Symmetry-protected superexchange yields long coherence times in silicon rare-earth spin qubits
Phys. Rev. B 113, 174440 – Published 28 May, 2026
DOI: https://doi.org/10.1103/8kv8-v2nq
Abstract
Rare-earth dopants in silicon offer a direct photonic interface for quantum networking, yet their deployment in dense, scalable arrays is hindered by decoherence arising from magnetic dipolar and exchange interactions between neighboring spins. Here we propose a symmetry-protected superexchange design in which local ligand symmetry is engineered to suppress the dominant decoherence channel from anisotropic exchange in the regime where the unsigned short-range exchange scale exceeds the dipolar background. Focusing on erbium-doped silicon with oxygen bridges, we decompose the exchange interaction into its irreducible representation—scalar, Dzyaloshinskii–Moriya (DM), and symmetric anisotropy—and identify a crossover regime at subnanometer separations where this unsigned short-range exchange diagnostic exceeds the dipolar background. We demonstrate that inversion-symmetric oxygen motifs strictly forbid the antisymmetric DM vector and constrain the symmetric tensor, effectively decoupling isotropic exchange from noise-generating anisotropic terms. A perturbative treatment of superexchange, benchmarked against first-principles calculations, reveals that optimized symmetric geometries retain substantially higher RB fidelity than asymmetric configurations—for example, the protected two-spin benchmark remains near 0.97–0.98 after 50 gates, whereas symmetry-broken cases fall to roughly 0.6—while the weaker magnetic dipolar background remains as a residual noise source. Furthermore, statistical modeling indicates finite projected protected-motif probabilities under implantation-scale placement disorder, providing a practical route to high-density, optically addressable quantum registers.
