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 | National Institute for Materials Science, University of Tsukuba |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Highly Sensitive NV Magnetometry
Section titled “Technical Documentation & Analysis: Highly Sensitive NV Magnetometry”This document analyzes the research paper “Highly Sensitive Magnetometer with Continuous Excitation Ramsey Protocol by Utilizing Long Dephasing Time of Millimeter-Scale Spin Ensembles in (111) Diamond” to provide technical specifications and demonstrate how 6CCVD’s advanced MPCVD diamond materials and customization capabilities can support and extend this critical quantum sensing research.
Executive Summary
Section titled “Executive Summary”The research successfully demonstrates a highly sensitive NV-ensemble magnetometer utilizing Continuous Excitation (CE) Ramsey protocol in (111) diamond, achieving performance metrics essential for bio-magnetic field applications.
- Record Sensitivity: Achieved a sensitivity of 15.8 pT/√Hz (2-200 Hz range) using the CE-Ramsey protocol in (111) diamond, setting a new benchmark for simply implementable magnetometers.
- Long Dephasing Time: Measured long dephasing times ($T_{2}^{*}$) up to 20.5 µs (Pulsed Double Quantum (DQ) + Spin Bath Driving (SBD)), significantly exceeding previous (111) diamond reports.
- Millimeter-Scale Ensembles: The high sensitivity was achieved using millimeter-scale spin ensembles (1.3 mm optical path), demonstrating scalability for practical industrial and biological sensing volumes.
- Advanced Quantum Control: Effective quantum manipulation was enabled by an original magnet array and antenna design, realizing a uniform bias field (≈ 3.5 mT) necessary for DQ Ramsey and SBD techniques.
- Material Optimization: The study utilized $^{12}$C enriched diamond with precisely controlled [P1] (0.28 ppm) and [NV$^-$] (0.07 ppm) concentrations, confirming that $^{13}$C nuclear spins and P1 electron spins remain the primary limiting dephasing channels.
- Pathway to Ultra-High Sensitivity: The work identifies that further reduction of $^{13}$C concentration and optimization of optical power density (e.g., beam shaping) are the key next steps to achieve ultra-high sensitivity.
Technical Specifications
Section titled “Technical Specifications”The following hard data points were extracted from the experimental results, detailing the performance and material characteristics of the NV magnetometer.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Magnetic Sensitivity (CE-Ramsey) | 15.8 | pT/√Hz | Magnetically insensitive mode (2-200 Hz) |
| Magnetic Sensitivity (Pulsed-Ramsey) | 14.8 | pT/√Hz | Magnetically insensitive mode (2-200 Hz) |
| Photon Shot Noise Limit (CE) | 14.7(2) | pT/√Hz | Theoretical limit at 160 mW |
| Dephasing Time T2* (Pulsed DQ+SBD) | 20.5 | µs | Longest T2* achieved |
| Dephasing Time T2* (CE DQ+SBD) | 16.4 | µs | Measured at 160 mW optical power |
| Excitation Optical Power | 160 | mW | Standard operating power (532 nm) |
| Bias Magnetic Field (B-field) | ≈ 3.5 | mT | Applied using custom SmCo magnet array |
| Diamond Crystal Orientation | (111) | N/A | Optimized for single-axis NV centers |
| Probing Volume Length | 1.3 | mm | Optical excitation path length |
| $^{13}$C Concentration | 50(10) | ppm | Primary dephasing source identified |
| P1 Concentration | 0.28(8) | ppm | Optimized for effective Spin Bath Driving (SBD) |
| NV- Concentration | 0.07(2) | ppm | Active sensing centers |
| Spin Initialization Dephasing Rate Slope | 0.11 | kHz/mW | Rate increase due to continuous laser excitation |
Key Methodologies
Section titled “Key Methodologies”The experiment relied on precise material engineering and advanced quantum control techniques to maximize the spin coherence time ($T_{2}^{*}$) in a large-volume ensemble.
- Material Preparation: Use of $^{12}$C enriched HPHT diamond, cut along the (111) crystallographic plane. The material was subjected to electron beam irradiation and annealing to achieve specific, optimized concentrations of P1 centers (0.28 ppm) and NV$^-$ centers (0.07 ppm).
- Bias Field Homogeneity: A highly homogeneous static bias field (≈ 3.5 mT) was applied using a custom-designed, symmetrically positioned pair of circular SmCo magnet arrays, minimizing magnetic field inhomogeneity-induced dephasing.
- Microwave (MW) and Radio Frequency (RF) Delivery: Combined MW and RF signals were applied via a 50 Ω Coplanar Waveguide (CPW) antenna, aligned parallel to the 1.3 mm optical excitation path to ensure spatial uniformity of the quantum manipulation fields.
- Quantum Control Protocols: Double Quantum (DQ) Ramsey was used to suppress common-mode dephasing, and Spin Bath Driving (SBD) was implemented to mitigate magnetic noise arising from P1 centers, maximizing $T_{2}^{*}$.
- Optical Excitation and Detection: A 532 nm laser irradiated the diamond side-face. Fluorescence was collected via total internal reflection and detected by a large area photodiode in a differential current setup, with the resulting signal processed by a two-phase lock-in amplifier (LIA).
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”This research highlights the critical need for ultra-high purity, precisely oriented, and custom-processed diamond substrates to push the limits of NV-based quantum sensing. 6CCVD’s MPCVD capabilities are perfectly positioned to supply the next generation of materials required to replicate and surpass the reported sensitivity.
| Research Requirement | 6CCVD Solution & Capability | Technical Advantage for Researchers |
|---|---|---|
| Ultra-Low $^{13}$C Purity (Paper used 50 ppm, limiting $T_{2}^{*}$) | Isotopically Purified SCD Substrates: We offer MPCVD Single Crystal Diamond (SCD) with $^{12}$C enrichment down to < 1 ppm (or lower upon request). | Immediate $T_{2}^{*}$ Enhancement: Eliminates the dominant $^{13}$C nuclear spin bath noise, directly enabling $T_{2}^{*}$ extension toward the 40-50 µs range predicted for P1-free limits. |
| Precise Crystal Orientation (Paper used (111) for 1-axis NV) | Custom (111) SCD Plates: Standard and custom-oriented SCD plates up to 500 µm thickness, with precise orientation control (e.g., (111) ± 0.5°). | Optimized Quantum Alignment: Guarantees maximum fidelity for single-axis NV ensembles and efficient application of DQ Ramsey protocols. |
| Scalability & Volume (Paper used 1.3 mm length ensemble) | Large Area PCD and Thick SCD: SCD substrates up to 500 µm thick and Polycrystalline Diamond (PCD) wafers up to 125 mm diameter. Substrates available up to 10 mm thickness. | Scalable Sensor Platforms: Provides the large, high-quality volumes necessary for industrial and bio-magnetic field applications (e.g., MEG), enabling high-density NV ensembles. |
| Dopant Control (Specific [P1] and [NV$^-$] required) | Controlled Nitrogen Doping: MPCVD allows for highly precise, in-situ nitrogen incorporation during growth, followed by optimized electron irradiation and annealing recipes. | Maximized Sensitivity Product: Enables researchers to tune the [P1] and [NV$^-$] concentrations to maximize the $T_{2}^{*} \cdot [\text{NV}^-]$ product under SBD conditions, optimizing sensor responsivity. |
| Integrated Antenna Design (Paper used CPW) | Custom Metalization Services: Internal capability for deposition of Au, Pt, Pd, Ti, W, and Cu metal stacks directly onto the diamond surface. | Streamlined Device Fabrication: Allows for integrated Coplanar Waveguides (CPW) or microstrip antennas, ensuring high RF/MW field homogeneity and simplifying experimental setup. |
| Optical Quality (Required for 532 nm excitation) | Quantum Grade Polishing: SCD surfaces polished to ultra-low roughness, typically Ra < 1 nm. | Reduced Optical Noise: Minimizes laser scattering and improves fluorescence collection efficiency, crucial for achieving photon shot noise limited sensitivity. |
Engineering Support
Section titled “Engineering Support”6CCVD’s in-house PhD team specializes in the growth and processing of quantum-grade diamond. We offer comprehensive engineering consultation to assist researchers in selecting the optimal material specifications (purity, orientation, and doping levels) required for advanced Ramsey Magnetometry and other NV-based quantum sensing projects.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
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.