Magnetic‐Field Dependent VB⁻ Spin Decoherence in Hexagonal Boron Nitrides: A First‐Principles Study

The negatively charged boron vacancy (V B − ) in h‐BN is a spin‐1 defect functioning as an optically addressable spin qubit in 2D materials. A precise understanding of its spin decoherence is essential to advance it into a robust qubit platform. First‐principles quantum many‐body simulations are employed to investigate V B − decoherence in dense nuclear spin baths of h‐BN under magnetic fields from 0.01 to 3 T, considering isotopic variants h‐ 10 B 14 N, h‐ 11 B 14 N, h‐ 10 B 15 N, and h‐ 11 B 15 N. A transition boundary (TB) is observed where the dominant decoherence mechanism changes: below the TB, sub‐microsecond decoherence is governed by independent nuclear spin dynamics, whereas above it, pairwise flip‐flops dominate, extending T 2 to tens of microseconds. Analytical predictions place the TB at 0.502 T for h‐ 10 B 14 N and 0.205 T for h‐ 11 B 14 N. The larger TB in h‐ 10 BN results from the larger nuclear spin of 10 B ( I = 3), which produces stronger nuclear modulation over a wider field range. The analytical approach also explains the magnetic‐field‐insensitive fast modulation observed below the TB. These findings clarify the role of dense nuclear spin baths with large nuclear spins ( I ≥ 1) in V B − decoherence and provide design principles for isotopically engineered h‐BN spin qubits.