| 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 |
This research demonstrates a versatile, indirect method for characterizing and spatially mapping spin-active quantum defects using Nitrogen-Vacancy (NV) centers in diamond as a quantum probe.
- Indirect Defect Detection: The technique, NV-based T1 relaxometry, successfully detects the Electron Spin Resonance (ESR) spectrum of Boron Vacancy (VB) defects in hexagonal Boron Nitride (hBN) without requiring direct optical excitation or fluorescence readout of the VB centers.
- Enhanced Spectral Resolution: The T1-Magnetic Resonance (T1-MR) technique yields a fourfold narrower linewidth (78 MHz) and significantly higher contrast (7.8%) compared to conventional VB Continuous-Wave Optically Detected Magnetic Resonance (CW-ODMR) (305 MHz linewidth, 2.2% contrast).
- Nanoscale Mapping: By exploiting cross-relaxation (CR) between the NV and VB ensembles, the method enables nanoscale spatial mapping of VB defect density with sub-diffraction-limited precision (potential resolution ~10 nm).
- Hybrid Architecture Enablement: This approach decouples the sensing qubit (VB) from the readout qubit (NV), facilitating the discovery and characterization of optically dim defects or those emitting in the challenging near-infrared (NIR)/telecom range.
- Temporal Gating for Isolation: A âdark readoutâ scheme, leveraging the difference in radiative lifetimes between VB (~1.6 ns) and NV (~12 ns), effectively suppresses residual VB fluorescence, enhancing measurement contrast.
| Parameter | Value | Unit | Context |
|---|
| NV T1 (Off-Resonant) | 1.23 ± 0.09 | ms | Reference longitudinal relaxation time (3.47 GHz) |
| NV T1 (Cross-Relaxation) | 406 ± 34 | ”s | T1 under CR condition (no RF drive) |
| NV T1 (Driven CR) | 116 ± 13 | ”s | T1 under CR condition (3.8 GHz RF drive) |
| VB Zero-Field Splitting (ZFS) | 3.49 | GHz | VB ground state splitting |
| T1-MR Linewidth (FWHM) | 78 | MHz | Spectral resolution achieved via T1 relaxometry |
| CW-ODMR Linewidth (FWHM) | 305 | MHz | Conventional VB ODMR linewidth |
| T1-MR Signal Contrast | 7.8 | % | Enhanced contrast achieved by T1-MR |
| VB Radiative Lifetime | ~1.6 | ns | Used for temporal âdark readoutâ gating |
| NV Radiative Lifetime | ~12 | ns | Used for temporal âdark readoutâ gating |
| NV Depth | 9 ± 4 | nm | Position beneath the diamond surface |
| NV-Sample Distance | 11.5 ± 4.7 | nm | Factory-calibrated distance during contact mode |
| hBN Thickness Range | 90-300 | nm | Nonuniform CVD-grown hBN membrane |
| He Ion Dose/Energy | 50 He+/nm2 at 30 keV | N/A | Used for VB defect generation |
| VB Defect Density | ~0.1 defects per nm2 | per atomic layer | Approximate density post-irradiation |
| Bias Magnetic Field (CR) | ~127 | G | Field required for NV and VB resonance overlap |
- hBN Sample Synthesis and Preparation:
- Hexagonal Boron Nitride (hBN) flakes of variable thickness (90-300 nm) were synthesized via Chemical Vapor Deposition (CVD).
- The hBN was transferred onto a gold coplanar waveguide (CPW) patterned on a sapphire substrate.
- Defect Engineering:
- VB defects were generated by irradiating the hBN sample with a beam of He ions at an energy of 30 keV and a dose of 50 He+/nm2.
- Scanning NV Magnetometry Setup:
- Measurements were performed using a commercial Qnami ProteusQTM scanning NV microscope at room temperature.
- A single NV center, located approximately 9 nm beneath the diamond surface, was integrated into the apex of a tuning fork-based cantilever.
- Microwave fields (0.1 W input) for spin manipulation were delivered via a nearby shorted coaxial cable antenna (~15 ”m from the NV).
- Pulsed T1 Relaxometry Protocol:
- The NV spin was initialized into the ms = 0 state using a 3 ”s green laser pulse.
- The spin was allowed to evolve toward thermal equilibrium during a dark interval (Ï), during which resonant or off-resonant microwave excitation was applied to the VB ensemble.
- Time-Gated âDark Readoutâ Implementation:
- To suppress VB fluorescence (which peaks at ~850 nm) and enhance NV contrast, a time-gated readout was used.
- The photon counter was activated 6 ns after the 10 ns readout laser pulse turned off, exploiting the significantly shorter radiative lifetime of VB (~1.6 ns) compared to NV (~12 ns).
- T1-Magnetic Resonance (T1-MR) Spectroscopy:
- The relative spin relaxation signal (âPL) was recorded after a fixed evolution time (Ï = 480 ”s or 250 ”s) while sweeping the microwave frequency, providing a rapid proxy for the VB ESR spectrum.
- Nanoscale Spatial Mapping (Iso-T1):
- A spatial map of VB density was constructed by performing single-Ï T1 relaxometry (iso-T1 measurement) at each pixel (100 nm step size) under the cross-relaxation condition (CR, 3.8 GHz RF).
| Industry/Sector | Application | Relevance to Research Findings |
|---|
| Quantum Sensing & Metrology | Development of high-sensitivity, broadband quantum sensors. | NV T1 relaxometry provides a standardized, high-contrast method for characterizing new spin defects (like VB) for use in magnetometry, thermometry, and strain sensing. |
| 2D Materials & Defect Engineering | High-resolution mapping and quality control of quantum materials. | Enables sub-diffraction-limited spatial mapping of spin defect distributions (VB density) in 2D materials like hBN, critical for optimizing material growth and implantation processes. |
| Hybrid Quantum Architectures | Creation of robust quantum networks and processors. | The methodology demonstrates decoupling of sensing (VB) and readout (NV) qubits, allowing for hybrid systems where each component is optimized for its specific function. |
| Quantum Communication | Integration of spin defects into fiber-based networks. | The technique can discover and read out spin defects emitting in the telecom wavelength range, which are currently inaccessible to low-cost silicon-based detectors. |
| Scanning Probe Microscopy (SPM) | Advanced nanoscale ESR and NMR spectroscopy. | Extends the capability of scanning NV microscopy to perform wide-band magnetic resonance spectroscopy on external spin ensembles via cross-relaxation protocols. |
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.