= Nitrogen-vacancy center
A <nitrogen-vacancy center> is a <point defect> in <diamond> consisting of a <nitrogen> atom occupying a <carbon> lattice site next to a <vacancy defect>. The surrounding <diamond> remains the host crystal; the adjacent impurity and missing atom create a localized electronic system. Its <electron spin> and optical response make individual defects accessible to <quantum control>.<Observation of coherent oscillation of a single nuclear spin and realization of a two-qubit conditional quantum gate by F. Jelezko and others>[Author preprint, page2 and Figure1, defect structure and spin states]{ref}
A2004 experiment coupled the defect's <electron spin> to one nearby <carbon-13> <nuclear spin>. Laser pulses provided optical preparation/readout, while <microwave> and <radio frequency> pulses manipulated the spin system. The researchers observed coherent nuclear-spin oscillations and implemented a conditional two-<qubit> operation.<Observation of coherent oscillation of a single nuclear spin and realization of a two-qubit conditional quantum gate by F. Jelezko and others>[Abstract and Figure2, electron/nuclear system and pulse/readout arrangement]{ref} Detecting <fluorescence> thus provided access to a spin experiment inside a solid, rather than requiring electrical contact to the individual defect.
For an ideal resonantly driven two-state model, a <Rabi oscillation> gives transition probability $P(t)=\sin^2(\Omega t/2)$, where $\Omega$ is the angular <Rabi frequency>. A pulse lasting $\pi/\Omega$ ideally transfers the population, while $2\pi/\Omega$ returns it. This illustrates controlled coherent evolution; actual fluorescence counts and gate accuracy also depend on preparation, readout and loss of coherence.
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