  A nitrogen-vacancy (NV) center in a diamond enables the access to an electron
spin, which is expected to present highly sensitive quantum sensors. Although
exploiting a nitrogen nuclear spin improves the sensitivity, manipulating it
using a resonant pulse requires a long gate time owing to its small
gyromagnetic ratio. Another technique to control nuclear spins is a conditional
rotation gate based on dynamical decoupling, which is faster but unavailable
for nitrogen spins owing to the lack of transverse hyperfine coupling with the
electron spin. In this study, we generated effective transverse coupling by
applying a weak off-axis magnetic field. An effective coupling depends on the
off-axis field; the conditional rotation gate on the nitrogen-14 spins of an NV
center was demonstrated within 4.2 {\mu}s under an 1.8% off-axis field and a
longitudinal field of approximately 280 mT. We estimated that a population
transfer from the electron to nitrogen spins can be implemented with 8.7
{\mu}s. Our method is applicable to an ensemble of NV centers, in addition to a
single NV center.
