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	Tokyo Institute of Technology, National Institutes for Quantum Science and Technology
Citations	14
Analysis	Full AI Review Included

Executive Summary

Core Innovation: Development of a negatively charged Nitrogen-Vacancy (NV) center gradiometer using two spatially separated diamond sensors coupled via optical fibers, enabling a variable base length.
Noise Performance: Achieved background magnetic noise reduction comparable to that of a commercial three-layer permalloy magnetically shielded enclosure.
Environmental Robustness: Successfully canceled environmental magnetic noise, including low-frequency fluctuations (below 1 Hz) and commercial power line frequencies (50 Hz and harmonics).
Cancellation Efficacy: A spatially homogeneous AC magnetic field (20 Hz, 31 µT) was reduced to less than 1/50 in the differential signal.
Deep Signal Sensing: Demonstrated effective detection of a target signal (30 Hz, 10 µT) at a 50 mm depth, with the differential signal increasing dramatically as the base length was extended up to 100 mm.
Portability Advantage: The system eliminates the need for bulky magnetic shielding, making it suitable for high-sensitivity magnetometry in noisy, non-laboratory environments (e.g., outdoor or in vehicles).
Sensor Noise Floor: The measured noise floor of the gradiometer was 34 nT, limited primarily by the sensor sensitivity above 10 Hz.

Technical Specifications

Parameter	Value	Unit	Context
Sensor Configuration	Gradiometer	N/A	Two spatially isolated NV diamond sensors.
Noise Floor (W/ Gradiometer)	34	nT	Comparable to three-layer permalloy shield.
Homogeneous Noise Cancellation	< 1/50	Ratio	Reduction of 20 Hz, 31 µT field in differential signal.
Target Signal Detection	10	µT	Detected 30 Hz signal amidst 31 µT noise.
Base Length (Variable Range)	Up to 100	mm	Tested for sensing deep targets (50 mm depth).
Diamond Type	Type Ib	N/A	Nitrogen concentration > 10¹⁹ atoms/cm³.
Electron Irradiation Energy	2	MeV	Used for vacancy creation.
Electron Irradiation Fluence	1 x 10¹⁸	/cm²	Dose applied to the diamond.
Irradiation/Annealing Temperature	750	°C	Performed during irradiation to create NV centers.
NV Center Volume (per sensor)	1.2	mm³	Volume of diamond illuminated/measured.
Laser Wavelength	532	nm	Excitation source.
Laser Power	300	mW	Optical power delivered through fiber.
MW Power	25	dBm	Continuous-wave power to coplanar waveguide.
MW Modulation Frequency	2	kHz	Frequency modulation rate.
ODMR Linewidth (Ch. 1 / Ch. 2)	12.8 / 13.1	MHz	Resonance width.
ODMR Contrast (Ch. 1 / Ch. 2)	1.7 / 1.8	%	Signal intensity relative to fluorescence.

Key Methodologies

Diamond Fabrication:
- Started with Type Ib diamond (high nitrogen concentration).
- Irradiated with 2 MeV electrons at a fluence of 1 x 10¹⁸ /cm².
- Irradiation was performed at 750 °C to prevent crystal damage accumulation and facilitate NV center formation.
- The exposed diamond was cut into two pieces to ensure equal quantum properties (coherence time, NV density) for Sensor 1 and Sensor 2.
Sensor Assembly and Optical Coupling:
- Each diamond piece was attached to a separate optical fiber, enabling flexible, variable base length configuration.
- Each diamond was mounted on a non-resonant coplanar waveguide (CPW) antenna for homogeneous microwave control across a wide frequency range.
Optically Detected Magnetic Resonance (ODMR) Setup:
- A 532 nm laser (300 mW) was delivered via the optical fiber to excite the NV centers in both diamonds.
- Fluorescence (> 600 nm) was collected via the same fiber, separated by a dichroic mirror, and detected by photodetectors.
Microwave (MW) Control:
- Two independent MW sources (25 dBm) were used, allowing the frequency for each sensor to be independently tuned to the steepest point of its respective ODMR spectrum (ms = -1 state).
- The MW signal was frequency modulated at 2 kHz with an 8 MHz deviation width.
Gradiometer Signal Processing:
- The magnetic field strength was calculated from the fluorescent signal change at the steepest point of the ODMR spectrum.
- The differential signal (Sensor 1 minus Sensor 2) was calculated digitally by a computer after measurement, providing noise cancellation.
Noise Evaluation:
- Homogeneous noise (20 Hz, 31 µT) was generated by a solenoid coil.
- Inhomogeneous target signal (30 Hz, 10 µT) was generated by a copper wire placed near Sensor 1.
- Performance was benchmarked against a single sensor operating inside a three-layer permalloy magnetic shield.

Commercial Applications

Biomedical Imaging (MEG/MCG): Enables Magnetoencephalography (MEG) and Magnetocardiography (MCG) at room temperature without the need for expensive, bulky magnetic shielding rooms, significantly increasing accessibility.
Field-Portable Quantum Sensing: Suitable for constructing portable, high-sensitivity magnetometers for use in noisy, unshielded environments.
Geophysical Exploration: Application in natural resource exploration, where the portability and robustness of diamond sensors are critical for field deployment.
Automotive Safety: Monitoring driver consciousness or physiological signals in vehicles, leveraging the diamond’s stability against harsh environments and magnetic noise cancellation.
Security and Defense: Development of robust, high-dynamic-range vector magnetometers for magnetic anomaly detection (MAD) systems.

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

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