  We demonstrate vector magnetometry with an ensemble of nitrogen-vacancy (NV)
centers in diamond without the need for an external bias field. The anisotropy
of the electric dipole moments of the NV center reduces the ambiguity of the
optically detected magnetic resonances upon polarized visible excitation.
Further lifting of the remaining ambiguities is achieved via application of an
appropriately linearly polarized microwave field, which enables suppression of
spin-state transitions of a certain crystallographic NV orientation. This
allows for the full vector reconstruction of small (less 0.1 mT) magnetic
fields without an external bias field having to interfere with the magnetic
structure.
