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	National Institute for Materials Science, Centre for Quantum Computation and Communication Technology
Citations	27
Analysis	Full AI Review Included

Executive Summary

This study experimentally validates the feasibility of using dense, shallow Nitrogen-Vacancy (NV) center ensembles in diamond to hyperpolarize external nuclear spins, aiming to significantly enhance Nuclear Magnetic Resonance (NMR) sensitivity.

Scalability Demonstrated: Polarization transfer was successfully achieved from a dense NV ensemble (addressing ~10⁶ NVs in parallel) to external hydrogen targets over a 50x50 µm field of view, scaling up previous single-NV results.
Maximum Transfer Rate: A peak polarization transfer rate of ~7500 spins per second per NV was measured for a solid biphenyl target, confirming sufficient coupling strength in current “best case” shallow ensembles.
Protocol Robustness: The PulsePol sequence was utilized, demonstrating robustness against broadband noise inherent to dense, near-surface NV ensembles, achieving NV coherence times (T₂) in excess of 10 µs.
Liquid Target Feasibility: Polarization transfer efficiency to sufficiently viscous liquid targets (glycerol) was only reduced by a factor of 2-3 compared to solids, suggesting NV-based liquid-state hyperpolarization is viable, especially if surface effects reduce molecular diffusion near the diamond.
Material Limitations Identified: Current NV yield (1.4%) and coherence degradation due to surface defects and nitrogen spin bath limit the steady-state polarization enhancement to three orders of magnitude over Boltzmann levels.
Future Enhancement Potential: The authors estimate that realistic improvements in diamond material quality (especially surface control and NV yield) could lead to a total enhancement of up to two orders of magnitude beyond current results, making the technique competitive for NMR enhancement.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Substrate	Electronic Grade CVD	N/A	Isotopically enriched (99.95% ¹²C)
¹²C Layer Thickness	1	µm	Overgrown layer
Implant Species/Energy	¹⁵N	2.5 keV	Used to create shallow NVs
Implant Fluence (D1)	1 x 10¹³	cm^-2	Sample D1
Implant Fluence (D2)	2 x 10¹³	cm^-2	Sample D2 (higher N density)
Annealing Temperature	1100	°C	Ramped anneal
Mean NV Depth (d_NV)	~6	nm	Measured via XY8-k spectroscopy
Areal NV Density (σ_NV)	~1500	µm^-2	Estimated average density
NV Yield (D1 / D2)	1.4% / 0.9%	N/A	Overall N to NV conversion ratio
NV Coherence Time (T_NV)	~22	µs	D1, off-resonance PulsePol decay
PulsePol Coherence Time (T₂)	> 10	µs	Achieved using the decoupling sequence
Maximum Cooling Rate (u)	~7500	spins/s per NV	Measured for biphenyl solid target
Steady-State Polarization (P)	2.7 x 10^-4	N/A	Predicted in 1 µm layer (3 orders enhancement)
External Bias Field (B)	~450	G	Low magnetic field operation
Laser Wavelength	532	nm	NV excitation and initialization
Laser Incident Intensity	~300	mW	Used for widefield NV microscope
Target Diffusion (Biphenyl, D_n)	~571	nm²/s	Estimated value for solid target
Target T_1,n (Fit)	~0.88	s	Fitted value for biphenyl polarization buildup

Key Methodologies

Diamond Substrate Preparation: Electronic grade CVD diamond substrates were used, featuring a 1 µm thick layer of isotopically enriched ¹²C (99.95%) to minimize spin noise from the carbon lattice.
Shallow NV Ensemble Creation: NVs were created using low-energy (2.5 keV) ¹⁵N ion implantation (fluence 1-2 x 10¹³ cm^-2), followed by a high-temperature ramped anneal (up to 1100 °C) to activate the NVs.
Surface Termination: The diamond surface was cleaned and oxygen-terminated using a boiling mixture of sulfuric and nitric acid to ensure a clean, stable surface environment.
Target Deposition: Solid targets (biphenyl, PMMA) and liquid targets (glycerol/water mixtures, viscous oil) were deposited directly onto the diamond surface. Biphenyl crystals were formed by drying an isopropanol solution and then encapsulated in UV-curing epoxy.
Widefield NV Microscopy: Experiments were conducted using a custom widefield NV microscope setup, allowing simultaneous addressing and readout of ~10⁶ NVs over a 50x50 µm field of view using a 532 nm laser and sCMOS camera.
PulsePol Protocol Implementation: The robust PulsePol sequence, a Hamiltonian engineering protocol, was delivered via a gold ring-shaped microwave resonator to achieve resonant flip-flop coupling between the NV electron spins and the external nuclear spins (hydrogen).
Polarization Measurement: Polarization transfer efficiency was inferred by measuring the normalized NV photoluminescence (PL) decay difference between resonant (on-resonance) and non-resonant (off-resonance) PulsePol sequences. The NV depolarisation rate was used as a proxy for the hydrogen cooling rate.

Commercial Applications

This research directly supports the development of next-generation hyperpolarization technology, primarily targeting enhanced sensitivity in NMR and related quantum sensing fields.

NMR Sensitivity Enhancement: The core application is improving the sensitivity of conventional bulk NMR by using the NV ensemble as a room-temperature, low-field hyperpolarizer, potentially replacing complex cryogenic Dynamic Nuclear Polarization (DNP) systems.
Micron-Scale NMR (NV-NMR): Enables high-resolution NMR spectroscopy and imaging of extremely small liquid or solid samples (picoliter volumes), crucial for microfluidics, chemical analysis, and biological studies where sample volume is limited.
Quantum Sensing Platforms: The methodology validates the use of dense, shallow NV ensembles for external spin manipulation, a key requirement for advanced quantum magnetometry and spectroscopy applications.
High-Purity Diamond Materials: Drives demand for high-quality, electronic-grade CVD diamond substrates with precise isotopic enrichment (¹²C) and optimized low-energy ion implantation recipes for creating stable, shallow NV layers.
Pharmaceutical and Chemical Analysis: Enhanced NMR sensitivity allows for faster analysis of low-concentration compounds or small samples, accelerating drug discovery and chemical reaction monitoring.

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