  Rabi nutations of a single nuclear spin in a solid have been observed. The
experiments were carried out on a single electron and a single 13C nuclear spin
of a single nitrogen vacancy defect center in diamond. The system was used for
implementation of quantum logical NOT and a conditional two-qubit gate (CROT).
Density matrix tomography of the CROT gate shows that the gate fidelity
achieved in our experiments is up to 0.9, good enough to be used in quantum
algorithms.
