  Spin degrees of freedom of charged nitrogen-vacancy (NV$^-$) centers in
diamond have large decoherence times even at room temperature, can be
initialized and read out using optical fields, and are therefore a promising
candidate for solid state qubits. Recently, quantum manipulations of NV$^-$-
centers using RF fields were experimentally realized. In this paper we show;
first, that such operations can be controlled by varying the frequency of the
signal, instead of its amplitude, and NV$^-$- centers can be selectively
addressed even with spacially uniform RF signals; second, that when several \NV
- centers are placed in an off-resonance optical cavity, a similar application
of classical optical fields provides a controlled coupling and enables a
universal two-qubit gate (CPHASE). RF and optical control together promise a
scalable quantum computing architecture.
