  The negatively charged silicon vacancy (SiV) color center in diamond has
recently proven its suitability for bright and stable single photon emission.
However, its electronic structure so far has remained elusive. We here explore
the electronic structure by exposing single SiV defects to a magnetic field
where the Zeeman effect lifts the degeneracy of magnetic sublevels. The similar
response of single centers and a SiV ensemble in a low strain reference sample
proves our ability to fabricate almost perfect single SiVs, revealing the true
nature of the defect's electronic properties. We model the electronic states
using a group-theoretical approach yielding a good agreement with the
experimental observations. Furthermore, the model correctly predicts
polarization measurements on single SiV centers and explains recently
discovered spin selective excitation of SiV defects.
