An NV− center in magnesium oxide as a spin qubit for hybrid quantum technologies

At a Glance

Metadata	Details
Publication Date	2025-03-17
Journal	npj Computational Materials
Authors	Vrindaa Somjit, Joel Davidsson, Yu Jin, Giulia Galli
Citations	1
Analysis	Full AI Review Included

Technical Analysis of NV^- Center in Magnesium Oxide for Quantum Technologies

This document analyzes the findings of the research paper regarding the Nitrogen-Vacancy (NV^-) center in Magnesium Oxide (MgO) and connects the material requirements and engineering challenges to the advanced capabilities offered by 6CCVD.

Executive Summary

The research identifies the NV^- center in MgO as a promising, yet challenging, candidate for hybrid quantum technologies, particularly when compared to the established NV^- in diamond.

Defect Identification: A negatively charged complex between a nitrogen interstitial and a magnesium vacancy (Int_NVac_Mg) in MgO was identified using high-throughput first-principles calculations.
Qubit Potential: The defect exhibits favorable electronic properties, including a stable triplet ground state (³A₂) and a large Zero-Field Splitting (ZFS) (up to 46.38 GHz), suitable for spin initialization and readout.
Optical Properties: Absorption and ZPL are predicted in the Ultraviolet (UV) range, while emission is in the visible/red-infrared range (1.76 eV), making detection straightforward.
Critical Challenge (Vibronic Coupling): The NV^- in MgO suffers from strong vibronic coupling (Huang-Rhys Factor S ≈ 44.49) due to a strong pseudo-Jahn Teller (pJT) effect and low-frequency phonon modes.
Undetectable ZPL: This strong coupling results in a negligible Debye-Waller Factor (DWF ≈ 10^-20), making the Zero-Phonon Line (ZPL) undetectable and rendering the defect unsuitable for single-photon emission (quantum communication).
Engineering Strategies: Biaxial compressive strain (up to 4%) and alternative substitutional defect complexes (e.g., Al_Mg-Al_O) are proposed to reduce vibronic coupling and increase the DWF, though significant increases are required.
Conclusion for Qubit Hosts: The findings underscore the inherent superiority of the Single Crystal Diamond (SCD) NV^- center for applications requiring robust optical addressability and high DWF.

Technical Specifications

Extracted hard data points characterizing the NV^- defect in MgO and comparative metrics.

Parameter	Value	Unit	Context
Host Material	Magnesium Oxide (MgO)	N/A	CMOS-compatible host with high dielectric constant.
Defect Type	Nitrogen Interstitial-Magnesium Vacancy (NV^-)	N/A	Int_NVac_Mg complex.
Ground State	Triplet (³A₂)	N/A	Stable spin state for qubit operation.
Zero-Field Splitting (D)	38.48 / 46.38	GHz	PBE level / DDH level calculation. Large ZFS aids sublevel isolation.
ZPL Energy (Calculated)	3.19	eV	Zero-Phonon Line (UV range).
Emission Energy (Calculated)	1.76	eV	Visible/Red-Infrared range.
Absorption Energy (Calculated)	4.98	eV	UV range.
Huang-Rhys Factor (S)	44.49	Dimensionless	High value indicating strong electron-phonon coupling.
Debye-Waller Factor (DWF)	≈ 10^-20	Dimensionless	Negligible DWF for unstrained system.
Predicted Coherence Time (T₂)	0.60	ms	Predicted for single electron spin in MgO (Kanai et al.).
Effective e-Phonon Frequency (ħω_eff)	25	meV	Low frequency contributing to strong vibronic coupling.
Biaxial Compressive Strain Tested	1% and 4%	%	Strategy to increase DWF (increased DWF by 4 and 7 orders of magnitude, respectively).

Key Methodologies

The study utilized a sophisticated computational workflow combining high-throughput screening with advanced electronic structure methods.

Defect Screening (ADAQ/httk): A high-throughput framework screened 2917 isolated defects and complexes in MgO, filtering based on stability (defect hull), spin triplet ground state, and ZPL presence.
Ground State Refinement (Hybrid DFT): The NV^- center was characterized using Density Functional Theory (DFT) with the dielectric-dependent hybrid (DDH) exchange-correlation functional (exact exchange fraction $\alpha$ = 0.34).
Structural Parameters: Calculations used 216-atom supercells and a fixed lattice constant (a₀ = 4.19 Å) corresponding to the experimental value extrapolated to 0 K.
Excited State Calculation (TDDFT/QDET): Time-Dependent DFT (TDDFT-DDH) and Quantum Defect Embedding Theory (QDET) were employed to analyze triplet and singlet excited states, confirming the possibility of an optical initialization cycle.
Spin Parameter Calculation: The Zero-Field Splitting (ZFS) tensor was calculated using the pyZFS package, and hyperfine parameters were calculated using the GIPAW method.
Vibronic Coupling Analysis: One-dimensional Configurational Coordinate Diagrams (CCD) were constructed, and the Adiabatic Potential Energy Surface (APES) was fitted to quantify the strong pseudo-Jahn Teller (pJT) stabilization energy and low effective phonon frequencies responsible for the high Huang-Rhys Factor.
Strain Simulation: Biaxial compressive strain (1% and 4%) was applied to the optimized ground and excited state geometries to evaluate its effect on the vibronic coupling and DWF.

6CCVD Solutions & Capabilities

The research highlights the critical material requirements for quantum defects, particularly the need for low vibronic coupling and high DWF—properties where the NV^- in diamond excels. 6CCVD provides the necessary materials and customization to replicate and advance this research, focusing on the superior performance of diamond-based qubits and the engineering solutions required for hybrid integration.

Applicable Materials

To achieve robust quantum performance (high DWF, long T₂), the optimal material remains the Single Crystal Diamond (SCD) NV^- center, which 6CCVD specializes in.

Application Focus	Recommended 6CCVD Material	Key Specification
Quantum Communication (High DWF)	Optical Grade SCD (Isotopically Purified)	Ultra-low ¹³C concentration for maximum coherence time (T₂). Inherently low vibronic coupling (high DWF) compared to MgO.
Quantum Sensing/Computing	High-Purity SCD Plates	Thicknesses available from 0.1µm up to 500µm. Polished to Ra < 1nm for optimal surface integration.
Hybrid Integration (Spintronics/Ferroelectrics)	Polycrystalline Diamond (PCD) Wafers	Custom dimensions up to 125mm diameter, suitable for scalable device fabrication and integration with CMOS processes.
Strain Engineering Substrates	Custom Substrates (SCD/PCD)	Substrates up to 10mm thick, ideal for mounting and applying controlled biaxial or uniaxial strain to optimize ZFS and DWF.

Customization Potential

The paper discusses engineering strategies involving strain and integration with spintronic devices (Magnetic Tunnel Junctions). 6CCVD directly supports these advanced requirements:

Custom Metalization: The integration of NV^- centers into hybrid classical-quantum devices (e.g., MTJs) requires precise metal contacts. 6CCVD offers in-house deposition of critical metals, including Ti, Pt, Au, Pd, W, and Cu, tailored to specific device geometries.
Precision Fabrication: 6CCVD provides custom dimensions for plates and wafers up to 125mm (PCD) and offers laser cutting services to achieve unique geometries required for nanophotonic structures or strain application mechanisms.
Ultra-Smooth Surfaces: For integration with high-Q cavities (as discussed in the paper regarding Er³⁺-doped MgO), 6CCVD guarantees ultra-low surface roughness: Ra < 1nm for SCD and Ra < 5nm for inch-size PCD.

Engineering Support

6CCVD’s in-house team of PhD material scientists and engineers are experts in MPCVD diamond growth and defect engineering. We offer comprehensive support for projects focused on NV^- center quantum sensing and computing.

Material Selection Consultation: We assist researchers in selecting the optimal diamond grade (SCD vs. PCD, purity level, doping) to maximize coherence time (T₂) and minimize decoherence sources.
Defect Creation Guidance: Our team can advise on post-processing techniques (e.g., nitrogen implantation, annealing protocols) necessary to create high-yield, high-quality NV^- centers in our SCD substrates.
Global Logistics: We ensure reliable, global shipping (DDU default, DDP available) to support international research timelines.

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/s41524-025-01558-w