Strong spin–orbit quenching via the product Jahn–Teller effect in neutral group IV qubits in diamond
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-10-30 |
| Journal | npj Quantum Materials |
| Authors | Christopher J. Ciccarino, Johannes Flick, Isaac Harris, Matthew E. Trusheim, Dirk Englund |
| Institutions | Harvard University, Massachusetts Institute of Technology |
| Citations | 20 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Group IV Neutral Qubits in MPCVD Diamond
Section titled “Technical Documentation & Analysis: Group IV Neutral Qubits in MPCVD Diamond”Executive Summary
Section titled “Executive Summary”This research provides critical first-principles calculations detailing the complex electronic and spin structure of neutral Group IV vacancy centers (SiV⁰, GeV⁰, SnV⁰, PbV⁰) in diamond, essential candidates for solid-state quantum information science (QIS).
- Qubit Focus: The study targets neutral Group IV defects, which offer a triplet ground state conducive to long spin coherence times and symmetry-protected optical transitions, crucial for quantum network nodes.
- Core Challenge Addressed: Accurate theoretical modeling of the excited state manifold, which is dominated by the collective product Jahn-Teller (pJT) effect and strong electron-phonon coupling.
- Methodology: Nonperturbative treatment combining second-order pJT coupling with Spin-Orbit Coupling (SOC) via direct Hamiltonian diagonalization.
- Key Finding (Quenching): The dominant pJT interaction results in a strong quenching of the expected spin-orbit splitting, with reduction factors (ρu/g) found to be less than 0.05 for all centers.
- Quantitative Results: Precise prediction of Zero-Phonon Line (ZPL) transition energies and fine structure details, including ms-resolved splittings (up to 11.31 meV for PbV⁰), necessary for experimental identification.
- Material Requirement: Replication and extension of this work require ultra-high purity, low-strain Single Crystal Diamond (SCD) wafers, a core specialization of 6CCVD.
Technical Specifications
Section titled “Technical Specifications”The following parameters were calculated from first principles, detailing the vibronic and spin-orbit properties of the neutral Group IV defects (Table 1 in the source paper).
| Parameter | Value (SiV⁰) | Value (SnV⁰) | Value (PbV⁰) | Unit | Context |
|---|---|---|---|---|---|
| Zero-Phonon Line (ZPL) | 1.361 | 1.833 | 2.216 | eV | Predicted ZPL (pure electronic ³Eu) |
| ZPL (³Eu) + SOC | 1.361 | 1.825 | 2.170 | eV | Predicted ZPL including SOC effects |
| Vibronic Splitting (γ(2)) | 3.21 | 6.22 | 7.90 | meV | Splitting between lowest vibronic states (2nd order pJT) |
| SO Splitting (λu + λg) | 0.089 | 3.15 | 11.31 | meV | Splitting between ms = ±1 levels of lowest Eu vibronic eigenstates |
| SO Quenching Factor (ρu) | 0.012 | 0.032 | 0.043 | Dimensionless | Indicates strong quenching (< 0.05) |
| Effective Vibrational Energy (ħωe) | 87.3 | 87.7 | 90.8 | meV | Energy of Eg phonon modes |
| First Order Instability (EJT(1)) | 258 | 217 | 200 | meV | Energy instability due to constructive pJT interference |
| Displacement Amplitude (ρ(1)) | 0.171 | 0.154 | 0.145 | Å | Displacement from D3d high-symmetry point |
Key Methodologies
Section titled “Key Methodologies”The theoretical framework relies on advanced computational material science techniques to accurately model the complex electron-phonon and spin-orbit interactions within the diamond lattice.
- First-Principles Calculations: Constrained Kohn-Sham Density Functional Theory (DFT) calculations were performed using the VASP code (version 5.4.4).
- Basis Set: Calculations utilized a plane wave basis set with Projector-Augmented Wave (PAW) pseudopotentials, employing a stringent energy cutoff of 400 eV (verified up to 800 eV).
- Functional: The hybrid HSE06 exchange-correlation functional was employed to accurately describe the energetics of the defect systems.
- Defect Modeling: Defects were modeled in a cubic supercell equivalent to 512 carbon atoms of the diamond lattice (lattice constant 3.545 Å), sampling only the Γ point of the Brillouin zone.
- Ionic Relaxation: Ionic relaxation was performed until forces on all atoms fell below 10-2 eV/Å, enforcing D3d (high-symmetry) and C2h (low-symmetry) constraints to capture the Jahn-Teller distortions.
- Hamiltonian Solving: The combined Spin-Orbit and Product Jahn-Teller Hamiltonian was solved numerically via direct diagonalization, including up to 40 phonons in the expansion, providing a nonperturbative treatment of the coupled spin-vibronic system.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”The successful experimental realization and manipulation of neutral Group IV qubits (SiV⁰, SnV⁰, GeV⁰, PbV⁰) depend critically on the quality and preparation of the host diamond material. 6CCVD is uniquely positioned to supply the necessary high-specification MPCVD diamond required to replicate and advance this quantum research.
Applicable Materials
Section titled “Applicable Materials”| Research Requirement | 6CCVD Material Solution | Technical Justification |
|---|---|---|
| Intrinsic Diamond Host | Optical Grade Single Crystal Diamond (SCD) | Qubit coherence and ZPL fidelity require extremely low nitrogen content (< 1 ppb) and minimal strain/birefringence, which 6CCVD’s MPCVD process achieves. |
| Heavy Dopant Integration | Custom SCD Substrates | Creation of SnV⁰ and PbV⁰ centers often involves ion implantation into high-purity SCD. Our material provides the necessary low-defect starting platform for high-yield defect creation. |
| Qubit Control Structures | Boron-Doped Diamond (BDD) | For integrated quantum devices, BDD layers can be grown for on-chip microwave/RF transmission lines or electrodes, leveraging diamond’s high thermal conductivity. |
Customization Potential
Section titled “Customization Potential”The complex spin-vibronic physics described in this paper necessitates precise material engineering for device integration. 6CCVD offers full customization capabilities to meet these advanced QIS requirements:
- Custom Dimensions and Thickness: We supply SCD plates up to 500 µm thick, and custom substrates up to 10 mm thick, allowing researchers flexibility in device architecture and defect creation depth. We also offer Polycrystalline Diamond (PCD) wafers up to 125 mm in diameter for large-scale sensor arrays.
- Ultra-Low Roughness Polishing: Experimental observation of the predicted fine structure (e.g., the 3-11 meV splittings) requires high-fidelity optical coupling. 6CCVD guarantees surface roughness of Ra < 1 nm for SCD, ensuring minimal scattering losses at the diamond-air interface.
- Integrated Metalization: Qubit manipulation requires microwave control structures. We offer in-house deposition of custom metal stacks, including Ti, Pt, Au, Pd, W, and Cu, essential for creating the RF waveguides necessary to address the ms-resolved spin sublevels.
- Laser Processing: For creating photonic structures (e.g., waveguides or nanocavities) to enhance ZPL collection efficiency, 6CCVD provides precision laser cutting and shaping services.
Engineering Support
Section titled “Engineering Support”The interplay between the Jahn-Teller effect, electron-phonon coupling, and spin-orbit interaction is highly complex. 6CCVD’s in-house team of PhD material scientists specializes in the growth and characterization of diamond for quantum applications.
- Material Selection for QIS: Our experts can assist researchers in selecting the optimal diamond grade (e.g., isotopic purity, nitrogen concentration, strain profile) to maximize the coherence times and optical stability of Group IV vacancy centers.
- Defect Engineering Consultation: We provide consultation on post-processing techniques (e.g., annealing protocols, implantation parameters) to achieve high concentrations of the desired neutral charge state (SiV⁰, SnV⁰) while maintaining material integrity.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Abstract Artificial atom qubits in diamond have emerged as leading candidates for a range of solid-state quantum systems, from quantum sensors to repeater nodes in memory-enhanced quantum communication. Inversion-symmetric group IV vacancy centers, comprised of Si, Ge, Sn, and Pb dopants, hold particular promise as their neutrally charged electronic configuration results in a ground-state spin triplet, enabling long spin coherence above cryogenic temperatures. However, despite the tremendous interest in these defects, a theoretical understanding of the electronic and spin structure of these centers remains elusive. In this context, we predict the ground-state and excited-state properties of the neutral group IV color centers from first principles. We capture the product Jahn-Teller effect found in the excited state manifold to second order in electron-phonon coupling, and present a nonperturbative treatment of the effect of spin-orbit coupling. Importantly, we find that spin-orbit splitting is strongly quenched due to the dominant Jahn-Teller effect, with the lowest optically-active 3 E u state weakly split into m s -resolved states. The predicted complex vibronic spectra of the neutral group IV color centers are essential for their experimental identification and have key implications for use of these systems in quantum information science.