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# NV center qubits

**NV center qubits** use the nitrogen-vacancy defect in diamond, a substitutional nitrogen atom adjacent to a missing carbon atom, as a solid-state spin qubit. The defect's electronic ground state has a spin-1 triplet, and the $m_s = 0$ and $m_s = \pm1$ sublevels form a natural two-level (or three-level) system that can be initialized, manipulated, and read out optically at room temperature. This makes NV centers unusual among qubit platforms: most others require cryogenic temperatures to function at all.

The spin state is initialized and read out through spin-dependent fluorescence: green laser excitation followed by red photoluminescence, where the emitted photon rate depends on which spin state the electron was in before excitation. Manipulation between spin sublevels is done with microwave pulses resonant with the zero-field splitting, in a scheme directly analogous to electron spin resonance.

$$H = D S_z^2 + \gamma_e \vec{B} \cdot \vec{S}$$

$D \approx 2.87\,\text{GHz}$ is the zero-field splitting between $m_s=0$ and $m_s=\pm1$, $S_z$ is the spin-1 operator along the NV axis, $\gamma_e$ is the electron gyromagnetic ratio, and $\vec{B}$ is an applied magnetic field that Zeeman-splits the $m_s = \pm1$ levels so a single one of them can be addressed as the qubit's $\lvert 1 \rangle$ state.

## Applications beyond computing

NV centers double as extremely sensitive magnetometers and thermometers, since the spin resonance frequency shifts with local magnetic field and temperature. This sensing use case has arguably seen more real-world deployment than NV centers as a quantum computing platform, where connecting many NV centers into a large, coupled register remains an open engineering problem.

 - Room-temperature operation, unlike superconducting or trapped-ion platforms
 - Long coherence times (milliseconds) thanks to diamond's low spin-noise environment
 - Nearby nuclear spins (nitrogen, carbon-13) serve as auxiliary qubits or quantum memory
 - Scaling to large qubit counts is harder than for lithographically patterned platforms