Parallel detection and spatial mapping of large nuclear spin clusters
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-03-10 |
| Journal | Nature Communications |
| Authors | K. S. Cujia, Konstantin Herb, Jonathan Zopes, John M. Abendroth, Christian L. Degen |
| Institutions | ETH Zurich |
| Citations | 25 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Core Achievement: Demonstrated parallel detection and three-dimensional (3D) spatial mapping of large nuclear spin clusters (up to 29 13C nuclei) surrounding a single Nitrogen-Vacancy (NV) center in diamond.
- Single-Molecule MRI Advancement: This strategy fulfills a critical requirement for developing single-molecule Magnetic Resonance Imaging (MRI) by efficiently localizing multiple spins at ambient (room) temperature.
- Spatial Range: Successfully mapped 13C spins within a 2.4 nm radius. Extrapolation based on electron spin coherence time (T2,e) suggests a potential mapping radius of 5-6 nm for 1H (proton) nuclei.
- Methodology: The technique combines weak quantum measurements (sampling the Free Induction Decay, FID), phase encoding, and a Generalized Simulated Annealing (GSA) algorithm for robust, parallel extraction of 3D hyperfine parameters.
- Spatial Selectivity: The protocol is spatially selective, allowing engineers to tune the sensitive slice radius (rslice) by varying the interaction time (tbeta). This avoids interference from strongly-coupled proximal nuclei.
- Resolution: Statistical analysis confirms that the method achieves sub-Angstrom precision for nuclei located near the maximum sensitivity radius, with volume uncertainties often less than the volume per carbon atom (5.67 A3).
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Operating Temperature | Ambient | N/A | Room temperature operation, compatible with biological samples. |
| Nuclear Spin Mapped | Carbon-13 (13C) | N/A | Natural isotope abundance (1.1%) in diamond. |
| Maximum Mapped Radius (13C) | 2.4 | nm | Demonstrated distance from the NV center. |
| Extrapolated Radius (1H) | 5-6 | nm | Potential range for proton detection, limited by T2,e. |
| NV Center Depth (Tested) | ~10 | nm | Shallow depth, demonstrating compatibility with near-surface sensing. |
| Electron Spin Coherence Time (T2,e) | ~50 | ”s | Measured for a 3.5 nm deep NV center. |
| Bias Magnetic Field (B0) | 188.89 to 201.29 | mT | External field aligned to the NV symmetry axis. |
| 13C Larmor Frequency | 2.156 | MHz | Reference frequency used for spectral calibration. |
| Number of Spins Mapped (Max) | 29 | N/A | Total 13C nuclei successfully localized in one cluster (NV2). |
| Diamond Lattice Volume per 12C | 5.67 | A3 | Used as a benchmark for single-nucleus resolution. |
| Spatial Precision (Best Case) | < 1 | Angstrom | Achieved for 13C spins located near the sensitive slice maximum. |
Key Methodologies
Section titled âKey Methodologiesâ-
Sample Preparation:
- Used electronic-grade diamond plates with natural 13C abundance (1.1%).
- NV centers created via 15N+ ion implantation (5 keV energy) followed by annealing at 850 °C.
- Nano-pillars etched into the surface to enhance photon collection efficiency.
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Spin Control and Setup:
- NV electronic spins manipulated using microwave pulses (~2.5 GHz) delivered via a coplanar waveguide.
- Nuclear spins manipulated using broadband radio-frequency (RF) pulses delivered via a planar micro-coil (3 dB bandwidth ~19 MHz).
- A permanent magnet provided a bias field (B0 ~ 200 mT) aligned to the NV symmetry axis.
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Nuclear Hyperpolarization:
- Nuclear spins were hyperpolarized using a repeated NOVEL (Nuclear spin Orientation via Electron spin Locking) sequence, transferring polarization from the optically-aligned electronic spin.
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Weak Measurement Detection Protocol:
- The Free Induction Decay (FID) signal was acquired by simultaneously exciting all nuclei with a broad-band pi/2 pulse.
- The transverse nuclear magnetization was sampled using repeated weak measurements, consisting of a Carr-Purcell-Meiboom-Gill (CPMG) pulse train (4-24 pulses) followed by optical readout.
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Spatial Selectivity Tuning:
- The radius of the âsensitive sliceâ (rslice) was tuned by varying the interaction time (tbeta) of the CPMG sequence.
- This tuning exploits the balance between signal gain (proportional to hyperfine coupling) and quantum back-action (which causes rapid decay for strongly coupled, proximal spins).
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Data Analysis and Localization:
- Four FID spectra, recorded at different tbeta values, were combined for redundancy and volume coverage.
- Hyperfine parameters (aparallel, aperpendicular, and phase phi) were extracted by minimizing a total cost function using the Generalized Simulated Annealing (GSA) algorithm.
- GSA was run on a high-performance cluster to globally optimize the fit parameters (up to 3n + 3 unknowns, where n is the number of spins).
- 3D spatial locations (r, theta, phi) were calculated by inverting the dipolar hyperfine interaction equation using the extracted hyperfine vector a.
Commercial Applications
Section titled âCommercial Applicationsâ| Industry/Sector | Application | Technical Relevance |
|---|---|---|
| Single-Molecule MRI | Direct structural determination of individual molecules (proteins, polymers) with 3D resolution and elemental specificity. | High spatial resolution (< 5 nm radius potential for 1H) and ambient operating conditions. |
| Quantum Computing & Simulation | Characterization and calibration of large nuclear spin registers (qubit registers) used in solid-state quantum processors. | Provides an efficient tool for mapping the coupling network and cross-talk in multi-qubit systems. |
| Biotechnology & Drug Discovery | Nanoscale surface NMR for monitoring chemical binding, conformational changes, and surface reactions (e.g., enzyme active sites). | Compatibility with near-surface NV centers (< 5 nm depth) and room temperature operation is crucial for biological samples. |
| Quantum Network Nodes | Mapping the spin environment of central electronic spins used in quantum network nodes and quantum interconnects. | Essential for engineering robust, long-coherence quantum memory registers (e.g., 29Si or 13C). |
| Materials Science | Nanoscale surface NMR spectroscopy for characterizing thin films, deposited molecular layers, and interfaces on diamond substrates. | High sensitivity allows detection of weak signals from external spins or dilute surface ensembles. |