Parallel detection and spatial mapping of large nuclear spin clusters

At a Glance

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

Technical Documentation: Nanoscale MRI and Quantum Sensing Substrates

Executive Summary

This research demonstrates a significant advancement in nanoscale Magnetic Resonance Imaging (MRI) by achieving parallel detection and 3D spatial mapping of large nuclear spin clusters using Nitrogen-Vacancy (NV) centers in diamond. This capability is critical for developing single-molecule MRI platforms and characterizing large quantum registers.

Core Achievement: Successful 3D spatial mapping of clusters containing 20 and 29 Carbon-13 (¹³C) nuclear spins surrounding near-surface NV centers at ambient (room) temperature.
Methodology: Combines weak quantum measurements, phase encoding, and Generalized Simulated Annealing (GSA) to efficiently extract hyperfine parameters and 3D positions in parallel.
Spatial Resolution: Achieved mapping radii between 0.7 nm and 2.4 nm, with spatial precision better than 1 Å for optimally located spins.
Material Requirement: Requires high-purity, electronic-grade Single Crystal Diamond (SCD) substrates compatible with shallow NV center implantation (depths ~10 nm).
Future Applications: The technique is scalable and compatible with the demanding environment required for prospective single-molecule MRI investigations and the characterization of large qubit registers for quantum computing and network nodes.
6CCVD Value Proposition: 6CCVD provides the ultra-high purity SCD substrates necessary to maximize electronic coherence time (T_2,e) and enable the next generation of nanoscale quantum sensing experiments.

Technical Specifications

The following hard data points were extracted from the experimental demonstration of ¹³C spin cluster mapping:

Parameter	Value	Unit	Context
Detected Nuclei (NV1)	20	spins	Carbon-13 (¹³C)
Detected Nuclei (NV2)	29	spins	Carbon-13 (¹³C)
Mapping Radius	0.7 - 2.4	nm	Distance from NV center
Extrapolated Radius (¹H/¹⁹F)	5 - 6	nm	Requires T_2,e ~ 40 µs
NV Center Depth	~10	nm	Near-surface compatibility
Electronic Coherence Time (T_2,e)	~50	µs	For 3.5-nm-deep NV center
Bias Magnetic Field (B₀)	188.89 - 201.29	mT	Aligned to NV symmetry axis
Operating Temperature	Ambient	°C	Room temperature
¹³C Isotope Abundance	1.1	%	Natural abundance diamond
Spatial Precision (Best Case)	< 1	Å	For ¹³C located near sensitive slice maximum
RF Circuit Bandwidth	~19	MHz	3-dB bandwidth of planar micro-coil
¹³C Rabi Frequency	~25	kHz	Typical experimental value

Key Methodologies

The experiment relies on a weak-measurement protocol combined with advanced computational analysis to achieve parallel detection and 3D localization.

Nuclear Spin Polarization: Nuclear spins (¹³C) are hyperpolarized through a polarization transfer from the optically-aligned electronic spin using a repeated NOVEL sequence.
Simultaneous Excitation: All nuclei are simultaneously excited using a broad-band π/2 radio-frequency (RF) pulse transmitted via a planar micro-coil.
Weak Measurement Detection: The Free Induction Decay (FID) signal is detected by repeatedly sampling the transverse nuclear magnetization using weak measurements.
CPMG Pulse Train: Each weak-measurement read-out block consists of a Carr-Purcell-Meiboom-Gill (CPMG) pulse train (4-24 equidistant pulses) to define the interaction time (t_s).
Spatial Selectivity: The interaction time (t_β) is varied to tune the radius of the “sensitive slice” (r_slice), allowing selective probing of nuclei at different distances while avoiding interference from strongly-coupled proximal nuclei.
Hyperfine Parameter Extraction: The FID trace is analyzed to extract the parallel (a_||,i) and perpendicular (a_⊥,i) components of the hyperfine vector, which encode the 3D position (r, $\vartheta$, $\phi$).
Maximum Likelihood Estimation (MLE): A cost function based on the negative likelihood function is minimized using the Generalized Simulated Annealing (GSA) algorithm on a high-performance computer cluster to determine the optimal number of spins (n) and their 3D locations.

6CCVD Solutions & Capabilities

This research highlights the critical role of high-quality diamond substrates in enabling next-generation quantum sensing and nanoscale MRI. 6CCVD is uniquely positioned to supply and customize the materials required to replicate and extend this work, particularly for transitioning to single-molecule imaging (Fig. 6).

Applicable Materials

To replicate the high-coherence, near-surface NV centers demonstrated in this paper, researchers require the highest quality SCD material.

Electronic Grade Single Crystal Diamond (SCD): Essential for maximizing the electronic coherence time (T_2,e) of the NV centers. 6CCVD provides ultra-low strain, high-purity SCD plates, minimizing native defects that contribute to decoherence.
Isotope Control (Optional Extension): While the paper used natural abundance (1.1% ¹³C), future experiments targeting specific nuclear spin registers or requiring extended T_2,e may benefit from Isotopically Purified SCD (e.g., <0.01% ¹³C) to reduce background nuclear spin noise.
Boron-Doped Diamond (BDD): For applications requiring integrated electrical contacts or surface charge control (crucial for stabilizing shallow NV centers), 6CCVD offers Boron-Doped Diamond (BDD) films, which can be tailored for specific conductivity requirements.

Customization Potential

The complexity of nanoscale MRI requires highly customized substrates, often involving specific dimensions, surface preparation, and integrated contacts.

Requirement from Paper/Future Work	6CCVD Custom Capability	Technical Advantage
Substrate Dimensions	Plates/wafers up to 125 mm (PCD) or custom SCD sizes.	Supports large-scale fabrication of quantum devices and micro-coil integration.
Thickness Control	SCD thickness from 0.1 µm up to 500 µm.	Allows precise control over substrate rigidity and thermal properties for integrated RF circuits.
Surface Quality	Polishing to Ra < 1 nm (SCD).	Essential for minimizing surface noise and enabling reliable immobilization of external molecules (e.g., proteins, Fig. 6).
Integrated Contacts	Custom metalization (Au, Pt, Pd, Ti, W, Cu).	Enables integration of the planar micro-coils and microwave transmission lines used for spin manipulation (Fig. 1a, 2b).
Nanostructuring Compatibility	High-quality SCD base material.	Provides the necessary foundation for subsequent etching processes (e.g., nanopillars) to enhance photon collection efficiency.

Engineering Support

The transition from mapping internal ¹³C clusters to imaging external molecules (single-molecule MRI) requires careful material engineering to maintain high T_2,e in near-surface NV centers.

Material Selection for T_2,e: 6CCVD’s in-house PhD team specializes in optimizing SCD growth parameters to minimize defects and maximize T_2,e, which is crucial for extending the detection radius to 5-6 nm for ¹H or ¹⁹F nuclei.
Surface Functionalization Consultation: We provide expert guidance on material preparation necessary for subsequent surface functionalization steps, ensuring compatibility with the immobilization of biological or chemical samples.
Support for Quantum Network Nodes: Our materials are ideal for the characterization of large nuclear spin registers in the context of quantum simulators and quantum network nodes, where precise control over the spin environment is paramount.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41467-022-28935-z