Subpicotesla Diamond Magnetometry

At a Glance

Metadata	Details
Publication Date	2015-10-05
Journal	Physical Review X
Authors	Thomas Wolf, Philipp Neumann, Kazuo Nakamura, Hitoshi Sumiya, Takeshi Ohshima
Institutions	University of Stuttgart, University of Tsukuba
Citations	344
Analysis	Full AI Review Included

Diamond Magnetometry: Achieving Sub-Picotesla Sensitivity with NV Ensembles

Executive Summary

This documentation analyzes the research demonstrating the highest room-temperature sensitivity achieved by a diamond NV-ensemble magnetometer to date, and outlines how 6CCVD’s proprietary MPCVD diamond capabilities meet the strict material requirements needed for replication and advancement of this technology.

Benchmark Achievement: A record room-temperature magnetic field sensitivity of 0.9 pT/√Hz was achieved using NV defect centers in diamond, placing this sensor among the best per-volume solid-state magnetometers.
Ensemble Requirements: The device relied on a large ensemble of 10¹¹ NV defect centers within a small effective sensing volume (8.5e-4 mm³), necessitating highly controlled Nitrogen (N) doping and defect conversion.
Noise Mitigation Strategy: High sensitivity was attained by systematically mitigating non-white noise sources (laser intensity fluctuation, microwave amplitude/phase noise) through advanced referencing sequences, allowing the system to scale its sensitivity proportional to √t (time).
Scaling Potential: The ultimate theoretical spin projection noise limit is calculated to be 6 fT/√Hz. Achieving this requires further material optimization and implementation of dynamic decoupling sequences.
6CCVD Value Proposition: 6CCVD offers custom Single Crystal Diamond (SCD) substrates tailored for high-density, low-strain NV ensemble creation, critical for reaching the projected fT/√Hz sensitivity future research demands.
Material Specification: Research requires extremely high-quality, high-purity single-crystal diamond with highly controlled nitrogen concentration (approximately 0.9 ppm N) for optimal NV generation uniformity and coherence.

Technical Specifications

The following hard data points were extracted from the study detailing the NV ensemble magnetometer performance and operational parameters.

Parameter	Value	Unit	Context
Achieved Field Sensitivity (B_min)	0.9	pT/√Hz	Measured sensitivity at room temperature (Photon Shot Noise Limited)
Effective Sensor Volume (V_eff)	8.5e-4	mm³	Volume containing the measured 10¹¹ NV centers
Total NV Defect Count (N)	10¹¹	defects	Number of centers contributing to the ensemble signal
Calculated Spin Projection Noise Limit	6.0	fT/√Hz	Theoretical maximum sensitivity limit (at T_φ = 50 µs)
Smallest Measured Field	100	fT	Lowest magnetic field detectable by the device
Nitrogen (N) Precursor Concentration	0.9	ppm	N concentration in the HPHT diamond source material
Laser Excitation Wavelength	532	nm	Used for optical polarization and readout
Phase Accumulation Time (T_φ)	50	µs	Used during the Hahn Echo sequence for sensing
Single Sequence Length (T_seq)	160	µs	Total duration of one pulsed measurement sequence
Microwave Zero-Field Splitting (D)	2.87	GHz	NV electron spin property
NV Electron Gyromagnetic Ratio (γ/2π)	28.7	GHz/T	Conversion factor for magnetic field detection

Key Methodologies

The core of the successful demonstration rested upon engineering the diamond material and implementing advanced pulsed sequences to suppress technical noise.

Material Preparation: A 0.9 ppm Nitrogen (N) HPHT-grown diamond was used. NV centers were created/converted starting 500 µm from the front surface, establishing a high-density ensemble.
Optical and Spin Initialization: The NV ensemble spin state was polarized and read out using pulsed 532 nm laser excitation, leveraging spin state-dependent fluorescence modulation.
Microwave Control: Microwave (MW) pulses were applied to coherently control the electron spin state in the NV triplet ground state (m_s = 0, ±1 sublevels).
AC Magnetometry Sequence: Measurements utilized a phase-locked Hahn Echo pulse sequence: (π/2)_x - (π)_x - (π/2)_y. This sequence employed a phase accumulation time (T_φ) of 50 µs and a total sequence length (T_seq) of 160 µs.
Noise Referencing (Signal $S_D$): Sensitivity scaling was optimized by implementing a self-referencing scheme (signal $S_D$) that provided two filtering steps against optical noise and one referencing step against microwave-related noise. This approach proved crucial for reducing the impact of microwave amplitude noise, which typically dominates measurement error on longer timescales.

6CCVD Solutions & Capabilities

This research validates the market need for ultra-high quality, precisely engineered diamond materials. 6CCVD is uniquely positioned to supply the materials required to replicate, scale, and surpass the sensitivity metrics achieved in this study.

Applicable Materials

To achieve optimal NV ensemble performance—requiring low background strain, high chemical purity, and precise nitrogen precursor concentration—the following 6CCVD materials are recommended:

Optical Grade Single Crystal Diamond (SCD): Researchers must move beyond standard HPHT material to mitigate strain, which limits coherence time. 6CCVD supplies low-strain, high-purity SCD plates (up to 500 µm thick).
Custom N-Doped SCD: 6CCVD specializes in introducing controlled nitrogen impurities during the MPCVD growth process. We can tailor the N concentration (e.g., to the required ~0.9 ppm or higher/lower, depending on the desired ensemble density) to ensure uniform NV formation across the sensing volume.
High-Purity Substrates: For researchers utilizing irradiation/annealing post-growth (as often done with HPHT) to create NVs, 6CCVD can provide ultra-low N, high-purity CVD diamond substrates (up to 10 mm thick) necessary for subsequent high-temperature processing.

Customization Potential

The optimization path toward fT/√Hz sensitivity noted by the authors explicitly involves material optimization. 6CCVD offers the engineering control necessary for these future requirements:

Research Requirement	6CCVD Capability	Specification & Benefit
Increased Sensitivity & Volume	Custom Dimension Wafers/Plates	MPCVD growth allows plates/wafers up to 125 mm (PCD) or custom-sized SCD plates, enabling larger sensing volumes (N > 10¹¹) and superior noise averaging.
Efficient Photon Collection	High-Quality Polishing & Finish	SCD plates can be polished to a surface roughness of Ra < 1 nm, minimizing optical scattering losses and maximizing fluorescence collection efficiency.
On-Chip Signal Routing	Custom Metalization Services	6CCVD offers internal metalization (e.g., Au, Pt, Ti) for fabricating on-chip microwave delivery lines (RF antennas) directly onto the diamond surface, improving MW field uniformity and reducing noise.
Future Waveguide Integration	Precision Thickness Control	We offer SCD thickness control from 0.1 µm to 500 µm, crucial for fabricating micro-optical structures or light-trapping diamond waveguides (Ref. 16, 22) mentioned in the study to enhance readout efficiency.

Engineering Support

Achieving the next milestone of fT/√Hz sensitivity requires meticulous material design. 6CCVD’s in-house PhD team provides specialized engineering consultation to assist with material selection for similar NV-ensemble magnetometry and quantum sensing projects. Our expertise ensures optimal nitrogen doping profiles, minimized lattice strain, and ideal surface termination for maximizing NV spin coherence time (T₂*) and T₂.

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

View Original Abstract

Diamond defect centers are promising solid state magnetometers. Single centers allow for high spatial resolution field imaging but are limited in their magnetic field sensitivity to around 10 nT/Hz^(1/2) at room-temperature. Using defect center ensembles sensitivity can be scaled as N^(1/2) when N is the number of defects. In the present work we use an ensemble of 1e11 defect centers for sensing. By carefully eliminating all noise sources like laser intensity fluctuations, microwave amplitude and phase noise we achieve a photon shot noise limited field sensitivity of 0.9 pT/Hz^(1/2) at room-temperature with an effective sensor volume of 8.5e-4 mm^3. The smallest field we measured with our device is 100 fT. While this denotes the best diamond magnetometer sensitivity so far, further improvements using decoupling sequences and material optimization could lead to fT/Hz^(1/2) sensitivity.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevx.5.041001