Probing Spin Defects via Single Spin Relaxometry
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2025-08-22 |
| Authors | Huan Zhao, Alex L. Melendez, YuehâChun Wu, Steven Randolph, Sujoy Ghosh |
| Institutions | Oak Ridge National Laboratory |
| Analysis | Full AI Review Included |
Technical Documentation: NV-Based Quantum Relaxometry for Spin Defect Characterization
Section titled âTechnical Documentation: NV-Based Quantum Relaxometry for Spin Defect CharacterizationâThis document analyzes the research paper âProbing Spin Defects via Single Spin Relaxometry,â which leverages the Nitrogen-Vacancy (NV) center in diamond to characterize and map Boron Vacancy ($V_B$) defects in hexagonal Boron Nitride (hBN). The findings validate the use of NV $T_1$ relaxometry as a powerful, non-optical readout method for emerging quantum spin systems, directly aligning with 6CCVDâs mission to supply high-purity diamond materials for advanced quantum sensing and networking.
Executive Summary
Section titled âExecutive SummaryâThe following points summarize the core technical achievements and material requirements of the research:
- Versatile Quantum Sensing: Demonstrated the use of a single Nitrogen-Vacancy (NV) center in diamond as a scanning quantum probe to indirectly detect and characterize Boron Vacancy ($V_B$) centers in hBN.
- Non-Optical Readout: Achieved Electron Spin Resonance (ESR) detection of $V_B$ defects by monitoring changes in the NV centerâs spin relaxation time ($T_1$), eliminating the need for direct optical excitation or fluorescence readout of the target defect.
- Enhanced Spectral Resolution: The $T_1$-Magnetic Resonance ($T_1$-MR) technique yielded a fourfold narrower linewidth (78 MHz FWHM) compared to conventional continuous-wave Optically Detected Magnetic Resonance (CW-ODMR) (305 MHz FWHM).
- Nanoscale Mapping: Performed high-resolution spatial mapping of $V_B$ defect density using iso-$T_1$ relaxometry, achieving sub-diffraction-limited precision (~10 nm resolution).
- Hybrid Architecture Validation: Confirmed the viability of hybrid quantum architectures where the sensing qubit (NV) and the readout mechanism are decoupled from the target spin system ($V_B$).
- Material Requirement: Success relies critically on high-purity Single Crystal Diamond (SCD) containing a shallow, integrated NV center for optimal proximity and coherence.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the experimental results and methodology:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Diamond Material | Single NV in Nanopillar | N/A | Scanning probe sensor |
| NV Center Depth | 9 ± 4 | nm | Proximity to diamond surface |
| NV-Sample Distance (Operating) | 11.5 ± 4.7 | nm | Calibrated distance to hBN |
| hBN Thickness (CVD) | 90 - 300 | nm | Nonuniform sample thickness |
| He Ion Implantation Energy | 30 | keV | Used for VB defect creation |
| He Ion Dose | 50 | He+/nm2 | Defect creation density |
| VB Defect Density (Approx.) | 0.1 | defects/nm2/atomic layer | Result of implantation |
| NV T1 (Off-Resonant) | 1.23 ± 0.09 | ms | Reference T1 at 3.47 GHz |
| NV T1 (CR Resonant) | 674 ± 99 | ”s | Reduced T1 at 3.68 GHz (VB resonance) |
| T1-MR Linewidth (FWHM) | 78 | MHz | Enhanced spectral resolution achieved |
| CW-ODMR Linewidth (FWHM) | 305 | MHz | Conventional VB ODMR |
| Spatial Resolution (Iso-T1 Map) | ~10 | nm | Limited by NV-sample distance |
| Microwave Input Power | ~0.1 | W | Supplied via RF antenna |
| Magnetic Field (CR Condition) | 127 | G | Bias field for NV/VB resonance overlap |
Key Methodologies
Section titled âKey MethodologiesâThe experiment combined advanced material synthesis, defect engineering, and high-precision scanning probe techniques:
- hBN Synthesis and Substrate Preparation: Thick hBN flakes (90-300 nm) were grown via Chemical Vapor Deposition (CVD) and transferred onto a lithographically patterned gold coplanar waveguide (CPW) on a sapphire substrate.
- Defect Engineering: Boron Vacancy ($V_B$) defects were generated in the hBN using controlled Helium ion (He$^{+}$) irradiation at 30 keV energy and a dose of 50 He$^{+}$/nm2.
- Scanning NV Sensor Fabrication: A single NV center was integrated into the apex of a tuning fork-based scanning probe cantilever. The NV center was positioned shallowly, approximately 9 ± 4 nm beneath the diamond surface.
- Spin Manipulation and Readout: NV measurements were performed using a commercial scanning NV microscope (Qnami ProteusQ). The NV was excited using a 520 nm laser (~20 ”W), and microwaves (~0.1 W) were delivered via an integrated RF antenna.
- Pulsed $T_1$ Relaxometry: The NV spin was initialized (3 ”s laser pulse), allowed to evolve during a dark interval (Ï) under microwave excitation, and then read out.
- âDark Readoutâ Scheme: A time-gated detection sequence (10 ns laser pulse + 30 ns counter delay) was implemented. This temporal gating exploited the difference in radiative lifetimes (NV ~12 ns vs. $V_B$ ~1.6 ns) to suppress $V_B$ fluorescence, significantly enhancing the $T_1$ measurement contrast.
- Nanoscale Mapping: Iso-$T_1$ relaxometry was performed by fixing the evolution time (Ï = 250 ”s) and scanning the NV probe across the hBN sample with a 100 nm step size to construct a high-resolution spatial map of $V_B$ density.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe success of this researchâpioneering hybrid quantum architectures and nanoscale defect mappingâis fundamentally dependent on the quality and customization of the diamond material used for the NV sensor. 6CCVD is uniquely positioned to supply the necessary high-specification MPCVD diamond required to replicate and advance this work.
Applicable Materials for Quantum Sensing
Section titled âApplicable Materials for Quantum Sensingâ| Research Requirement | 6CCVD Material Solution | Technical Specification Alignment |
|---|---|---|
| High-Purity NV Host | Optical Grade Single Crystal Diamond (SCD) | Provides the ultra-low strain, high-purity lattice essential for long NV coherence times ($T_2$ and $T_1$), critical for sensitive relaxometry measurements. |
| Shallow NV Precursor | SCD Plates (0.1 ”m - 500 ”m thickness) | Ideal starting material for precise, shallow NV implantation (required depth: ~9 nm) and subsequent nanopillar fabrication for scanning probes. |
| Advanced Spin Engineering | Boron-Doped Diamond (BDD) | Essential for future extensions of this work, such as controlled charge state manipulation or scanning-probe NMR spectroscopy via cross-relaxation protocols. |
| Large-Area Substrates | PCD Wafers (up to 125 mm) | Provides scalable, large-area substrates for high-throughput hBN growth, transfer, and defect engineering experiments. |
Customization Potential for Hybrid Device Fabrication
Section titled âCustomization Potential for Hybrid Device FabricationâThe integration of the NV sensor with the hBN/CPW structure demands precise material engineering and fabrication capabilities, which 6CCVD provides:
- Custom Dimensions and Geometry: The paper utilized a single NV integrated into a cantilever tip. 6CCVD offers custom dimensions and laser cutting services for diamond plates up to 125 mm, enabling researchers to define specific geometries for scanning probe tips or specialized substrates.
- Surface Preparation: Achieving the optimal NV-to-sample distance (~11.5 nm) requires an atomically flat diamond surface. 6CCVD guarantees ultra-low surface roughness (Ra < 1 nm for SCD), minimizing surface noise and maximizing the magnetic coupling strength ($W_d$) required for efficient cross-relaxation.
- Integrated Microwave Structures: The experiment utilized a gold coplanar waveguide (CPW) for microwave delivery. 6CCVD offers internal metalization capabilities (Au, Pt, Pd, Ti, W, Cu), allowing researchers to fabricate custom CPW structures or contact pads directly onto the diamond substrate, streamlining the assembly of hybrid quantum devices.
Engineering Support
Section titled âEngineering Supportâ6CCVDâs in-house team of PhD material scientists specializes in MPCVD growth parameters optimized for quantum applications. We can assist researchers in selecting the optimal diamond specifications (e.g., nitrogen concentration, crystal orientation, thickness) required for similar nanoscale quantum sensing and defect characterization projects.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
<title>Abstract</title> Spin defects in solids offer promising platforms for quantum sensing and memory due to their long coherence times and compatibility with quantum networks. Here, we integrate a single nitrogen vacancy (NV) center in diamond with scanning probe microscopy to discover, read out, and spatially map arbitrary spin-based quantum sensors at the nanoscale. Using the boron vacancy (V<sub>B</sub><sup>-</sup>) center in hexagonal boron nitrideâan emerging two-dimensional spin systemâas a model, we detect its electron spin resonance through changes in the spin relaxation time (T<sub>1</sub>) of a nearby NV center, without requiring direct optical excitation or readout of the V<sub>B</sub><sup>-</sup> fluorescence. Cross relaxation between the NV and V<sub>B</sub><sup>-</sup> ensembles results in a pronounced NV T<sub>1</sub> reduction, enabling nanoscale mapping of spin defect distributions beyond the optical diffraction limit. This approach highlights NV centers as versatile quantum probes for characterizing spin systems, including those emitting at wavelengths beyond the range of silicon-based detectors. Our results open a pathway to hybrid quantum architectures where sensing and readout qubits are decoupled, facilitating the discovery of otherwise inaccessible quantum defects for advanced sensing and quantum networking.