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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...

The phase relation between quantum states represents an essential resource for the storage and processing of quantum information. While quantum phases are commonly controlled dynamically by tuning energetic interactions, utilizing geometric phases that accumulate during cyclic evolution may offer superior robustness to noise. To date, demonstrations of geometric phase control in solid-state systems rely on microwave fields that have limited spatial resolution. Here, we demonstrate an alloptical method based on stimulated Raman adiabatic passage to accumulate a geometric phase, the Berry phase, in an individual nitrogen-vacancy (NV) center in diamond. Using diffraction-limited laser light, we guide the NV center's spin along loops on the Bloch sphere to enclose arbitrary Berry phase and characterize these trajectories through time-resolved state tomography. We investigate the limits of this control due to loss of adiabiaticity and decoherence, as well as its robustness to noise intentionally introduced into the experimental control parameters, finding its resilience to be independent of the amount of Berry phase enclosed. These techniques set the foundation for optical geometric manipulation in future implementations of photonic networks of solid state qubits linked and controlled by light.When a quantum mechanical system evolves slowly along a closed loop in its parameter space, a given eigenstate may acquire a phase consisting of both a dynamic and geometric contribution. First proposed by S. Pancharatnam 1 in his study of cyclic rotations of the polarization of light, and later generalized by M. V. Berry 2 , this adiabatic geometric phase is determined solely by the geometry of the traversed loop, in contrast to the dynamic phase that accumulates from the energetics and travel time of the intervening state evolution. Since the Berry phase is proportional to the area enclosed by the path in parameter space, it is intrinsically resilient to noise that causes deviations to the path but conserve the total enclosed area 3,4 . Geometric control thus represents a promising avenue for constructing faulttolerant quantum logic gates 5,6 .Control over geometric phases, occurring both when the cyclic evolution is traversed adiabatically 2 and non-adiabatically 7 , has been demonstrated in a variety of physical platforms, including liquid nuclear magnetic resonance 8 , trapped atoms 9 , and more recently in the solid-state in superconducting qubits 10,11 and defect spins 12-14 . However, current implementations of geometric phase control in solid-state systems have utilized microwaves that are difficult to localize and thus concede the ability to selectively

Atom-scale defects in semiconductors are promising building blocks for quantum devices, but our understanding of their material-dependent electronic structure, optical interactions, and dissipation mechanisms is lacking. Using picosecond resonant pulses of light, we study the coherent orbital and spin dynamics of a single nitrogen-vacancy center in diamond over time scales spanning six orders of magnitude. We develop a time-domain quantum tomography technique to precisely map the defect's excited-state Hamiltonian and exploit the excited-state dynamics to control its ground-state spin with optical pulses alone. These techniques generalize to other optically addressable nanoscale spin systems and serve as powerful tools to characterize and control spin qubits for future applications in quantum technology.

Homoepitaxial growth of single crystal diamond membranes is demonstrated employing a microwave plasma chemical vapor deposition technique. The membranes possess excellent structural, optical, and spin properties, which make them suitable for fabrication of optical microcavities for applications in quantum information processing, photonics, spintronics, and sensing.

| Deep-level defects are usually considered undesirable in semiconductors as they typically interfere with the performance of present-day electronic and optoelectronic devices. However, the electronic spin states of certain atomicscale defects have recently been shown to be promising quantum bits for quantum information processing as well as exquisite nanoscale sensors due to their local environmental sensitivity. In this review, we will discuss recent advances in quantum control protocols of several of these spin defects, the negatively charged nitrogen-vacancy (NV À ) center in diamond and a variety of forms of the neutral divacancy ðVV 0 Þ complex in silicon carbide (SiC). These defects exhibit a spintriplet ground state that can be controlled through a variety of techniques, several of which allow for room temperature operation. Microwave control has enabled sophisticated decoupling schemes to extend coherence times as well as nanoscale sensing of temperature along with magnetic and electric fields. On the other hand, photonic control of these spin states has provided initial steps toward integration into quantum networks, including entanglement, quantum state teleportation, and all-optical control. Electrical and mechanical control also suggest pathways to develop quantum transducers and quantum hybrid systems. The versatility of the control mechanisms demonstrated should facilitate the development of quantum technologies based on these spin defects.

The nitrogen-vacancy (NV) center in diamond has been recognized as a high-sensitivity nanometer-scale metrology platform. Thermometry has been a recent focus, with attention largely confined to room temperature applications. Temperature sensing at low temperatures, however, remains challenging as the sensitivity decreases for many commonly used techniques, which rely on a temperature dependent frequency shift of NV center's spin resonance and its control with microwaves. Here we use an alternative approach that does not require microwaves, ratiometric all-optical thermometry, and demonstrate that it may be utilized to liquid nitrogen temperatures without deterioration of the sensitivity. The use of an array of nanodiamonds embedded within a portable polydimethylsiloxane (PDMS) sheet provides a versatile temperature sensing platform that can probe a wide variety of systems without the configurational restrictions needed for applying microwaves. With this device, we observe a temperature gradient over tens of microns in a ferromagnetic-insulator substrate (yttrium iron garnet, YIG) under local heating by a resistive heater. This thermometry technique provides a cryogenically compatible, microwavefree, minimally invasive approach capable of probing local temperatures with few restrictions on the substrate materials.I.