diamond 318 is cut from the synthetic diamond 312 (see FIG. 11) synthesized in the third step. The cut diamond 318 is a region that is within a single growth sector (e.g., sector 316) that does not include the sector boundary 314 and has 10 or fewer dislocations detected by X-ray topography. The cut diamond 318 is cut using laser processing in the same manner as in the second step described above.

(5th step)
The cut diamond 318 cut from the synthetic diamond 312 in the fourth step is irradiated with an electron beam having an energy of 300 KeV or more and 1.2 MeV or less. This ionizes the orbital electrons of carbon in the cut diamond 318, ejecting carbon nuclei and forming vacancies in the cut diamond 318. Alternatively, an electron beam having an energy of 500 KeV or more and 1 MeV or less may be irradiated. The electron beam irradiation dose can be changed in the range of 1×10 ¹⁸ cm ⁻² to 4×10 ¹⁹ cm ⁻² depending on the amount of NV ⁻ desired to be generated.

(6th step)
The cut diamond 318 that has been subjected to the fifth step is annealed in a vacuum at a temperature of 1100° C. to 1400° C. for 0.1 to 0.5 hours, thereby moving the nitrogen in the cut diamond 318 and forming NV centers consisting of nitrogen and vacancies.

As a result, it is possible to manufacture a diamond spin sensor 100 that is larger in size than conventional ones and has an increased transverse relaxation time T2 without reducing the NV center concentration C.

The irradiation step in step 5 and the annealing step in step 6 may be repeated two or more times. This allows the desired amount of NV centers to be formed in the cut diamond 318.

As mentioned above, a nitrogen getter is used in the third step. The nitrogen getter can contain any of titanium (Ti), zirconium (Zr), hafnium (Hf), gallium (Ga), aluminum (Al), copper (Cu), silver (Ag), and gold (Au). This increases the product of the transverse relaxation time T2 and the NV center concentration C, resulting in a more sensitive sensor.

As mentioned above, the cut diamond 318 may include only one sector boundary, or may not include any sector boundaries. This increases the product of the transverse relaxation time T2 and the NV center concentration C, resulting in a more sensitive sensor.

Note that, with reference to FIG. 12 , the above synthesis step (see step 3) may be performed using a larger seed crystal 320. For example, in a rectangular seed crystal 320, the length L of one side of the rectangle of the (001) crystal face or the (111) crystal face is 3 mm or more. This allows the sector boundary 324, which is prone to impurities, to be separated from the center of the sector 326. Therefore, a larger cut diamond 328 can be cut out from the sector 326 of the synthetic diamond 322 grown from the seed crystal 320. The cut cut diamond 328 is then subjected to electron beam irradiation (step 5) and annealing (step 6) as described above. This allows a larger diamond spin sensor 100 to be manufactured.

A diamond synthesized as described above is shown schematically in Figure 13. Figure 13 is a plan view of the (001) plane. The left-right direction in Figure 13 corresponds to the <100> direction. Sectors 332, which are triangular regions at the four corners, represent (111) sectors. The boundaries between different sectors are sector boundaries with different impurity concentrations (see dot pattern), which create distortion and introduce impurities. As shown in Figure 12, by increasing the size of the seed crystal, the impurity-contaminated region can be positioned more toward the periphery from the center. Therefore, a larger region can be cut out and a larger diamond spin sensor can be fabricated. Referring to Figure 13, a diamond spin sensor can be fabricated by cutting out the shaded portion of one sector 330 and using it as a seed crystal. Alternatively, as shown by the dashed lines, a larger seed crystal can be fabricated by cutting out a region that includes dislocation defects and two sector regions, and a diamond spin sensor can be fabricated using this. Furthermore, the entire (111) sector 330 can be used as a seed crystal to fabricate a diamond spin sensor.

As a comparative example, Figure 14 shows a synthetic diamond for a conventional NV spin sensor. Figure 14 is a plan view similar to Figure 13. Sector 342 is a (111) sector. In the synthetic diamond of Figure 14, a large sector boundary (see dot pattern) with different impurity concentrations is formed around the periphery of the largest (001) sector 340, and it can be seen that it is not possible to cut out an area large enough for a diamond spin sensor. This is because different sectors, such as the (001) sector and the (111) sector, form a boundary.

The effectiveness of the diamond spin sensor disclosed herein will be demonstrated below through examples. The transverse relaxation time T2 and NV center concentration C were measured using multiple diamond spin sensors fabricated using the above-described manufacturing method. The manufacturing conditions are shown in Figure 15, and the measurement results are shown in Figures 15 and 16.

Figure 15 shows diamond seed crystals of a specified size cut using a laser processing machine from the synthetic diamond produced as described above (see step 2), shown as Samples 1 to 8. For example, Samples 1 to 6 are 3mm x 3mm x 0.5mm seed crystals. Sample 9 is a commercially available synthetic diamond (DNV-B14 manufactured by ElementSix) produced by CVD (Chemical Vapor Deposition). The various crystals were cut from within the sectors listed in the "Seed Crystal Cutting Sector" column of Figure 15. In the "Seed Crystal Cutting Sector" column, "(100)" means cut from the (100) sector. Furthermore, "(100) + (111)" means cut to include both the (100) and (111) sectors. The presence or absence of defects in the resulting diamond seed crystals was confirmed using X-ray topography images. The number of dislocation defects confirmed is shown in the "Dislocation Defects" column in Figure 15.

Next, a diamond crystal was grown on the diamond seed crystal using the temperature gradient method to obtain a single-crystal diamond for each sample (see step 3). This is indicated by HPHT (High-Pressure High-Temperature) in Figure 15. Diamond powder containing 100 ppm to 200 ppm of nitrogen and 0.5 ppm to 1 ppm of boron as impurities was used as the carbon source. High-purity iron (Fe) and cobalt (Co) were used as the solvent metal, with a solvent composition of Fe:Co = 55:45 (weight ratio). 1.75 mass% titanium was added to the solvent metal. The temperature gradient method for each sample was performed under the following conditions: titanium was used as the nitrogen getter, the temperature difference between the high-temperature section (carbon source) and the low-temperature section (seed crystal) was 23°C, the pressure was 5.3 GPa, and the holding temperature of the low-temperature section was 1350°C. The holding time was 150 hours.

Subsequently, steps 4 to 6 described above were carried out to fabricate multiple diamond spin sensors. The electron beam irradiation energy was 0.95 MeV, the electron beam dose was 8 x 10 ¹⁸ cm ^-2 , the annealing temperature was 1100°C, and the annealing time was 15 minutes. The diamonds cut by step 4 are shown in the "Method for cutting sensor material" column in Figure 15. Regarding the "position of growth sector,""above" represents the shaded area in Figure 13, and "above + adjacent" represents the dashed area in Figure 13. The measured nitrogen concentration (in ppm) and NV ^- center concentration (in ppm) are shown in the "N concentration" and "NV ^- concentration" columns, respectively.

The transverse relaxation time T2 was measured using the fabricated diamond spin sensor. The results are shown in Figure 16. Samples 1 to 9 are the same as those in Figure 15. In Figure 16, the phase difference represents the average phase difference (unit: nm/mm) over the entire surface. (V ⁰ +V ^- )/NV ^- means the ratio of the sum of the number of uncharged isolated vacancies and the number of negatively charged isolated vacancies to the number of NV ^- centers. The product α means the product of the transverse relaxation time T2 (unit: μsec) and the NV ^- center concentration (unit: ppm).

As shown in Figure 16, the product α is greater than 65 for all of Samples 1 to 6. For Samples 1, 2, 5, and 6, the product α is greater than 120. For Samples 5 and 6, the product α is greater than 160. The product α for Sample 6 is greater than 200 and greater than 250. It can be seen that Samples 1 and 6 all achieve a greater product α than Samples 7 to 9, and also have longer transverse relaxation times T2. The ^{NV -center} concentrations for Samples 1 to 6 are lower than those for Samples 7 and 8. Therefore, the transverse relaxation times T2 of Samples 1 to 6 are longer than those of Samples 7 and 8. On the other hand, when Samples 1 to 3 are compared with Sample 9, the NV ⁻ center concentrations are approximately the same, but the transverse relaxation times T2 of Samples 1 to 3 are longer than that of Sample 9. Therefore, it can be seen that a diamond with an increased transverse relaxation time T2 has been achieved without reducing the NV ⁻ center concentration.

With regard to the NV ^- center concentration, Samples 1 to 6 all achieved 0.02 ppm to 10 ppm, Sample 6 achieved 0.02 ppm to 1.2 ppm, and Samples 5 and 6 achieved 0.02 ppm to 2 ppm. With regard to the average phase difference over the entire surface, Samples 1 to 6 achieved 6 nm/mm or less, and Samples 1, 2, 5, and 6 achieved an average phase difference of 4 nm/mm or less. With regard to (V ⁰ +V ^- )/NV ^- , Samples 1 to 6 achieved 10% or less. Regarding the number of dislocation defects, Sample 1 to Sample 6 can achieve 10 or less, while Sample 5 and Sample 6 can achieve 0.

The present disclosure has been described above by explaining the embodiments, but the above-described embodiments are merely examples, and the present disclosure is not limited to the above-described embodiments. The scope of the present disclosure is indicated by the claims in the scope of the claims, taking into consideration the detailed description of the invention, and includes all modifications that are equivalent in meaning and scope to the wording set forth therein.

100 Diamond spin sensor 102 First surface 104 First edge 202 Electromagnetic wave irradiating unit 204 Excitation light 206 Fluorescence 210 Excitation light generating unit 212 Filter 214 Light collecting element 216 Optical waveguide 218 LPF
220 Light detection unit 230 Control unit 232 Electromagnetic wave generation unit 250 Pressure medium 252 Graphite heater 254 Insulating member 256 Carbon source 258 Solvent metal 300, 310, 320 Seed crystal 302, 312, 322 Synthetic diamond 304, 318, 328 Cut diamond 314, 324 Sector boundary 316, 326, 330, 332, 340, 342 Sector A1, A2, A3, A4, A5, A6, A7, A8 Point C Carbon L Length N Nitrogen P1, P2, P3 Pulse t, t1, t2, t3, τ Time interval V Vacancy X, Y, Z Axis Δf Frequency difference φ Angle

Claims (8)

Diamond containing an NV ^- center having an electron spin,
A diamond spin sensor, wherein the product of T2 and C is greater than 65, where T2 is the transverse relaxation time of the electron spin measured by the Hahn echo method, and C is the concentration of the NV ⁻ center in the diamond. The diamond spin sensor of claim 1, wherein the product is greater than 200. the concentration of the ^NV center is 0.02 ppm or more and 10 ppm or less,
3. The diamond spin sensor according to claim 1, wherein the average phase difference over the entire surface of the diamond is 6 nm/mm or less. The diamond spin sensor of claim 3, wherein the average phase difference is 4 nm/mm or less. 5. The diamond spin sensor according to claim 1, wherein the concentration of the NV ⁻ center is 0.02 ppm or more and 1.2 ppm or less. the concentration of the ^NV center is 0.02 ppm or more and 10 ppm or less,
5. The diamond spin sensor according to claim 1, wherein the number of dislocation defects in the entire diamond detected by an X-ray topography image is 10 or less. the concentration of the ^NV center is 0.02 ppm or more and 2 ppm or less,
7. The diamond spin sensor according to claim 6, wherein the number of dislocation defects is zero. 8. The diamond spin sensor according to claim 1, wherein the ratio of the concentration of isolated vacancies in the diamond to the concentration of the NV ⁻ centers is 10% or less.