Highly Sensitive Magnetometer with Continuous Excitation Ramsey Protocol by Utilizing Long Dephasing Time of Millimeter‐Scale Spin Ensembles in (111) Diamond
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-06-25 |
| Journal | Advanced Quantum Technologies |
| Authors | Ikuya Fujisaki, 李泽汉 Li Zehan, Yuji Hatano, T. Sekiguchi, Naota Sekiguchi |
| Institutions | Institute of Science Tokyo, Tokyo Institute of Technology |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research details the development of a highly sensitive, millimeter-scale magnetometer utilizing Nitrogen-Vacancy (NV) centers in (111) diamond, optimized for Continuous Excitation (CE) Ramsey protocols.
- Record Dephasing Time: Achieved a long dephasing time (T*2) of 20.5 µs (Pulsed-Ramsey) and 16.4 µs (CE-Ramsey) in millimeter-scale NV ensembles, significantly exceeding previous reports for (111) diamond.
- High Sensitivity: Demonstrated a magnetic sensitivity of 15.8 pT/√Hz using the CE-Ramsey protocol in magnetically insensitive mode, paving the way for simpler, high-performance magnetometers.
- Engineered Homogeneity: Sensitivity was enabled by an original magnet array and antenna (CPW) design, ensuring highly uniform bias fields and efficient application of quantum control techniques (Spin Bath Driving, SBD, and Double Quantum, DQ, Ramsey) over the 1.3 mm probing length.
- Dephasing Analysis: Decomposition of dephasing factors revealed that 13C nuclear spins and P1 electron spins are the dominant limiting channels, guiding future material improvements.
- CE vs. Pulsed Comparison: The study quantified the noise induced by laser pulsing, showing that CE-Ramsey avoids this noise source, although its T*2 is slightly reduced due to continuous spin initialization.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| CE-Ramsey Sensitivity (η) | 15.8 | pT/√Hz | Magnetically insensitive mode (2-200 Hz) |
| Pulsed-Ramsey Sensitivity (η) | 14.8 | pT/√Hz | Magnetically insensitive mode (2-200 Hz) |
| CE-Ramsey T2 (DQ+SBD)* | 16.4 | µs | Continuous Excitation, 160 mW optical power |
| Pulsed-Ramsey T2 (DQ+SBD)* | 20.5 | µs | Pulsed Excitation, 160 mW optical power |
| Photon Shot Noise Limit (CE) | 14.7(2) | pT/√Hz | Calculated sensitivity limit |
| Magnetic Bias Field (Bz) | ~3.5 | mT | Applied via SmCo magnet array |
| Optical Excitation Power (P) | 160 | mW | Standard measurement power |
| Diamond Temperature (T) | ~34 | °C | Estimated during 160 mW excitation |
| Probing Volume (Length x Diameter) | 1.3 x 0.3 | mm | Side-face laser irradiation |
| Responsivity (CE-Ramsey) | 1.43 | A/T | Maximized at interrogation time topt = 10.0 µs |
| Diamond Orientation | (111) | N/A | Surface cut along the crystallographic plane |
| NV- Concentration | 0.07(2) | ppm | Optimized for SBD efficiency |
| P1 Concentration | 0.28(8) | ppm | Optimized for SBD efficiency |
| 13C Concentration | 50(10) | ppm | Isotope concentration |
| TIA Gain / Cutoff Frequency | 9 kV A-1 / 15 kHz | N/A | Signal detection electronics |
| Spin Initialization Dephasing Rate Slope | 0.11 | kHz/mW | Rate increase per unit laser power (dΓ*2,CE/dP) |
Key Methodologies
Section titled “Key Methodologies”The high performance was achieved through specialized material selection and engineering of the magnetic and RF environment for millimeter-scale sensing.
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Diamond Material Selection and Preparation:
- Used 12C enriched HPHT diamond cut along the (111) plane, which simplifies sensor alignment (1-axis NV centers).
- NV and P1 concentrations (0.07 ppm and 0.28 ppm, respectively) were optimized to maximize the product of T*2 and NV concentration, ensuring efficient Spin Bath Driving (SBD) under available RF power.
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Bias Field Homogeneity Engineering:
- A custom, symmetrically positioned pair of circular SmCo magnet arrays was used to generate a highly homogeneous bias field (~3.5 mT) across the 1.3 mm probing volume.
- The inner (170 mm) and outer (188 mm) ring diameters, and their ring-to-ring distances (288 mm and 318 mm), were numerically optimized to minimize magnetic field inhomogeneity-induced dephasing.
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Quantum Manipulation Implementation:
- A Coplanar Waveguide (CPW) antenna was aligned parallel to the 1.3 mm optical excitation path to ensure spatial uniformity of Microwave (MW) and Radio Frequency (RF) fields.
- Double Quantum (DQ) Ramsey: Used the ms = {-1, +1} basis to suppress common-mode dephasing (e.g., temperature fluctuations, longitudinal strain inhomogeneity).
- Spin Bath Driving (SBD): Applied RF fields to decouple the NV spins from the P1 electron spin bath, significantly extending T*2.
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Continuous Excitation (CE) Protocol:
- The CE-Ramsey protocol was employed, where the 532 nm laser was continuously irradiated throughout the Ramsey sequence. This eliminates the noise induced by laser pulsing, which was identified as a significant noise source in the pulsed-Ramsey setup.
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Differential Detection and Noise Mitigation:
- Fluorescence was collected via total internal reflection and detected using a large area photodiode.
- Laser intensity noise was suppressed using a differential current measurement between the signal photodiode and a reference photodiode (nonpolarizing beam splitter pickup).
Commercial Applications
Section titled “Commercial Applications”The development of a highly sensitive, large-volume, low-power magnetometer using robust (111) diamond is critical for several emerging quantum sensing applications.
- Biomagnetism (MEG/MCG):
- High sensitivity (15.8 pT/√Hz) in the near-DC to 200 Hz range is essential for applications like Magnetoencephalography (MEG) and Magnetocardiography (MCG), which require sensing weak magnetic fields generated by neural and cardiac activity.
- Geophysics and Materials Science:
- Large-volume sensing (millimeter-scale) is suitable for non-destructive evaluation (NDE) and geological surveys where a larger sensing area is beneficial for detecting bulk magnetic anomalies or defects.
- Industrial Monitoring and Quality Control:
- The simple implementation and robust nature of the (111) diamond sensor, combined with high sensitivity, make it viable for industrial magnetic field monitoring and quality control systems that require ambient operation.
- Quantum Sensing Technology:
- Advancement of NV-based quantum platforms, particularly demonstrating effective quantum control (DQ, SBD) in large-scale ensembles, which is a prerequisite for scaling up quantum sensors.
View Original Abstract
Abstract We developed a highly sensitive magnetometer with continuous excitation (CE) Ramsey protocol for (111) diamonds. The distinctive feature of the magnetometer is the magnet array and antenna design to realize a uniform bias field and efficient application of quantum manipulation techniques of spin bath driving and double quantum Ramsey in millimeter‐scale spin ensembles directed to the surface normal. CE‐ and pulsed‐Ramsey show dephasing times of and , respectively, with an excitation power of 160 mW. Analysis of the compositions of dephasing factors unveils the dephasing channels limiting the dephasing time. Thanks to the long dephasing time, a sensitivity of is achieved in CE‐Ramsey. Furthermore, the comparison of sensitivity between CE‐ and pulsed‐Ramsey indicated that the noise induced by laser pulsing exists. This work paves the way for realizing ultra‐high sensitivity magnetometers with simple implementation.