  Negatively charged boron vacancies (VB-) 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 VB- 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 VB- 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 VB- spin by a factor of three. In addition, the T2 time of
the VB- can be enhanced by a factor of 1.3 by inducing a curvature around VB-.
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 VB-
T2, yielding 207.2 and 161.9 {\mu}s 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.
