Diamond Magnetometry and Gradiometry Towards Subpicotesla dc Field Measurement

At a Glance

Metadata	Details
Publication Date	2021-06-30
Journal	Physical Review Applied
Authors	Chen Zhang, Farida Shagieva, Matthias Widmann, Michael Kübler, Vadim V. Vorobyov
Institutions	Tokyo Gas (Japan), University of Stuttgart
Citations	93
Analysis	Full AI Review Included

Technical Documentation: Subpicotesla DC Magnetometry using High-Purity MPCVD Diamond

This document analyzes the key findings and methodologies presented in the research paper “Diamond magnetometry and gradiometry towards subpicotesla DC field measurement.” It serves as technical documentation and a guide for engineers and scientists seeking to replicate or advance this work using 6CCVD’s specialized Microwave Plasma Chemical Vapor Deposition (MPCVD) diamond materials.

Executive Summary

The research successfully demonstrates high-sensitivity DC magnetometry and gradiometry using Nitrogen Vacancy (NV) ensembles in high-purity, ¹²C enriched Single Crystal Diamond (SCD) under ambient conditions.

Record Sensitivity: Achieved a minimum detectable DC magnetic field of 0.3-0.7 pT in a 73 s measurement time by integrating a ferrite flux guide (FG).
Bandwidth Normalized Performance: The corresponding magnetic field sensitivity is 2.6-6 pT/√Hz, setting a new benchmark for NV ensemble magnetometers operating at room temperature.
Low Power Operation: High sensitivity was maintained using low-intensity optical excitation (laser power below 100 mW), mitigating thermal noise and technical constraints common in high-power NV systems.
Optimized Methodology: Comparison of Continuous-Wave Optically Detected Magnetic Resonance (CW-ODMR) and Continuously Excited Ramsey (CE-Ramsey) sequences confirmed that CE-Ramsey offers superior sensitivity potential under low-power continuous excitation.
Material Requirements: The experiment relied on a high-quality, (111)-oriented, 99.97% ¹²C enriched SCD cube (0.5 mm)³ with a long dephasing time (T₂* = 8.5 µs) and narrow ODMR linewidth (28 kHz).
Sensing Volume: The results were achieved within a compact sensing volume of 0.125 mm³, demonstrating excellent spatial resolution capabilities.

Technical Specifications

The following hard data points were extracted from the experimental results and material characterization:

Parameter	Value	Unit	Context
Minimum Detectable Field	0.3-0.7	pT	With Flux Guide (73 s measurement)
Bandwidth Normalized Sensitivity	2.6-6	pT/√Hz	With Flux Guide
Intrinsic Magnetic Noise Level	17	pT/√Hz	Without Flux Guide (1 Hz normalization)
Diamond Crystal Orientation	(111)	N/A	Single Crystal Diamond (SCD)
Diamond Volume	0.125 (0.5 mm)³	mm³	Sensing volume
Carbon Enrichment	99.97	% ¹²C	High-purity SCD
Initial Nitrogen Concentration	1.4	ppm	Pre-irradiation
Final NV Concentration	0.4	ppm	Post-annealing
Minimum ODMR Linewidth	28	kHz	FWHM, experimental minimum
Dephasing Time (T₂*)	8.5	µs	Measured on the SCD sample
Optimized Rabi Frequency (Ω_R)	17	kHz	For CW-ODMR method
Excitation Laser Power	< 100	mW	Low-intensity operation
Flux Guide Enhancement Factor	6.3	Times	Experimental enhancement
Gradiometer Intrinsic Noise	4-6	pT	In 20-200 Hz frequency range

Key Methodologies

The high-sensitivity DC magnetometry relied on precise material engineering and optimized quantum control sequences combined with technical noise suppression.

Material Preparation:
- Growth: (111)-oriented Single Crystal Diamond (SCD) cubes (0.5 mm)³ were grown using the High-Pressure/High-Temperature (HPHT) temperature gradient method.
- Enrichment: The crystal utilized 99.97% ¹²C enrichment to minimize spin bath noise and maximize T₂*.
- NV Creation: The crystal was irradiated with 2 MeV electrons and subsequently annealed at 1000 °C for 2 hours in vacuum to convert substitutional nitrogen (1.4 ppm) into NV centers (0.4 ppm).
Optical and Microwave Setup:
- Excitation: Low-noise 532 nm laser (Sprout-G) was used for initialization and readout.
- Fluorescence Collection: A Compound Parabolic Concentrator (CPC) was used, achieving a high fluorescence collection efficiency exceeding 60%.
- MW Delivery: A dielectric resonator antenna generated a uniform MW field, driven by vector signal generators for Double Resonance (DR) driving.
Measurement Sequences:
- Noise Suppression: Lock-in detection (LIA) was employed to avoid low-frequency 1/f noise.
- CW-ODMR: Optimized for low laser power, driving both hyperfine and DR transitions for maximum scalar factor enhancement (1.3 times).
- CE-Ramsey (Continuous Excitation): A modified Ramsey sequence where the laser remains continuously on during the magnetic field sensing time (T_m), simplifying optics and improving thermal stability compared to traditional pulsed Ramsey.
Sensitivity Enhancement:
- Magnetic Shielding: The entire setup was enclosed in a magnetic shield cube (inner µ-metal, outer aluminum) to attenuate external magnetic field noise.
- Flux Guide (FG): A ferrite rod (MN60, permeability 6500) with a 2 mm tip diameter was placed near the diamond to concentrate and guide the magnetic field, achieving a 6.3x sensitivity enhancement.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the foundational materials and custom engineering required to replicate and advance this subpicotesla magnetometry research. Our MPCVD diamond expertise ensures the highest quality substrates necessary for achieving world-class NV quantum sensing performance.

Applicable Materials

The core requirement of this research is ultra-high-purity, isotopically enriched Single Crystal Diamond (SCD) with low nitrogen content, which is a flagship product of 6CCVD.

Research Requirement	6CCVD Material Solution	Key Benefit for Quantum Sensing
High Purity, ¹²C Enriched SCD	Optical Grade SCD (99.99% ¹²C available)	Maximizes T₂* and T₁ coherence times, crucial for achieving pT sensitivity.
(111) Orientation	Custom Oriented SCD Wafers/Plates	Enables optimal alignment of the NV axis relative to the applied magnetic field for vector magnetometry.
High Thickness/Volume (0.5 mm)³	SCD Substrates up to 500 µm thick	Provides the necessary sensing volume (V) for ensemble magnetometry, where sensitivity scales with 1/√V.
Thermal Stability/Heat Sinking	High Thermal Conductivity SCD	Essential for managing heat load from the laser, especially when scaling up to Watt-level pumping for fT sensitivity goals.

Customization Potential

The paper highlights the use of a specific (0.5 mm)³ cube and the integration of a ferrite flux guide (FG) requiring precise geometry and integration. 6CCVD offers comprehensive customization services to meet these demanding specifications:

Custom Dimensions and Geometry: We provide custom laser cutting and shaping of SCD plates up to 500 µm thickness, ensuring precise dimensions (e.g., 0.5 mm cubes or custom geometries for flux concentrator integration) with high accuracy.
Large Area Substrates: While the paper used a small cube, 6CCVD can supply SCD substrates up to 500 µm thick and PCD wafers up to 125 mm in diameter, enabling scaling of sensing arrays or larger volume magnetometers.
Advanced Metalization: The integration of MW delivery structures (like the dielectric resonator antenna) often requires precise metal contacts. 6CCVD offers in-house metalization services including deposition of Au, Pt, Pd, Ti, W, and Cu, tailored for robust electrical contact and MW transmission on diamond surfaces.
Surface Quality: Achieving optimal optical readout requires minimal scattering. Our SCD polishing capability guarantees surface roughness of Ra < 1 nm, ensuring high fluorescence collection efficiency (as required by the 60% CPC efficiency cited in the paper).

Engineering Support

6CCVD’s in-house PhD team specializes in the material science and quantum properties of MPCVD diamond. We offer direct consultation to assist researchers in:

Material Selection: Optimizing ¹²C enrichment levels and initial nitrogen concentration (N_s) to balance NV density (for signal strength) against T₂* coherence time (for sensitivity).
Integration Design: Providing guidance on the mechanical and thermal integration of diamond sensors into complex setups, such as those involving flux guides or cryogenic systems (though this paper focused on ambient conditions).
Recipe Optimization: Assisting with material specifications for similar NV Ensemble DC Magnetometry projects, ensuring the supplied diamond meets the exact requirements for subsequent irradiation and annealing processes.

Call to Action: For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available) to ensure your research proceeds without delay.

View Original Abstract

Nitrogen vacancy (NV) centers in diamond have developed into a powerful\nsolid-state platform for compact quantum sensors. However, high sensitivity\nmeasurements usually come with additional constraints on the pumping intensity\nof the laser and the pulse control applied. Here, we demonstrate high\nsensitivity NV ensemble based magnetic field measurements with low-intensity\noptical excitation. DC magnetometry methods like, e.g., continuous-wave\noptically detected magnetic resonance and continuously excited Ramsey\nmeasurements combined with lock-in detection, are compared to get an\noptimization. Gradiometry is also investigated as a step towards unshielded\nmeasurements of unknown gradients. The magnetometer demonstrates a minimum\ndetectable field of 0.3-0.7 pT in a 73 s measurement by further applying a flux\nguide with a sensing dimension of 2 mm, corresponding to a magnetic field\nsensitivity of 2.6-6 pT/Hz^0.5. Combined with our previous efforts on the\ndiamond AC magnetometry, the diamond magnetometer is promising to perform wide\nbandwidth magnetometry with picotesla sensitivity and a cubic-millimeter\nsensing volume under ambient conditions.\n

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.15.064075