First-principles theory of extending the spin qubit coherence time in hexagonal boron nitride

Negatively charged boron vacancies (V_B⁻) in hexagonal boron nitride (h-BN) are a rapidly developing qubit platform in two-dimensional materials for solid-state quantum applications. However, their spin coherence time (T₂) is very short, limited to a few microseconds owing to the inherently dense nuclear spin bath of the h-BN host. As the coherence time is one of the most fundamental properties of spin qubits, the short T₂ time of V_B⁻ could significantly limit its potential as a promising spin qubit candidate. In this study, we theoretically proposed two materials engineering methods, which can substantially extend the T₂ time of the V_B⁻ spin by four times more than its intrinsic T₂. We performed quantum many-body computations by combining density functional theory and cluster correlation expansion and showed that replacing all the boron atoms in h-BN with the ¹⁰B isotope leads to the coherence enhancement of the V_B⁻ spin by a factor of three. In addition, the T₂ time of the V_B⁻ can be enhanced by a factor of 1.3 by inducing a curvature around V_B⁻. Herein, we elucidate that the curvature-induced inhomogeneous strain creates spatially varying quadrupole nuclear interactions, which effectively suppress the nuclear spin flip-flop dynamics in the bath. Importantly, we find that the combination of isotopic enrichment and strain engineering can maximize the T₂ time of V_B⁻, yielding 207.2 μs and 161.9 μs for single- and multi-layer h-¹⁰BN, respectively. Furthermore, our results can be applied to any spin qubit in h-BN, strengthening their potential as material platforms to realize high-precision quantum sensors, quantum spin registers, and atomically thin quantum magnets.