Gradiometer Using Separated Diamond Quantum Magnetometers

At a Glance

Metadata	Details
Publication Date	2021-02-02
Journal	Sensors
Authors	Y. MASUYAMA, Katsumi Suzuki, Akira Hekizono, Mitsuyasu Iwanami, Mutsuko Hatano
Institutions	National Institutes for Quantum Science and Technology, Tokyo Institute of Technology
Citations	14
Analysis	Full AI Review Included

Technical Documentation & Analysis: NV Gradiometer for High-Sensitivity Magnetometry

This document analyzes the research paper “Gradiometer Using Separated Diamond Quantum Magnetometers” and outlines how 6CCVD’s specialized MPCVD diamond materials and fabrication services can support the replication, optimization, and commercialization of this high-sensitivity quantum sensing technology.

Executive Summary

The research successfully demonstrates a room-temperature, variable base length gradiometer utilizing negatively charged Nitrogen-Vacancy (NV) centers in diamond, achieving noise cancellation capabilities previously restricted to heavily shielded environments.

Noise Cancellation: The gradiometer configuration effectively cancels spatially homogeneous magnetic noise (including 50 Hz power line harmonics), achieving a noise floor of 34 nT, comparable to a three-layer permalloy shielded enclosure.
Homogeneous Noise Reduction: Spatially homogeneous AC magnetic noise (20 Hz) was reduced by a factor of less than 1/50 in the differential signal.
Deep Signal Sensing: The variable base length (tested up to 100 mm) proved crucial for sensing deep, inhomogeneous signals (e.g., 10 µT at 30 Hz) at a depth of 50 mm, validating its potential for Magnetoencephalography (MEG) applications.
Material Requirements: The sensors were fabricated from high-nitrogen concentration Type Ib diamond (> 10¹⁹ atoms/cm³), processed via 2 MeV electron irradiation at 750 °C, requiring highly matched quantum properties between the two sensors.
Portability Potential: The optical fiber configuration allows for flexible sensor placement and variable base length adjustment, paving the way for field-portable, high-sensitivity magnetometers without bulky magnetic shielding.
6CCVD Value Proposition: 6CCVD provides the necessary high-quality, high-nitrogen Single Crystal Diamond (SCD) precursor material, custom dicing, and metalization services required to manufacture matched, high-performance NV gradiometer pairs.

Technical Specifications

The following hard data points were extracted from the experimental results and methodology described in the paper:

Parameter	Value	Unit	Context
Diamond Precursor Material	Type Ib	N/A	Nitrogen concentration > 10¹⁹ atoms/cm³
NV Creation Method	Electron Irradiation	N/A	2 MeV energy, 1 x 10¹⁸ /cm² fluence
Annealing/Irradiation Temperature	750	°C	To avoid crystal damage accumulation
Sensor Fluorescence Volume	1.2	mm³	Volume of NV centers in each diamond
Excitation Wavelength	532	nm	Laser source
Excitation Power	300	mW	Optical power delivered via fiber
MW Frequencies (Ch. 1 / Ch. 2)	2.71 / 2.7	GHz	Optimum ODMR points for ms = -1 state
MW Modulation Frequency	2	kHz	Used with 8 MHz frequency deviation width
Base Length (Tested Range)	Up to 100	mm	Distance between Sensor 1 and Sensor 2
Target Depth (Tested)	50	mm	Distance from target magnet to Sensor 1
Gradiometer Noise Floor	34	nT	Comparable to 3-layer magnetic shield (below 1 Hz)
Homogeneous Noise Reduction	< 1/50	Ratio	Reduction of 20 Hz noise in differential signal
ODMR Linewidth (Average)	12.95	MHz	Average of Ch. 1 (12.8 MHz) and Ch. 2 (13.1 MHz)
ODMR Contrast (Average)	1.75	%	Average of Ch. 1 (1.7%) and Ch. 2 (1.8%)

Key Methodologies

The gradiometer relies on precise material preparation and a differential measurement setup to achieve high noise immunity.

Material Preparation: High-nitrogen concentration Type Ib diamond (N > 10¹⁹ atoms/cm³) was selected as the starting material.
NV Center Fabrication: The diamond was irradiated using a 2 MeV electron beam at a fluence of 1 x 10¹⁸ /cm². This process was carried out at a high temperature (750 °C) to promote NV center formation while mitigating crystal damage.
Sensor Matching: The exposed diamond was split into two pieces (Sensor 1 and Sensor 2) to ensure equal-quality quantum properties (NV density, coherence time) necessary for effective noise cancellation.
Sensor Integration: Each diamond sensor was attached to an optical fiber for remote placement and mounted on a coplanar waveguide antenna for homogeneous microwave (MW) delivery.
Optical Excitation and Collection: A 532 nm laser illuminated both sensors via the optical fiber. Fluorescence (> 600 nm) was collected and detected by separate photodiodes.
Magnetic Field Measurement: The magnetic field strength was determined by measuring the shift in the Optically Detected Magnetic Resonance (ODMR) spectrum at the steepest point of the ms = -1 magnetic sublevel.
Noise Cancellation: The differential signal (Sensor 1 minus Sensor 2) was calculated computationally to cancel the large, spatially homogeneous environmental magnetic noise, leaving the localized target signal.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the critical diamond materials and fabrication services necessary to advance this NV gradiometer research into robust, commercial devices. Our capabilities directly address the material quality, dimensional precision, and integration requirements highlighted in the paper.

Applicable Materials

Research Requirement	6CCVD Material Recommendation	Technical Justification
High Nitrogen Concentration (> 10¹⁹ atoms/cm³)	High-Nitrogen Single Crystal Diamond (SCD)	Our MPCVD process allows for precise control over nitrogen incorporation, providing the optimal precursor material for high-density NV ensemble creation via post-growth irradiation/annealing.
Equal-Quality Sensors	Optical Grade SCD Wafers	Starting with highly uniform SCD wafers (up to 500 µm thick) ensures that subsequent processing (irradiation) yields two sensors with highly matched quantum properties (coherence time, ODMR contrast), critical for maximizing noise cancellation efficiency.
Deep Signal Sensing (MEG)	Thick SCD Substrates (up to 10 mm)	For applications requiring larger NV volumes or robust mechanical support, 6CCVD offers substrates up to 10 mm thick, exceeding the 1.2 mm³ volume used in the study.

Customization Potential

The success of the gradiometer relies on precise physical dimensions and effective integration of microwave components. 6CCVD provides end-to-end customization services:

Custom Dimensions and Dicing: The paper required splitting the diamond into two equal-quality pieces. 6CCVD offers custom laser cutting and dicing services to produce matched sensor pairs with precise dimensions (e.g., 1.2 mm³ volume) from large SCD plates (up to 500 µm thick) or substrates (up to 10 mm).
Surface Quality: Efficient optical coupling (532 nm laser) and fluorescence collection require minimal surface scattering. 6CCVD guarantees ultra-low roughness polishing (Ra < 1 nm) for SCD, ensuring maximum photon throughput.
Microwave Integration (Antenna): The sensors utilized coplanar waveguide antennas. 6CCVD offers in-house custom metalization services, including standard stacks like Ti/Pt/Au, Ti/W, and Cu, deposited directly onto the diamond surface to facilitate robust microwave circuit integration.

Engineering Support

The paper notes that further sensitivity improvement requires optimizing electron irradiation, thermal treatment, and spin manipulation sequences (e.g., double quantum method).

Process Optimization: 6CCVD’s in-house PhD team specializes in MPCVD growth parameters and material characterization. We offer consultation to researchers and engineers seeking to optimize the starting material (SCD) for specific post-growth processing recipes (e.g., high-temperature irradiation at 750 °C) to achieve high concentration NV centers with long spin coherence times.
Application Focus: We provide expert material selection assistance for similar Quantum Magnetometry and Bio-Sensing projects, ensuring the diamond properties (N concentration, strain, surface finish) meet the demanding requirements of high-sensitivity quantum sensors.

Call to Action

To replicate this breakthrough research or develop next-generation field-portable NV gradiometers, high-quality, customized diamond material is essential. For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available).

View Original Abstract

The negatively charged nitrogen-vacancy (NV) center in diamonds is known as the spin defect and using its electron spin, magnetometry can be realized even at room temperature with extremely high sensitivity as well as a high dynamic range. However, a magnetically shielded enclosure is usually required to sense weak magnetic fields because environmental magnetic field noises can disturb high sensitivity measurements. Here, we fabricated a gradiometer with variable sensor length that works at room temperature using a pair of diamond samples containing negatively charged NV centers. Each diamond is attached to an optical fiber to enable free sensor placement. Without any magnetically shielding, our gradiometer realizes a magnetic noise spectrum comparable to that of a three-layer magnetically shielded enclosure, reducing the noises at the low-frequency range below 1 Hz as well as at the frequency of 50 Hz (power line frequency) and its harmonics. These results indicate the potential of highly sensitive magnetic sensing by the gradiometer using the NV center for applications in noisy environments such as outdoor and in vehicles.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s21030977

References

2008 - High-Sensitivity Diamond Magnetometer with Nanoscale Resolution [Crossref]
2013 - The Nitrogen-Vacancy Colour Centre in Diamond [Crossref]
2014 - Nitrogen-Vacancy Centers in Diamond: Nanoscale Sensors for Physics and Biology [Crossref]
2015 - High-Frequency and High-Field Optically Detected Magnetic Resonance of Nitrogen-Vacancy Centers in Diamond [Crossref]
2014 - Electronic Properties and Metrology Applications of the Diamond NV−Center under Pressure [Crossref]
2016 - Quantitative Nanoscale Vortex Imaging Using a Cryogenic Quantum Magnetometer [Crossref]
2019 - Measuring Magnetic Field Texture in Correlated Electron Systems under Extreme Conditions [Crossref]
2005 - Bright Fuorescent Nanodiamonds: No Photobleaching and Low CY-Totoxicity [Crossref]
2012 - Real-Time Background-Free Selective Imaging Fluorescent Nanodiamonds in Vivo [Crossref]
2016 - Fluorescent Nanodiamond: A Versatile Tool for Long-Term Cell Tracking, Super-Resolution Imaging, and Nanoscale Temperature Sensing [Crossref]