  Nitrogen-vacancy (NV) centers in diamond have attracted significant interest
because of their excellent spin and optical characteristics for quantum
information and metrology. To take advantage of the characteristics, the
precise control of the orientation of the N-V axis in the lattice is essential.
Here we show that the orientation of more than 99 % of the NV centers can be
aligned along the [111]-axis by CVD homoepitaxial growth on (111)-substrates.
We also discuss about mechanisms of the alignment. Our result enables a
fourfold improvement in magnetic-field sensitivity and opens new avenues to the
optimum design of NV center devices.
