  We report a versatile method to efficiently polarize single nuclear spins in
diamond, which is based on optical pumping of a single NV color center and
mediated by a level-anti crossing in its excited state. A nuclear spin
polarization higher than 98% is achieved at room temperature for the 15N
nuclear spin associated to the NV center, corresponding to $\mu$K effective
nuclear spin temperature. We then show simultaneous deterministic
initialization of two nuclear spins (13C and 15N) in close vicinity to a NV
defect. Such robust control of nuclear spin states is a key ingredient for
further scaling up of nuclear-spin based quantum registers in diamond.
