Diamond Magnetometry and Gradiometry Towards Subpicotesla dc Field Measurement
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-06-30 |
| Journal | Physical Review Applied |
| Authors | Chen Zhang, Farida Shagieva, Matthias Widmann, Michael KĂŒbler, Vadim V. Vorobyov |
| Institutions | National Institutes for Quantum and Radiological Science and Technology, Tokyo Gas (Japan) |
| Citations | 93 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research demonstrates the achievement of subpicotesla DC magnetic field sensitivity using Nitrogen-Vacancy (NV) center ensembles in diamond, optimized for low optical power and ambient operation.
- Core Achievement: Minimum detectable field of 0.3 - 0.7 pT achieved in a 73 s measurement time by integrating a ferrite flux guide (FG).
- Sensitivity Record: The normalized magnetic field sensitivity reached 2.6 - 6 pT/âHz when utilizing the flux guide enhancement (6.3x factor).
- Compact Sensing: The sensor utilizes a small (0.5 mm)3 diamond cube, corresponding to a sensing volume of 0.125 mm3.
- Low Power Operation: High sensitivity is maintained using low optical excitation power (below 100 mW), mitigating thermal noise and complexity associated with high-power lasers.
- Methodology Optimization: The Continuously Excited (CE)-Ramsey sequence with lock-in detection proved superior to optimized Continuous-Wave Optically Detected Magnetic Resonance (CW-ODMR) at low laser power.
- Gradiometry Capability: The setup successfully demonstrated gradiometry, achieving a minimum detectable differential field noise of 4 - 6 pT, suppressing common-mode line shifts.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Minimum Detectable Field | 0.3 - 0.7 | pT | With ferrite flux guide (FG), 73 s measurement |
| Normalized Sensitivity (with FG) | 2.6 - 6 | pT/âHz | 1 Hz normalization |
| Intrinsic Noise Floor | 2 - 3 | pT | Without FG, 0 - 200 Hz bandwidth |
| Normalized Sensitivity (without FG) | 17 | pT/âHz | 1 Hz normalization |
| Gradiometry Noise (Differential) | 4 - 6 | pT | 73 s measurement time |
| Sensing Volume | 0.125 | mm3 | (0.5 mm)3 diamond cube |
| Dephasing Time (T2*) | 8.5 | ”s | NV ensemble characteristic |
| Relaxation Time (T1) | 6 | ms | NV ensemble characteristic |
| Minimum ODMR Linewidth | 28 | kHz | FWHM, weak MW driving |
| Excitation Laser Power | < 100 | mW | Low-intensity operation |
| Flux Amplification Factor | 6.3 | (unitless) | Experimental enhancement using MN60 ferrite rod |
| Magnetometer Bandwidth (3 dB) | ~1.5 | kHz | CW-ODMR operation |
| Ferrite Rod Permeability | 6500 | (unitless) | MN60 material |
| Fluorescence Collection Efficiency | > 60 | % | Using Compound Parabolic Concentrator (CPC) |
| NV Concentration | 0.4 | ppm | After irradiation and annealing |
Key Methodologies
Section titled âKey MethodologiesâThe high-sensitivity measurements relied on optimized material preparation, advanced spin manipulation, and robust noise mitigation techniques.
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Diamond Sample Preparation:
- Material: (111)-oriented, 99.97% 12C enriched single crystal.
- Growth: High Pressure High Temperature (HPHT) method.
- NV Creation: 2 MeV electron irradiation followed by high-temperature annealing (1000 °C for 2 hours in vacuum).
- Result: NV concentration of 0.4 ppm, yielding long T2* (8.5 ”s) and narrow ODMR linewidth (28 kHz).
-
Spin Manipulation and Readout:
- Excitation: Low-noise 532 nm laser (Lighthouse Sprout-G) operating at low power (near 80 mW).
- MW Driving: Uniform MW field generated by a Dielectric Resonator Antenna (DRA).
- Measurement Sequence: Continuously Excited (CE)-Ramsey sequence used, where the laser remains on during the field acquisition time (Tm) to simplify optics and improve thermal stability.
- Detection: Fluorescence collected via a Compound Parabolic Concentrator (CPC) and processed using a Lock-in Amplifier (LIA, Zurich Instruments HF2LI) for differential detection.
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Sensitivity Enhancement and Noise Mitigation:
- Flux Concentration: A MN60 ferrite rod (2 mm tip diameter) was placed near the diamond to guide and concentrate the magnetic flux, providing a 6.3x signal amplification.
- Gradiometry: Two identical diamond sensors were constructed to measure the differential magnetic field, suppressing common-mode noise (e.g., thermal drift).
- Shielding: The entire setup was enclosed in a magnetic shield cube (inner ”-metal, outer aluminum) to attenuate external magnetic field noise.
- Bias Field: Generated by 12 V lead-acid batteries to ensure low magnetic field noise in the shields during intrinsic noise measurements.
Commercial Applications
Section titled âCommercial ApplicationsâThe demonstrated NV diamond magnetometer, characterized by its picotesla sensitivity, small sensing volume, and ambient operation, is highly relevant for several high-tech sectors.
- Biomedical and Clinical Diagnostics:
- Non-Cryogenic Biomagnetism: Development of compact, room-temperature sensors for Magnetoencephalography (MEG) and Magnetocardiography (MCG), replacing bulky SQUID systems.
- Cellular and Neural Activity: High-resolution sensing of magnetic fields generated by single neurons or small biological samples.
- Material Science and Nondestructive Evaluation (NDE):
- Magnetic Imaging: High-spatial resolution mapping of magnetic domains, defects, and current distributions in microelectronic circuits and advanced materials.
- Quality Control: Ultra-sensitive detection of magnetic impurities or structural flaws in semiconductor wafers and components.
- Quantum Technology and Metrology:
- Compact Quantum Sensors: Integration into portable or drone-mounted systems requiring high sensitivity without cryogenic cooling.
- Vector Magnetometry: Potential for wide-bandwidth vector field measurements when combined with multi-orientation driving schemes.
- Defense and Security:
- Unshielded Sensing: Gradiometry combined with flux concentration enables high-sensitivity measurements in unshielded, noisy environments.
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