Subnanotesla Magnetometry with a Fiber-Coupled Diamond Sensor

At a Glance

Metadata	Details
Publication Date	2020-10-30
Journal	Physical Review Applied
Authors	R. L. Patel, Li Zhou, Angelo Frangeskou, G. A. Stimpson, Ben G. Breeze
Institutions	Engineering and Physical Sciences Research Council, Element Six (United Kingdom)
Citations	62
Analysis	Full AI Review Included

Technical Documentation & Analysis: Fiber-Coupled Diamond Magnetometry

Executive Summary

This research demonstrates a significant advancement in compact, high-sensitivity magnetometry using ensemble Nitrogen Vacancy (NV) centers in diamond, specifically targeting applications like Magnetocardiography (MCG).

Record Sensitivity: Achieved a sensitivity of (310 ± 20) pT/√Hz in the 10-150 Hz range, representing the best reported sensitivity for a fiber-coupled diamond magnetometer using applied test fields.
Core Material: The sensor relies on a 4 mm x 4 mm x 0.6 mm, (100)-oriented Single Crystal Diamond (SCD) enriched to 99.995% ¹²C, grown via Chemical Vapor Deposition (CVD).
Compact Design: The fiber-coupled setup allows the sensor head to be brought within 2 mm of the object under study, crucial for high-mobility medical diagnostic techniques.
Methodology: Sensitivity was achieved using continuous wave Optically Detected Magnetic Resonance (ODMR) combined with square-wave frequency modulation and lock-in detection.
Performance Gap: The achieved sensitivity is approximately six times worse than the estimated photon shot-noise limit (50 pT/√Hz), primarily due to poor photon collection efficiency (0.03% conversion efficiency) caused by diamond’s high refractive index (n_d = 2.42).
Future Optimization: Improvements require advanced optical integration (e.g., total internal reflection lenses or waveguides) and more homogeneous microwave driving fields.

Technical Specifications

Parameter	Value	Unit	Context
Achieved Sensitivity (Applied Field)	310 ± 20	pT/√Hz	Measured using known test fields (10-150 Hz range)
Photon Shot Noise Limit (Estimated)	50	pT/√Hz	Theoretical limit based on collected fluorescence (1.2x10¹⁵ photons/s)
Diamond Material	Single Crystal CVD	N/A	99.995% ¹²C enriched
Diamond Dimensions	4 x 4 x 0.6	mm	(100)-orientation
NV^- Concentration	4.6	ppm	Negatively charged Nitrogen Vacancy centers
Excitation Wavelength	532	nm	Green laser excitation
Excitation Power (Used)	1	W	Used to reduce laser noise
Zero-Field Splitting (NVC)	~2.87	GHz	Room temperature operation
Measurement Contrast (C)	1.76	%	Extracted from ODMR spectrum
Linewidth (Δν)	1.11	MHz	Extracted from ODMR spectrum
Optimal Microwave Power	~0.8	W	Power after amplification
Refractive Index (n_d)	2.42	N/A	Primary cause of low photon collection efficiency

Key Methodologies

The sub-nanotesla sensitivity was achieved using a highly optimized continuous wave ODMR setup focusing on noise reduction and precise microwave control.

Optical Excitation and Noise Cancellation:
- A 1 W, 532 nm laser beam was focused into a 5 m, 400 µm core optical fiber (0.22 N.A.).
- Laser intensity noise was actively cancelled using a Thorlabs PDB450A balanced detector, sampling approximately 1% of the incident beam.
Fiber-to-Diamond Coupling:
- The fiber output was collimated and focused onto the diamond using a pair of adjustable aspheric lenses (Thorlabs C171TMD-B and C330TMD-B). The same lenses collected the resulting fluorescence.
Microwave Generation and Delivery:
- Microwave excitation (2-4 GHz range) was provided by an Agilent N5172B source and amplified by a Mini-Circuits ZHL-16W-43-S+ amplifier.
- Microwaves were delivered via a 5 mm copper loop deposited onto an aluminum prototyping board, which also served for heat management.
Signal Optimization:
- Hyperfine excitation was utilized by mixing a 2.158 MHz sinewave to improve ODMR contrast.
- Microwaves were square-wave frequency modulated (FM depth 300 kHz, FM frequency 3.0307 kHz) to maximize the zero-crossing slope.
Magnetic Alignment:
- A permanent rare earth magnet was aligned to the (111) crystallographic orientation to maximize the projection of the external field onto the NVC symmetry axis.
Detection:
- The signal was processed by a Zurich MFLI DC - 500 kHz lock-in amplifier (LIA). Sensitivity was determined by analyzing 160 one-second Fast-Fourier Transforms (FFTs) of the fluorescence signal monitored at the zero-crossing point.

6CCVD Solutions & Capabilities

The successful replication and extension of this sub-nanotesla magnetometry research hinges on the availability of ultra-high-quality, custom-engineered diamond substrates. 6CCVD is uniquely positioned to supply the necessary materials and fabrication services required for next-generation NV-based quantum sensors.

Research Requirement	6CCVD Applicable Materials	Customization Potential & Sales Advantage
High-Purity Substrate	Optical Grade SCD (Single Crystal Diamond): We supply low-strain, high-purity MPCVD SCD, essential for maximizing the NV center coherence time (T₂).	We can provide isotopically enriched ¹²C SCD (as used in this paper) to minimize decoherence from ¹³C nuclear spins, ensuring optimal quantum performance.
Custom Dimensions & Thickness	SCD Plates/Wafers & Substrates: The paper used 4 mm x 4 mm x 0.6 mm. We offer custom dimensions up to 125 mm (PCD) and precise thickness control for SCD (0.1 µm - 500 µm) and substrates (up to 10 mm).	Precision Laser Cutting: Enables the fabrication of sensor heads with exact geometries required for compact fiber coupling and integration within 2 mm of the target object.
Optical Integration & Efficiency	Ultra-Low Roughness Polishing: SCD polishing capability to Ra < 1 nm.	The paper identifies collection efficiency (0.03%) as the biggest limitation. Our ultra-smooth surfaces are critical for implementing advanced optical solutions (e.g., total internal reflection lenses, solid immersion lenses, or waveguide coupling) suggested to reach the 50 pT/√Hz shot-noise limit.
Microwave Delivery Structures	Custom Metalization Services: In-house deposition of Au, Pt, Pd, Ti, W, and Cu.	Necessary for fabricating the high-homogeneity microwave driving fields required for advanced pulsed schemes like Ramsey magnetometry, which offer significant sensitivity improvements over continuous wave methods.
Target Application	Engineering Support: Our in-house PhD team specializes in material selection and defect engineering for Quantum Sensing and Magnetocardiography (MCG) projects.	We provide consultation on optimizing nitrogen incorporation and subsequent irradiation/annealing protocols to achieve the required [NV^-] concentration (4.6 ppm in this study) for ensemble sensing.

For custom specifications or material consultation regarding high-sensitivity NV magnetometry, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available).

View Original Abstract

Nitrogen-vacancy centers (NVCs) in diamond are being explored for future quantum technologies, and in particular ensembles of NVC are the basis for sensitive magnetometers. We present a fiber-coupled NVC magnetometer with an unshielded sensitivity of (310±20)pT/√Hz in the frequency range of 10-150 Hz at room temperature. This takes advantage of low-strain 12C diamond, lenses for fiber coupling and optimization of microwave modulation frequency, modulation amplitude, and power. Fiber coupling means the sensor can be conveniently brought within 2 mm of the object under study.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.14.044058