Homogeneous spin-dephasing time of NV− centre in millimetre-scale 12C-enriched high-pressure high-temperature diamond crystals

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

Technical Documentation & Analysis: Homogeneous NV Spin Dephasing in Millimetre-Scale Diamond

This document analyzes the findings of the research paper concerning the achievement of highly uniform spin dephasing time ($T_2^*$) in ¹²C-enriched HPHT diamond, and outlines how 6CCVD’s advanced MPCVD diamond materials and processing capabilities can replicate and extend this critical research for high-sensitivity quantum sensing applications.

Executive Summary

The following points summarize the core technical achievements and commercial implications of the research:

High $T_2^*$ Uniformity Achieved: The study successfully demonstrated highly uniform NV center spin dephasing time ($T_2^$) in millimetre-scale ¹²C-enriched HPHT diamond, achieving a median $T_2^$ of 4.5 µs.
Minimal Spatial Variance: Spatial variance of $T_2^*$ was limited to only 10% over a 1.1 mm x 1.1 mm area, validating the material quality for large-volume quantum sensing ensembles.
Application Target: This uniformity is essential for scaling up NV quantum sensors toward the target sensitivity required for human magnetoencephalography (<10 fT Hz^-1/2), which necessitates $T_2^* > 10$ µs.
Limiting Factor Identified: The primary factor limiting $T_2^*$ to 4.5 µs (approximately 2/3 of the theoretical limit based on nitrogen impurities) was identified as the residual strain gradient ($\Delta M_z = 0.06$ MHz) within the diamond crystal.
Material Strategy: The research confirms that reducing structural imperfections and strain gradients in the low nitrogen concentration regime (<1 ppm) is the key pathway to achieving the required $T_2^*$ for high-fidelity spin manipulation.
6CCVD Relevance: 6CCVD specializes in ultra-high purity, isotopically enriched MPCVD Single Crystal Diamond (SCD), offering the precise material control necessary to minimize both nitrogen impurities (DDI noise) and lattice strain (gradient noise).

Technical Specifications

The following hard data points were extracted from the research paper:

Parameter	Value	Unit	Context
Median Spin Dephasing Time (<T₂*>)	4.5	µs	¹²C-enriched HPHT diamond
T₂* Spatial Variance	10	%	Over 1.1 mm x 1.1 mm area
Target T₂* for High Sensitivity	>10	µs	Required for <10 fT sensitivity
Initial Nitrogen Concentration ([N_s⁰]_initial)	1.3 ± 0.4	ppm	{111} growth sector
Crystal Thickness	0.4	mm	{111} growth sector
Excitation Volume (Diameter x Depth)	20 x 400	µm	Columnar excitation
Strain Gradient ($\Delta M_z$ FWHM)	0.06	MHz	Spatial variance of ODMR shift
HPHT Synthesis Pressure	5.5	GPa	Modified belt-type apparatus
HPHT Synthesis Temperature Range	1300-1350	°C	Fluctuation < ±7.5 °C
Diamond Growth Rate	$\sim 1$	mg h^-1	Low rate to suppress metal inclusions
Microwave $\pi$-Pulse Time	$\sim 100$	ns	Required for high-fidelity spin rotation

Key Methodologies

The experiment relied on precise material synthesis and advanced quantum characterization techniques:

HPHT Synthesis: ¹²C isotopically enriched diamond single crystals were grown using a modified belt-type HPHT apparatus at 5.5 GPa and 1300-1350 °C.
Nitrogen Doping Control: Nitrogen concentration was controlled in the range of 0.7 to 14 ppm by adding nitrogen-getter metals (Ti or Al) to the Fe-Co-Ti-Cu or Fe-Co-Al solvent-metals.
Low Growth Rate: A slow growth rate ($\sim 1$ mg h^-1) was maintained over 40-80 hours to suppress the formation of metal inclusions and associated strain gradients.
Crystal Preparation: Synthesized crystals were laser-cut parallel to the {111} plane (400 ± 100 µm thickness) and subsequently treated with hot acid (H₂SO₄:HNO₃) to remove laser-induced surface defects.
NV Center Creation: NV centers were formed via electron-beam irradiation (1 x 10¹⁷ to 5 x 10¹⁷ e cm^-2 fluence) followed by post-annealing at 1000 °C.
T₂ Mapping:* Free-induction decay (FID) measurements were performed using a columnar excitation fluorescence microscope with a 532 nm laser, mapping $T_2^*$ across the millimetre-scale {111} growth sector with a 100 µm pixel step.
Strain Gradient Analysis: Optically Detected Magnetic Resonance (ODMR) measurements were used to map the spatial variance of the spin-strain interaction component ($M_z$), quantifying the internal strain gradient.

6CCVD Solutions & Capabilities

This research highlights the critical need for ultra-low strain, isotopically pure diamond to advance NV quantum sensing. 6CCVD’s expertise in MPCVD growth and precision processing directly addresses the limitations identified in this HPHT study (residual strain and DDI noise).

Applicable Materials

To replicate or extend this research and achieve the target $T_2^* > 10$ µs, researchers require materials with superior isotopic purity and lower intrinsic strain than the HPHT samples examined:

Optical Grade SCD (Single Crystal Diamond): Required for the highest possible $T_2^*$. 6CCVD provides SCD with isotopic ¹²C enrichment (<500 ppm ¹³C) to minimize nuclear spin bath noise.
Ultra-Low Nitrogen SCD: Our MPCVD process allows for precise control of nitrogen incorporation down to sub-ppb levels, minimizing the dipole-dipole interaction (DDI) noise that limits $T_2^*$ in the high [N] regime.
Custom {111} Orientation: The paper emphasizes the use of {111} oriented crystals for easier magnetic field alignment. 6CCVD offers custom SCD substrates grown and polished to {111} orientation.

Customization Potential

6CCVD provides the necessary engineering services to move from fundamental material characterization to integrated quantum devices:

Research Requirement / Challenge	6CCVD Customization Service	Technical Benefit to Project
Millimetre-Scale Dimensions	Custom Plates/Wafers up to 125 mm (PCD) and SCD plates up to 500 µm thick.	Enables scaling of the excitation volume (V) far beyond the 1.1 mm² area studied, directly improving the shot-noise limit ($\eta_{sp}^{ensemble}$).
Strain Mitigation & Surface Quality	SCD Polishing to Ra < 1 nm.	Minimizes surface-related strain and noise, which is critical for maintaining high $T_2^*$ uniformity across large ensembles.
Device Integration	Custom Metalization (Au, Pt, Ti, W, Cu) and Laser Cutting.	Allows for the integration of microwave delivery structures (e.g., coplanar waveguides) directly onto the diamond surface, necessary for high-fidelity spin rotation ($\pi$-pulse time $\sim 100$ ns).
Thickness Optimization	Substrates and SCD layers available from 0.1 µm to 10 mm.	Provides flexibility to optimize the columnar excitation depth (400 µm used in the paper) for maximum photon collection efficiency and NV ensemble count.

Engineering Support

The paper concludes that reducing the strain gradient in the low nitrogen concentration regime (<1 ppm) is the key to achieving the required $T_2^* > 10$ µs. This requires highly specialized material engineering.

6CCVD’s in-house PhD team can assist researchers with material selection and optimization for similar High-Sensitivity Magnetometry projects. We leverage our expertise in MPCVD growth kinetics to minimize lattice defects and intrinsic strain, providing materials engineered to overcome the strain limitations observed in the HPHT samples.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s43246-025-00782-7