Homogeneous spin-dephasing time of NV− centre in millimetre-scale 12C-enriched high-pressure high-temperature diamond crystals
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-04-08 |
| Journal | Communications Materials |
| Authors | Chikara Shinei, Y. MASUYAMA, Hiroshi Abe, Masashi Miyakawa, Takashi Taniguchi |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research demonstrates the successful synthesis of highly uniform, millimetre-scale 12C-enriched High-Pressure High-Temperature (HPHT) diamond crystals, achieving critical specifications necessary for next-generation quantum magnetometers.
- Core Achievement: Exceptional spatial uniformity of the NV center spin-dephasing time (T2*) was achieved, showing a variance of only 10% over a large 1.1 x 1.1 mm2 area.
- Performance Metric: The median T2* value (<T2*>) reached 4.5 µs in the 12C-enriched HPHT diamond with a low nitrogen concentration (1.3 ± 0.4 ppm).
- Limiting Factor Identified: The T2* performance is primarily limited by the residual strain gradient within the HPHT crystal, not the electron-spin bath of nitrogen impurities. The measured T2* (4.5 µs) is approximately 2/3 of the theoretical limit imposed by the nitrogen bath (7.6 µs).
- Strain Quantification: The strain gradient was quantified by mapping the spin-strain interaction (Mz), yielding a full width at half maximum (FWHM) of 0.06 MHz.
- Pathway to Femto-tesla Sensitivity: Achieving the target T2* > 10 µs—essential for high-fidelity spin manipulation (100 ns π pulse) and femto-tesla magnetic sensitivity (<1 pT Hz-1/2)—requires further reduction of this internal strain gradient.
- Synthesis Method: High crystal quality was maintained by using a low HPHT growth rate (~1 mg h-1) and precise nitrogen concentration control.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Median Spin Dephasing Time (<T2*>) | 4.5 | µs | Measured in 12C-enriched HPHT diamond. |
| T2* Spatial Variance | 10 | % | Over 1.1 x 1.1 mm2 area in the {111} growth sector. |
| Target T2* for High Sensitivity | > 10 | µs | Required for 10 fT human magnetoencephalography (MEG). |
| Nitrogen Concentration ([Ns0]initial) | 1.3 ± 0.4 | ppm | In the examined HPHT diamond sample. |
| Excitation Volume (Mapped Area) | 1.1 x 1.1 | mm2 | Spatial mapping region. |
| Crystal Thickness (Excited Depth) | 400 | µm | Depth of the {111} growth sector examined. |
| Spin-Strain Interaction (ΔMz FWHM) | 0.06 | MHz | Quantifies the spatial dispersion of strain. |
| Estimated Strain Gradient Slope | 0.05 | kHz µm-1 | Calculated from ΔMz over the mapped distance. |
| HPHT Synthesis Pressure | 5.5 | GPa | Growth pressure. |
| HPHT Synthesis Temperature | 1300-1350 | °C | Growth temperature range. |
| HPHT Growth Rate | ~1 | mg h-1 | Low rate used to suppress metal inclusions and strain. |
| Theoretical T2* Limit (Nitrogen DDI) | 7.6 ± 0.9 | µs | Calculated based on the nitrogen concentration (1.3 ppm). |
| Target Magnetic Sensitivity (MEG) | < 10 | fT | Required for human magnetoencephalography. |
| Spin π-Rotation Time | ~100 | ns | Typical time achieved in this study. |
Key Methodologies
Section titled “Key Methodologies”The study combined specialized HPHT synthesis techniques with high-resolution optical and microwave measurements to characterize the NV center ensemble properties.
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HPHT Crystal Synthesis:
- Apparatus: Modified belt-type high-pressure apparatus with cooling water for temperature stabilization (fluctuation less than ± 7.5 °C).
- Source Material: 12C isotopically enriched CVD polycrystalline diamond was used as the carbon source (assumed 13C concentration ~50 ppm).
- Growth Conditions: Crystals were grown at 5.5 GPa and 1300-1350 °C for 40-80 hours, maintaining a low growth rate (~1 mg h-1) to minimize structural defects and metal inclusions.
- Doping Control: Nitrogen concentration (1.3 ± 0.4 ppm) was controlled by adding nitrogen-getter metals (Ti or Al) to the Fe-Co-Cu or Fe-Co-Al solvent.
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NV Center Formation:
- NV centers were created via electron-beam irradiation (1 x 1017 to 5 x 1017 e cm-2 fluence) followed by post-annealing at 1000 °C for 2 hours.
- The maximum conversion efficiency ([NV-]/[Ns0]initial) achieved was approximately 20%.
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Sample Preparation and Orientation:
- Synthesized diamonds were laser-cut parallel to the {111} plane, which is the preferred orientation for NV sensing systems due to easier alignment of the external magnetic field with the NV axis.
- Acid treatment (H2SO4:HNO3) was used post-cutting to remove paramagnetic amorphous carbon defects from the surface.
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T2 Spatial Mapping:*
- Excitation: A columnar excitation fluorescence microscope was used to excite the NV ensemble throughout the crystal thickness (400 µm depth) with a 20 µm diameter laser spot.
- Measurement: Free-Induction Decay (FID) measurements were conducted at 100 µm steps across the 1.1 x 1.1 mm2 region in the {111} growth sector.
- Field Alignment: An external magnetic field (2.5 mT) was applied parallel to the [111] NV center axis to maximize spin coherence.
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Strain Gradient Measurement (Mz Mapping):
- Optically Detected Magnetic Resonance (ODMR) measurements were performed at each excited position to determine the resonance frequency shift (Mz), which corresponds to the local strain interaction.
- The spatial dispersion of Mz (ΔMz) was used to quantify the strain gradient limiting T2*.
Commercial Applications
Section titled “Commercial Applications”The development of millimetre-scale, highly uniform, low-strain diamond crystals is crucial for scaling up quantum technologies from laboratory experiments to practical commercial devices.
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Quantum Sensing and Metrology:
- Femto-tesla Magnetometry: Enabling high-sensitivity detection of extremely weak magnetic fields, necessary for advanced applications.
- Biomagnetism: Realization of human Magnetoencephalography (MEG) and Magnetocardiography (MCG) under ambient conditions, requiring sensitivity <10 fT.
- Electric Vehicle (EV) Battery Monitoring: High-resolution monitoring of current and temperature in EV batteries using compact diamond sensors.
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Quantum Information Technology:
- Quantum Computation: The long and uniform T2* is essential for maintaining stable quantum superposition states (QSS) and performing high-fidelity spin rotation operations.
- Quantum Telecommunication: Utilizing single NV centers with stable quantum states for robust quantum communication protocols.
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Advanced Material Science:
- High-Quality Substrates: Providing large-area, low-defect, isotopically pure 12C diamond substrates for various electronic and optical applications where strain must be minimized.
View Original Abstract
Abstract Negatively charged nitrogen vacancy (NV−) centres in diamond crystals are promising colour centres for high-sensitivity quantum sensors. A long dephasing time (T 2 * > 10 μs) is essential for achieving increased sensitivity and higher uniformity of T 2 * in millimetre-scale diamond is strongly desired for femto-tesla weak magnetic field detection. High uniformity of T 2 * for NV− centres is achieved herein. The median value of T 2 *, <T 2 *>, 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 T 2 * is only 10% over a millimetre-scale region (1.1 × 1.1 mm2) within the 0.4 mm thick {111} growth sector. <T 2 *> 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 T 2 *. Reducing the strain gradient in diamond crystals provide a pathway to achievement of high sensitivity magnetometry using NV quantum sensing.