  Nuclear spins nearby nitrogen-vacancy (NV) centers in diamond are excellent
quantum memory for quantum computing and quantum sensing, but are difficult to
be initialized due to their weak interactions with the environment. Here we
propose and demonstrate a magnetic-field-independent, deterministic and highly
efficient polarization scheme by introducing chopped laser pulses into the
double-resonance initialization method. With this method, we demonstrate
initialization of single-nuclear-spin approaching $98.1\%$ and a
$^{14}N$-$^{13}C$ double-nuclear-spin system approaching $96.8\%$ at room
temperature. The initialization is limited by a nuclear-spin depolarization
effect due to chopped laser excitation. Our approach could be extended to NV
systems with more nuclear spins and would be a useful tool in future
applications such as nano-MRI and single-cell NMR.
