Subnanotesla Magnetometry with a Fiber-Coupled Diamond Sensor
At a Glance
Section titled âAt a Glanceâ| 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 |
Executive Summary
Section titled âExecutive SummaryâThis analysis summarizes the development and performance of a highly sensitive, fiber-coupled diamond magnetometer designed for portable applications like magnetocardiography (MCG).
- Core Achievement: Demonstrated sub-nanotesla (sub-nT) sensitivity, achieving (310 ± 20) pT/âHz in the 10-150 Hz frequency range using applied test fields.
- Technology: Optically Detected Magnetic Resonance (ODMR) utilizing an ensemble of Nitrogen Vacancy (NV) centers in a high-purity, 12C-enriched CVD diamond.
- Key Innovation: Integration of aspheric lenses within the sensor head to efficiently couple excitation light into the diamond and collect fluorescence back into the fiber, significantly reducing optical losses.
- Portability: The fiber-coupled design separates the bulky control instrumentation (lasers, microwaves) from the sensor head, allowing the sensor to be placed conveniently within 2 mm of the object under study.
- Performance Context: This sensitivity represents a significant improvement over previous fiber-coupled NV magnetometers (35 nT/âHz using applied fields) and is a factor of ~6 away from the estimated photon shot-noise limit (50 pT/âHz).
- Future Outlook: Achieving the sensitivity required for MCG (estimated to be an order of magnitude better) will require improvements in photon collection efficiency (currently 0.03%) and implementation of noise cancellation techniques (e.g., gradiometry or dual-resonance methods).
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Achieved Sensitivity | 310 ± 20 | pT/âHz | Measured using applied test fields (10-150 Hz) |
| Photon Shot Noise Limit | 50 | pT/âHz | Calculated theoretical limit |
| Zero-Field Splitting (D) | ~2.87 | GHz | NV ground state at room temperature |
| Hyperfine Splitting (A) | ~2.16 | MHz | Interaction with 14N nuclear spin |
| Optimal Microwave Power | ~0.8 | W | Applied power after amplification |
| Optimal Modulation Frequency | 3.0307 | kHz | Frequency modulation rate |
| Optimal Frequency Depth | 300 | kHz | Frequency modulation deviation |
| Laser Wavelength | 532 | nm | Excitation source (Laser Quantum Gem-532) |
| Laser Power Used | 1 | W | Used to reduce laser noise |
| Excitation Fiber Core Diameter | 400 | ”m | Thorlabs FG400AEA fiber |
| Diamond Dimensions | 4 x 4 x 0.6 | mm | Single crystal sample size |
| Diamond Orientation | (100) | - | Crystallographic orientation |
| Diamond Isotope Purity | 99.995% | 12C | Carbon enrichment |
| NV- Concentration | 4.6 | ppm | Negatively charged NV concentration |
| Fluorescence Collection Efficiency | 0.03 | % | Calculated conversion efficiency (Green to Red) |
| Diamond Refractive Index (nd) | 2.42 | - | Limits light collection due to TIR |
| Measured Linewidth (ÎÎœ) | 1.11 | MHz | Extracted from ODMR spectrum |
| Measured Contrast (C) | 1.76 | % | Extracted from ODMR spectrum |
Key Methodologies
Section titled âKey MethodologiesâThe magnetometry system relies on continuous wave (CW) ODMR using a fiber-coupled setup optimized for high photon collection efficiency.
- Excitation and Noise Reduction: A 532 nm laser (1 W) is used for excitation. Approximately 1% of the beam is sampled and fed to a balanced detector (Thorlabs PDB450A) reference arm to actively cancel out laser intensity noise.
- Fiber Coupling and Focusing: The main laser beam is coupled into a 400 ”m core fiber. The fiber output is collimated by the first aspheric lens (C171TMD-B) and focused onto the diamond by the second lens (C330TMD-B). These same lenses collect the resulting NV fluorescence.
- Microwave Delivery: Microwaves (2-4 GHz) are generated, amplified (Mini-Circuits ZHL-16W-43-S+), and delivered via a coaxial circulator to a 5 mm copper loop antenna deposited on an aluminum prototyping board, which also serves for heat management.
- Magnetic Field Alignment: A permanent rare earth magnet is aligned to the (111) crystallographic axis of the NV ensemble to provide the necessary Zeeman splitting.
- Signal Detection and Modulation: The balanced detector output is fed to a lock-in amplifier (Zurich MFLI). The microwave frequency is square-wave modulated, and a 2.158 MHz sinewave is mixed in to utilize hyperfine excitation for improved contrast.
- Optimization: Sensitivity was optimized by systematically varying the microwave power (0.06 W to 3.16 W pre-amplification), frequency modulation depth (100 kHz to 600 kHz), and modulation frequency (1 kHz to 80 kHz) to maximize the zero-crossing slope of the derivative spectrum.
- Sensitivity Measurement: Sensitivity was determined using two methods: (1) calculating the zero-crossing slope (yielding 171 pT/âHz) and (2) applying calibrated test fields via a Helmholtz coil along the (100) direction (yielding 310 ± 20 pT/âHz).
Commercial Applications
Section titled âCommercial ApplicationsâThe combination of sub-nT sensitivity and high portability makes this fiber-coupled NV diamond sensor suitable for several advanced engineering and medical applications:
- Medical Diagnostics (MCG): The primary target application is Magnetocardiography (MCG), requiring highly sensitive, non-invasive magnetic field detection near the body. The compact, mobile sensor head is ideal for clinical settings.
- Geology and Materials Inspection: Sensing small magnetic fields for geological surveys or non-destructive testing of materials where the sensor must be brought into close proximity (less than 2 mm) to the sample.
- Portable Quantum Sensing: Development of robust, field-deployable quantum sensors that require high sensitivity without the need for cryogenic cooling (room temperature operation).
- Compact Instrumentation: Integration into complex systems where the control electronics must be physically separated from the sensing element, such as in boreholes, confined spaces, or integrated circuit testing.
- Temperature-Invariant Sensing: Future implementation of dual-resonance or double-quantum magnetometry techniques will enable highly stable, temperature-invariant sensors crucial for industrial monitoring and field use.
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/âHz 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.