Polarization Transfer to External Nuclear Spins Using Ensembles of Nitrogen-Vacancy Centers

At a Glance

Metadata	Details
Publication Date	2021-05-24
Journal	Physical Review Applied
Authors	A. J. Healey, L. T. Hall, G.A.L. White, T. Teraji, Sani Ma
Institutions	Centre for Quantum Computation and Communication Technology, University of Melbourne
Citations	27
Analysis	Full AI Review Included

Technical Documentation & Analysis: Hyperpolarisation using NV Centre Ensembles

Executive Summary

This research successfully demonstrates the scaling of NV-based hyperpolarisation from single defects to dense ensembles, a critical step toward achieving realistic Nuclear Magnetic Resonance (NMR) sensitivity enhancement.

Core Achievement: Experimental evidence of robust polarisation transfer from a shallow, dense Nitrogen-Vacancy (NV) ensemble to external nuclear targets (hydrogen spins) over micrometre scales.
Performance Metric: A maximum polarisation transfer rate of $\approx 7500$ spins per second per NV was measured in a solid biphenyl target.
Material Basis: The experiment utilized electronic grade MPCVD diamond substrates featuring a 1 µm thick, isotopically enriched ${}^{12}$C layer, implanted with ${}^{15}$N at 2.5 keV to create shallow NV ensembles ($d_{NV} \approx 6-7$ nm).
Protocol Success: The PulsePol sequence proved robust, enabling parallel operation of $\approx 10^{6}$ NVs and achieving NV coherence times ($T_2$) up to 22 µs in high-density ensembles, overcoming limitations of previous protocols.
Application Potential: The demonstrated cooling rate is sufficient to generate a predicted polarisation enhancement exceeding three orders of magnitude over Boltzmann polarisation at room temperature and low magnetic fields.
Material Challenge: Future implementation relies heavily on improving diamond material properties, specifically achieving higher NV yield, better surface control, and longer $T_2$ in high-density, near-surface ensembles.

Technical Specifications

The following hard data points were extracted from the experimental results and material characterization:

Parameter	Value	Unit	Context
Diamond Substrate Layer Thickness	1	µm	Isotopically enriched ${}^{12}$C CVD layer
Implantation Energy	2.5	keV	${}^{15}$N implantation for shallow NVs
Implantation Dose (D1)	$1 \times 10^{13}$	cm^-2	Sample D1
Implantation Dose (D2)	$2 \times 10^{13}$	cm^-2	Sample D2 (Higher N density)
Annealing Temperature	1100	°C	Ramped anneal to maximize NV yield
Mean NV Depth ($d_{NV}$)	$\approx 6$ to $7$	nm	Near-surface ensemble
NV Areal Density ($\sigma_{NV}$)	$\approx 1500$ to $1800$	µm^-2	Estimated for D1 and D2
Maximum NV $T_2$ (PulsePol)	22	µs	Achieved on D1 with Biphenyl target
Maximum Polarisation Transfer Rate	$\approx 7500$	spins/sec/NV	Measured on solid Biphenyl target
Predicted Steady-State Polarisation	$2.7 \times 10^{-4}$	(Unitless)	In 1 µm layer, 3+ orders enhancement over Boltzmann
NV $T_2$ Decay Exponent ($\beta$)	0.42 to 0.47	(Unitless)	Non-exponential decay observed

Key Methodologies

The experimental success relied on precise material engineering and advanced quantum control protocols:

High-Purity Substrate Selection: Electronic grade MPCVD diamond substrates featuring a 1 µm thick layer of 99.95% isotopically enriched ${}^{12}$C were used to minimize the intrinsic ${}^{13}$C spin bath noise.
Shallow NV Creation: Nitrogen-Vacancy ensembles were formed via low-energy ${}^{15}$N ion implantation (2.5 keV) to place NVs within 10 nm of the surface, followed by a high-temperature ramped anneal (1100 °C).
Surface Preparation: Diamond surfaces were cleaned and terminated using a boiling mixture of sulfuric and nitric acid to achieve an oxygen-terminated surface.
Widefield Microscopy: Experiments were conducted using a purpose-built widefield NV microscope, utilizing a 532 nm laser (≈ 300 mW) for NV excitation and initialization, and an sCMOS camera for photoluminescence (PL) readout.
Microwave Delivery: Spin state control and manipulation were achieved by delivering RF radiation via a gold ring-shaped resonator deposited on a glass coverslip.
PulsePol Protocol: The PulsePol pulse sequence was employed for robust polarisation transfer, chosen for its ability to decouple the NVs from broadband noise sources (nitrogen spin bath and surface defects) and achieve extended $T_2$ times.

6CCVD Solutions & Capabilities

The research highlights the critical dependence of NV hyperpolarisation efficiency on diamond material quality, particularly concerning $T_2$ coherence time, NV depth control, and surface engineering. 6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials and customization services required to replicate and significantly extend this research toward commercial NMR enhancement platforms.

Research Requirement / Challenge	6CCVD Solution & Capability	Technical Advantage
High-Purity Substrates (Low background noise, long $T_2$)	Optical Grade SCD or Isotopically Enriched ${}^{12}$C SCD wafers.	We provide ultra-low substitutional nitrogen concentration (PPM level) and high isotopic purity, minimizing the dominant spin noise bath and extending NV $T_2$ coherence times, essential for high-efficiency polarisation transfer.
Shallow NV Ensemble Creation (Precise depth control, high yield)	Custom Ion Implantation & Annealing Services.	6CCVD partners with leading facilities to offer precise low-energy implantation (e.g., 2.5 keV ${}^{15}$N) and optimized high-temperature annealing protocols (up to 1100 °C) to control NV depth ($d_{NV} \approx 6-7$ nm) and maximize NV yield and charge stability near the surface.
Custom Dimensions & Integration (50 × 50 µm FOV used)	Custom Dimensions up to 125mm (PCD) / Custom Laser Cutting.	We supply plates and wafers in custom sizes and shapes, ensuring compatibility with wide-field microscopy setups and integration into microfluidic or on-chip NMR platforms.
Surface Engineering & Passivation (Mitigating surface defects, improving $T_2$)	Advanced Polishing (Ra < 1 nm for SCD) and Surface Termination Consultation.	Our superior polishing minimizes surface roughness and defects. Our in-house PhD team provides consultation on optimal surface termination protocols (e.g., oxygen, hydrogen) to mitigate fast-fluctuating surface defects that severely degrade near-surface NV properties.
Microwave Resonator Integration (Gold ring resonator used)	In-House Custom Metalization (Au, Pt, Pd, Ti, W, Cu).	We offer precise deposition of thin film metal layers required for on-chip microwave delivery and RF control, streamlining the fabrication process for complex quantum sensing platforms.

Engineering Support: 6CCVD’s in-house PhD team specializes in optimizing MPCVD diamond growth recipes and post-processing techniques (implantation, annealing, surface termination) specifically for quantum sensing applications. We can assist researchers in selecting materials and protocols to achieve the required $T_2$ improvements necessary to scale up NV-based NMR hyperpolarisation projects.

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

View Original Abstract

The nitrogen-vacancy (N-V) center in diamond has emerged as a candidate to noninvasively hyperpolarize nuclear spins in molecular systems to improve the sensitivity of nuclear magnetic resonance (NMR) experiments. Several promising proof-of-principle experiments have demonstrated small-scale polarization transfer from single N-V centers to hydrogen spins outside the diamond. However, the scaling up of these results to the use of a dense N-V ensemble, which is a necessary prerequisite for achieving realistic NMR sensitivity enhancement, has not yet been demonstrated. In this work, we present evidence for a polarizing interaction between a shallow N-V ensemble and external nuclear targets over a micrometer scale, and characterize the challenges in achieving useful polarization enhancement. In the most favorable example of the interaction with hydrogen in a solid-state target, a maximum polarization transfer rate of approximately 7500 spins per second per N-V is measured, averaged over an area containing order 106 N-V centers. Reduced levels of polarization efficiency are found for liquid-state targets, where molecular diffusion limits the transfer. Through analysis via a theoretical model, we find that our results suggest that implementation of this technique for NMR sensitivity enhancement is feasible following realistic diamond material improvements.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.15.054052