Discovery of atomic clock-like spin defects in simple oxides from first principles

At a Glance

Metadata	Details
Publication Date	2024-06-06
Journal	Nature Communications
Authors	Joel Davidsson, Mykyta Onizhuk, Christian Vorwerk, Giulia Galli
Institutions	Argonne National Laboratory, University of Chicago
Citations	12
Analysis	Full AI Review Included

Technical Documentation & Analysis: Atomic Clock-Like Spin Defects in Simple Oxides

Reference: Davidsson et al., Discovery of atomic clock-like spin defects in simple oxides from first principles, Nature Communications (2024)15:4812.

Executive Summary

This research predicts a new class of NV-like spin defects (X_CaV_O) in Calcium Oxide (CaO) that exhibit exceptional coherence properties, positioning simple oxides as attractive platforms for quantum technologies.

NV-Like Defects Predicted: High-throughput calculations identified X_CaV_O complexes (X=Sb, Bi, I) in CaO with electronic structures remarkably similar to the Nitrogen-Vacancy (NV) center in diamond.
Telecommunication Emission: The Bismuth complex (Bi_CaV_O^-) is predicted to emit at 1624 nm (0.76 eV), placing its Zero Phonon Line (ZPL) directly in the telecommunication L-band (1565-1625 nm).
Superior Host Material: CaO is predicted to be an inherently low-noise host, yielding a baseline nuclear spin-limited Hahn-echo coherence time (T₂) of 34 ms, significantly exceeding that of natural diamond (0.89 ms).
Atomic Clock Transitions: The strong coupling of the high-spin ²⁰⁹Bi nucleus to the electron spin enables “atomic clock-like transitions” (CTs).
Record Coherence Time: Operating the Bi_CaV_O^- qubit at the clock transition (22.18 mT) is predicted to increase the T₂ time by two orders of magnitude, reaching 4.7 seconds.
Integration Advantage: CaO’s low refractive index (1.84) is closer to that of optical fibers (1.44) than diamond (2.42), facilitating efficient photon coupling in integrated quantum systems.

Technical Specifications

The following hard data points compare the predicted Bi_CaV_O^- defect in CaO (using the HSE functional) against the established NV center in diamond.

Parameter	Bi_CaV_O^- (CaO)	NV^- (Diamond)	Unit	Context
Host Material	Calcium Oxide (CaO)	Diamond (C)	N/A	SCD is 6CCVD’s core product for NV centers.
Crystal Symmetry	C_4v	C_3v	N/A	Defect symmetry
Band Gap (HSE)	5.32	5.47	eV	CaO is a wide band-gap insulator.
Refractive Index	1.84	2.42	N/A	Lower index favors fiber coupling.
ZPL (Zero Phonon Line)	0.76 (1624)	2.00 (619)	eV (nm)	Bi_CaV_O^- emits in the telecom L-band.
ZFS (Zero-Field Splitting)	2.46	3.42	GHz	Ground state splitting (S=1).
Radiative Lifetime	63	6.5	ns	Bi_CaV_O^- is predicted to be bright.
Baseline T₂ (Hahn-Echo)	34	0.89	ms	Nuclear spin-limited coherence time.
Clock Transition T₂	4.7	N/A	s	Achieved at B = 22.18 mT for Bi_CaV_O^-.
Transition Dipole Moment (TDM)	118	48	Debye²	High TDM suggests bright emission.

Key Methodologies

The research employed a multi-stage, high-throughput computational workflow combining Density Functional Theory (DFT) with advanced quantum embedding and spin dynamics calculations.

High-Throughput Screening (ADAQ): Used the Automatic Defect Analysis and Qualification (ADAQ) software to generate and screen 9077 potential single and double defects in CaO.
Stability and Selection: Defects were filtered based on formation energy (defect hull stability), resulting in 1149 stable defects. Further filtering required a triplet ground state (S=1) and detectable Zero Phonon Line (ZPL), yielding 5 final candidates (X_CaV_O, X=P, As, Sb, Bi, I).
DFT Calculations (VASP): Structural and electronic properties were calculated using the Vienna Ab initio Simulation Package (VASP) on 512-atom supercells.
Functional Selection: Three functionals were used: PBE, HSE06 (with 25% and 62.5% mixing parameters), and K-PBEO, with HSE (62.5%) providing the best agreement with experimental diamond NV data.
Excitation Energies: ZPLs and absorption energies were calculated using constrained DFT (Δ-SCF).
Many-Body States: Quantum Defect Embedding Theory (QDET) was employed to accurately determine the singlet-triplet (S-T) splitting and the many-body level diagram, confirming the optical cycle viability.
Coherence Time (CCE): Nuclear spin bath-limited spin coherence (T₂) was computed using the Coupled Cluster Expansion (CCE) method (PyCCE code) to model decoherence dynamics near clock transitions.

6CCVD Solutions & Capabilities

The research highlights the critical role of material engineering in achieving long-coherence quantum platforms, directly comparing predicted oxide performance to the established NV center in diamond. 6CCVD provides the necessary engineered diamond materials and customization services to immediately advance or benchmark this research.

Applicable Materials

While the paper focuses on CaO, the NV center in diamond remains the most mature solid-state qubit platform. 6CCVD specializes in providing the highest quality MPCVD diamond necessary to replicate and extend the diamond NV center benchmarks used in this study.

Research Requirement	6CCVD Material Solution	Technical Advantage
NV Center Benchmarking	Optical Grade Single Crystal Diamond (SCD)	Ultra-low nitrogen concentration (< 1 ppb) for maximizing T₂ and achieving high-fidelity spin initialization, essential for direct comparison to theoretical models.
Hybrid Optoelectronic Systems	Boron-Doped Diamond (BDD)	Provides conductive diamond substrates necessary for engineering hybrid quantum systems, Schottky diodes, or adjusting the Fermi level (E_F) as suggested in the paper (e.g., for n-type conditions).
Strain Field Engineering	Thin SCD/PCD Films	SCD or PCD layers (down to 0.1 µm thickness) for integration onto other substrates (like SiC or potentially CaO) to introduce specific strain fields, a key optimization parameter mentioned in the paper.

Customization Potential

The paper notes that nanostructuring and thin films may be required to optimize quantum efficiency (Debye-Waller factors) and that Fermi level adjustment is necessary for defect stability. 6CCVD offers comprehensive customization capabilities to meet these advanced engineering needs.

Custom Dimensions and Thickness: 6CCVD provides SCD and PCD plates/wafers in custom dimensions up to 125 mm (PCD). We offer precise thickness control for thin films, ranging from 0.1 µm to 500 µm (SCD/PCD), ideal for nanostructuring or creating hybrid heterostructures.
Surface Preparation: To minimize surface defects and noise, 6CCVD guarantees ultra-smooth polishing: Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD, critical for high-fidelity optical interfaces.
Advanced Metalization: For Fermi level control and device integration (e.g., creating Schottky diodes or contacts for applying electric fields), 6CCVD offers in-house metalization services including Au, Pt, Pd, Ti, W, and Cu deposition.
Laser Cutting and Shaping: We provide custom laser cutting and shaping services to create microstructures or specific geometries required for optimizing photon collection efficiency, especially relevant given the discussion of low refractive index hosts.

Engineering Support

The discovery of new spin defects requires deep expertise in material science and quantum physics. 6CCVD’s in-house PhD team specializes in the growth and characterization of MPCVD diamond for quantum applications.

Material Selection Consultation: Our experts can assist researchers in selecting the optimal diamond grade (SCD, PCD, BDD) and specification (doping, orientation, surface termination) required for similar solid-state spin qubit projects, ensuring the material meets the stringent requirements for long coherence times and high quantum efficiency.
Integration Strategy: We provide technical guidance on integrating diamond components into complex optoelectronic systems, leveraging our experience in custom metalization and thin-film growth.

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/s41467-024-49057-8