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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.^[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.^[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.

A missing atom at a lattice site that would be occupied in the reference crystal. A vacancy defect is a point defect and can contribute configurational entropy because many choices of empty sites can have the same macroscopic energy. Formation energy and pressure-volume work oppose this entropy gain. Vacancies can be present in thermal equilibrium; they need not all be preparation mistakes. Their allowed number and creation mechanism depend on whether the lattice size, atom number and surface reservoirs are fixed.