  We experimentally investigate the protection of electron spin coherence of
nitrogen vacancy (NV) center in diamond by dynamical nuclear polarization. The
electron spin decoherence of an NV center is caused by the magnetic ield
fluctuation of the $^{13}$C nuclear spin bath, which contributes large thermal
fluctuation to the center electron spin when it is in equilibrium state at room
temperature. To address this issue, we continuously transfer the angular
momentum from electron spin to nuclear spins, and pump the nuclear spin bath to
a polarized state under Hartman-Hahn condition. The bath polarization effect is
verified by the observation of prolongation of the electron spin coherence time
($T_2^*$). Optimal conditions for the dynamical nuclear polarization (DNP)
process, including the pumping pulse duration and depolarization effect of
laser pulses, are studied. Our experimental results provide strong support for
quantum information processing and quantum simulation using polarized nuclear
spin bath in solid state systems.
