  Implementation of many quantum information protocols require an efficient
initialization of the quantum register. In the present report, we optimize a
population trapping protocol for initializing a hybrid spin register associated
a single nitrogen vacancy (NV) center in diamond. We initialize the quantum
register by polarizing the electronic and the nuclear spins of the NV with a
sequence of microwave, radio-frequency and optical pulses. We use a rate
equation model to explain the distribution of population under the effect of
the optical pulses. The model is compared to the experimental data obtained by
performing partial quantum state tomography. To further increase the spin
polarization, we propose a recursive protocol with optimized optical pulses.
