Detection and control of single proton spins in a thin layer of diamond grown by chemical vapor deposition

At a Glance

Metadata	Details
Publication Date	2020-09-14
Journal	Applied Physics Letters
Authors	Kento Sasaki, Hideyuki Watanabe, Hitoshi Sumiya, Kohei M. Itoh, Eisuke Abe
Institutions	RIKEN Center for Emergent Matter Science, National Institute of Advanced Industrial Science and Technology
Citations	8
Analysis	Full AI Review Included

Executive Summary

Core Achievement: Successful detection, characterization, and full quantum control (polarization, Rabi rotation, free precession) of a single proton (¹H) nuclear spin using a Nitrogen-Vacancy (NV) center in isotopically pure ¹²C Chemical Vapor Deposition (CVD) diamond.
Spatial Determination: The NV-proton distance (r) was determined to be 1.44 nm, with a polar angle (θ) of 72.3°, providing atomic-scale localization of the impurity.
Material Origin: The proton was confirmed to be incorporated into the diamond lattice during the CVD growth process, rather than originating from surface contaminants (like immersion oil or H-termination).
Quantum Control Demonstrated: Full control sequences were executed, including Pulsed Dynamic Nuclear Polarization (PulsePol) achieving a saturation polarization (N_spin,sat) of 0.56, and coherent Rabi oscillation at 57.7 kHz.
High-Resolution Spectroscopy: Free Induction Decay (FID) measurements extended to the single-spin level revealed a fine structure splitting (1.2173 MHz and 1.2203 MHz), speculated to arise from dipolar coupling between the ¹H and a nearby ¹⁵N nucleus.
Engineering Significance: Protons offer a higher Larmor frequency compared to standard ¹³C nuclei, enabling potentially faster operations when utilized as built-in quantum memories coupled to the NV center.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Material	Single crystal CVD	N/A	As-grown, ¹²C enriched
Substrate Temperature	800	°C	CVD Growth
CVD Pressure	25	Torr	CVD Growth
Methane Source	Isotopically pure ¹²C	N/A	CH₄ source gas
Nitrogen Concentration	less than 0.1	ppm	Trace amount in gas mixture
Carbon-to-Hydrogen Ratio	24	N/A	Gas flow ratio [CH₄]/[H₂]
NV Center Areal Density	3 x 10⁶	cm^-2	Estimated from fluorescence images
Operating Magnetic Field (B₀)	28.7	mT	Single proton detection (NV1)
Proton Larmor Frequency (f_H)	1.2239	MHz	At B₀ = 28.7 mT
NV1-Proton Distance (r)	1.44	nm	Estimated spatial coordinate
NV1-Proton Polar Angle (θ)	72.3	°	Estimated spatial coordinate
Parallel Hyperfine (A_\|\|/2π)	-19.0	kHz	NV1 coupling constant
Perpendicular Hyperfine (A_⊥/2π)	22.9	kHz	NV1 coupling constant
Nuclear Rabi Frequency	57.7	kHz	Coherent control demonstration
Nuclear π/2 Pulse Length	4.115	µs	Derived from Rabi oscillation
Saturation Polarization (N_spin,sat)	0.56	N/A	Achieved for the single proton (NV1)

Key Methodologies

Diamond Growth and NV Creation: Single-crystal diamond was grown using CVD at 800 °C and 25 Torr. Isotopically pure ¹²C methane was used to minimize background ¹³C noise, and trace nitrogen (< 0.1 ppm) was introduced to create NV centers.
Confocal Microscopy and Initialization: A tabletop system utilizing an oil immersion objective lens was used for optical initialization and readout of the NV electron spin state.
NMR Spectroscopy (XY16-N): Standard NMR spectra were recorded using multipulse sequences (XY16-N) to identify the proton Larmor frequency (f_H) and the shifted resonance frequency (f_XY) due to hyperfine coupling.
Correlation Spectroscopy: A specialized sequence was employed to analyze the NV-proton interaction, resolving the two frequency components (f₀ and f₁) corresponding to the NV electron spin states (m_s = 0 and m_s = -1). This allowed for the calculation of the parallel (A_||) and perpendicular (A_⊥) hyperfine constants.
Pulsed Dynamic Nuclear Polarization (PulsePol): Sequences (PolY and PolX) were used to dynamically transfer polarization from the NV electron spin to the proton nuclear spin, demonstrating the ability to polarize the single nucleus.
Coherent Control (Rabi Oscillation): An RF pulse (Trf) was applied at the proton precession frequency (1.2151 MHz) to drive coherent rotation of the polarized nuclear spin, confirming quantum control.
High-Resolution Free Induction Decay (FID): A continuous readout technique was implemented to measure the free precession of the polarized proton, achieving sub-millihertz resolution and revealing fine structure splitting indicative of nuclear-nuclear interactions (likely ¹H-¹⁵N coupling).

Commercial Applications

Nanoscale Magnetic Resonance Imaging (MRI/NMR): The ability to localize and characterize single nuclear spins provides a pathway for atomic-scale NMR spectroscopy, crucial for resolving chemical structures and molecular dynamics in extremely small sample volumes (e.g., single proteins).
Solid-State Quantum Memory: Protons incorporated during CVD growth can serve as robust, built-in quantum memories coupled to the NV center qubit. Their high Larmor frequency allows for faster quantum operations compared to conventional ¹³C memories.
Quantum Sensing and Metrology: Utilizing the NV center as a highly sensitive quantum sensor for detecting and characterizing individual nuclear species, enhancing the sensitivity and resolution of magnetic field and material sensing applications.
Advanced Quantum Register Development: The system serves as a testbed for engineering multi-nuclear quantum registers (involving ¹H, ¹⁵N, and ¹³C) within diamond, essential for scaling up quantum computing architectures.
CVD Material Quality Control: The technique provides a direct, atomic-scale method to characterize the location and nature of hydrogen impurities and defects introduced during the CVD process, enabling precise control over quantum material fabrication.

View Original Abstract

We report detection and coherent control of a single proton nuclear spin using an electronic spin of the nitrogen-vacancy (NV) center in diamond as a quantum sensor. In addition to determining the NV-proton hyperfine parameters by employing multipulse sequences, we polarize and coherently rotate the single proton spin and detect an induced free precession. Observation of free induction decays is an essential ingredient for high resolution proton nuclear magnetic resonance, and the present work extends it to the atomic scale. We also discuss the origin of the proton as incorporation during chemical vapor deposition growth, which provides an opportunity to use protons in diamond as built-in quantum memories coupled with the NV center.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0016196

References

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