Towards dislocation-driven quantum interconnects

A central problem in the deployment of quantum technologies is the realization of robust architectures for quantum interconnects. We propose to engineer interconnects in semiconductors and insulators by patterning spin qubits at dislocations, thus forming quasi one-dimensional lines of entangled point defects. To gain insight into the feasibility and control of dislocation-driven interconnects, we investigate the optical cycle and coherence properties of nitrogen-vacancy (NV) centers in diamond, in proximity of dislocations, using a combination of advanced first-principles calculations. We show that one can engineer spin defects with properties similar to those of their bulk counterparts, including charge stability and a favorable optical cycle, and that NV centers close to dislocations have much improved coherence properties. Finally, we predict optically detected magnetic resonance spectra that may facilitate the experimental identification of specific defect configurations. Our results provide a theoretical foundation for the engineering of one-dimensional arrays of spin defects in the solid state.

💡 Research Summary

This paper proposes a novel strategy for building robust quantum interconnects by exploiting the one‑dimensional atomic arrangements that naturally occur at crystal dislocations. The authors focus on nitrogen‑vacancy (NV) centers in diamond as a model spin qubit and investigate how their formation, charge stability, electronic structure, optical cycle, and spin coherence are affected when the defects are placed near 30° and 90° glide partial dislocations. Using a combination of density‑functional theory (DFT), time‑dependent DFT (TDDFT), dielectric‑dependent hybrid functionals, quantum defect embedding theory (QDET), and cluster‑expansion techniques, they perform a high‑throughput screening of 90 (30°) and 112 (90°) possible NV configurations.

The screening reveals that the majority of NVs have lower formation energies than bulk NVs (by up to ~1.5 eV) and that roughly 60 % of them favor the triplet spin state required for qubit operation. Selected representative defects (e.g., G‑9‑G11 near the 30° core and G‑6‑G9 near the 90° core) are shown to be stable in the negative charge state (‑1) over a wide range of Fermi‑level positions, suggesting that conventional nitrogen doping can be used to tune the electronic environment without interference from the dislocation itself.

Electronic‑structure analysis demonstrates that the reduced symmetry at the dislocation core (C₁ instead of C₃ᵥ) splits the degenerate e‑levels of the bulk NV into two distinct states, yet the key level spacing (a₁‑to‑e) remains larger than 2 eV, preventing thermally activated intra‑defect transitions at room temperature. Additional states localized along the dislocation line appear near the band edges but remain well separated from the NV defect levels, minimizing unwanted carrier capture.

Optical properties are evaluated with TDDFT and QDET. For all examined configurations, the lowest spin‑conserving vertical excitation (³A₁→³A₂) lies below any defect‑to‑band transition, mirroring the bulk NV behavior. Three singlet excited states (1 A₁, 1 A₂, 1 A₃) are found between the two triplet states, enabling an optical initialization and readout cycle analogous to the bulk system. Zero‑phonon line (ZPL) energies vary widely (1.06–2.49 eV) and Debye‑Waller factors range from 0.55 % to 10.17 %, indicating that many of these centers should be experimentally observable via photoluminescence.

Inter‑system crossing (ISC) rates, crucial for spin polarization, are computed by combining SOC matrix elements from QDET with vibrational overlap functions. The authors find that the upper ISC (³A₂→1 A₃) and lower ISC (1 A₁→³A₁) can be either suppressed or enhanced relative to bulk, depending on the specific geometry. For example, the G9‑G8 configuration shows a markedly reduced upper ISC (favoring longer excited‑state lifetimes) while its lower ISC is faster and more selective for the |0⟩ sub‑level, potentially improving spin‑polarization efficiency. Overall, about 22 % of the 18 representative NVs exhibit ISC characteristics that satisfy the requirements for a high‑fidelity optical cycle.

To assess practical performance, the authors simulate a seven‑state kinetic model (ground‑state triplet, excited‑state triplet, and an effective singlet manifold) and evaluate optical spin initialization, continuous‑wave optically detected magnetic resonance (cw‑ODMR), and photoluminescence readout. The G‑9‑G8 defect can be initialized into the |0⟩ sub‑level with ~70 % probability and yields an ODMR contrast of ~18 %, comparable to or better than bulk NVs.

In summary, the study demonstrates that crystal dislocations can serve as natural templates for arranging spin qubits into quasi‑one‑dimensional arrays without compromising, and in some cases even enhancing, their quantum properties. The work provides a comprehensive first‑principles framework, predicts experimentally accessible ODMR spectra and ZPL signatures, and establishes a solid theoretical foundation for engineering dislocation‑driven quantum interconnects in diamond and potentially other wide‑bandgap materials.