  Nitrogen-Vacancy (NV) centers in diamond have useful properties for detecting
both AC and DC magnetic fields with high sensitivity at nano-scale resolution.
Vector detection of AC magnetic fields can be achieved by using NV centers
having three different orientations. Here, we propose a method to achieve this
by using NV centers of single orientation. In this method, a static magnetic
field is applied perpendicular to the NV axis, leading to strong mixing of the
$m_{s}=-1$ and $1$ electron spin states. As a result, all three electron spin
transitions of the triplet ground state have non-zero dipole moments, with each
transition coupling to a single component of the magnetic field. This can be
used to measure both strength and orientation of the applied AC field. To
validate the technique, we perform a proof of principle experiment using a
subset of ensemble NV centers in diamond, all having the same orientation.
