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We demonstrate precise control over the zero-phonon optical transition energies of individual nitrogen-vacancy (NV) centers in diamond by applying multiaxis electric fields, via the dc Stark effect. The Stark shifts display surprising asymmetries that we attribute to an enhancement and rectification of the local electric field by photoionized charge traps in the diamond. Using this effect, we tune the excited-state orbitals of strained NV centers to degeneracy and vary the resulting degenerate optical transition frequency by >10 GHz, a scale comparable to the inhomogeneous frequency distribution. This technique will facilitate the integration of NV-center spins within photonic networks.

The study of individual quantum systems in solids, for use as quantum bits (qubits) and probes of decoherence, requires protocols for their initialization, unitary manipulation, and readout. In many solid-state quantum systems, these operations rely on disparate techniques that can vary widely depending on the particular qubit structure. One such qubit, the nitrogen-vacancy (NV) center spin in diamond, can be initialized and read out through its special spin-selective intersystem crossing, while microwave electron spin resonance techniques provide unitary spin rotations. Instead, we demonstrate an alternative, fully optical approach to these control protocols in an NV center that does not rely on its intersystem crossing. By tuning an NV center to an excited-state spin anticrossing at cryogenic temperatures, we use coherent population trapping and stimulated Raman techniques to realize initialization, readout, and unitary manipulation of a single spin. Each of these techniques can be performed directly along any arbitrarily chosen quantum basis, removing the need for extra control steps to map the spin to and from a preferred basis. Combining these protocols, we perform measurements of the NV center's spin coherence, a demonstration of this full optical control. Consisting solely of optical pulses, these techniques enable control within a smaller footprint and within photonic networks. Likewise, this unified approach obviates the need for both electron spin resonance manipulation and spin addressability through the intersystem crossing. This method could therefore be applied to a wide range of potential solid-state qubits, including those which currently lack a means to be addressed. quantum control | quantum optics | semiconductor defects | spintronics T o explore control of individual quantum states, our experiments exploit coherent dark resonances that occur in a basic quantum mechanical-level configuration known as a lambda (Λ) system. This configuration, consisting of two lower energy states coherently coupled to a single excited state, has been observed in a wide array of systems including atoms (1), trapped ions, diamond nitrogenvacancy (NV) centers (2-4), quantum dots (5), superconducting phase quantum bits (qubits) (6), and optomechanical resonators (7). In trapped ions, Λ systems can additionally be exploited to drive stimulated Raman transitions (SRTs) providing unitary rotations of the qubit state (8, 9). This versatile structure also forms the framework for a variety of other important advances in quantum science such as electromagnetically induced transparency (10), slow light (11), atomic clocks (12), laser cooling (13), and spin-photon entanglement (14).Here, we use time-resolved methods and quantum state tomography to explore the dynamics of various optically driven processes within a solid-state Λ system (Fig. 1A). This allows us to demonstrate three all-optical quantum control (9, 15, 16) protocols for a single NV center: initialization, unitary rotation, and readout of its spin state. Our Λ s...