The negatively charged nitrogen-vacancy (NV-) center in diamond has been studied in recent years on account of possible applications in quantum information processing, spin-electronics and, e.g., biophotonics.

Particular focus has been drawn onto its optical properties. In that context, the triplet ground state forming a two-level quantum system can serve as a quantum bit whose state can be controlled coherently by electron spin resonance. The spin coherence time reaches 2 ms at room temperature; thus, many operations can be performed before dephasing occurs.

The talk focuses on polarization-dependent optical studies of NV- centers in diamond subjected to high magnetic fields of up to 10 Tesla and high-frequency microwaves (60 GHz), thus providing insight into their energy- and spin-level structure. We observe asymmetric Zeeman splitting of the zero-phonon line photoluminescence, a strong optical alignment as well as Faraday rotation at room temperature. Also, the optically detected magnetic resonance of the NV- centers sensitively depends on the crystal orientation and light polarization. Hereby, the spin transitions of the NV- centers are driven into saturation already at a microwatt power level of microwave pumping, evidencing that the NV- centers are well decoupled from their environment. By comparison, a coupling between neutral (NV0) and negatively charged NV centers is observed for resonant optical excitation of NV0 and is assumed to be based on resonant energy transfer.