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1. Spin polarization through intersystem crossing in the silicon vacancy of silicon carbide

   https://doi.org/doi.org/10.1103/PhysRevB.99.184102  

   Dong, Wenzheng ; Doherty, M. W. ; Economou, Sophia E. ( May 2019 , Physical review. B, Condensed matter)

   Silicon carbide (SiC)-based defects are promising for quantum communications, quantum information processing, and for the next generation of quantum sensors, as they feature long coherence times, frequencies near the telecom, and optical and microwave transitions. For such applications, the efficient initialization of the spin state is necessary. We develop a theoretical description of the spin-polarization process by using the intersystem crossing of the silicon vacancy defect, which is enabled by a combination of optical driving, spin-orbit coupling, and interaction with vibrational modes. By using distinct optical drives, we analyze two spin-polarization channels. Interestingly, we find that different spin projections of the ground state manifold can be polarized. This paper helps in understanding initialization and readout of the silicon vacancy and explains some existing experiments with the silicon vacancy center in SiC. 

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2. Enhanced cavity coupling to silicon vacancies in 4H silicon carbide using laser irradiation and thermal annealing

   https://doi.org/10.1073/pnas.2021768118  

   Gadalla, Mena N. ; Greenspon, Andrew S. ; Defo, Rodrick Kuate ; Zhang, Xingyu ; Hu, Evelyn L. ( March 2021 , Proceedings of the National Academy of Sciences)

   The negatively charged silicon monovacancy${\mathrm{V}}_{\text{Si}}^{-}$in 4H silicon carbide (SiC) is a spin-active point defect that has the potential to act as a qubit in solid-state quantum information applications. Photonic crystal cavities (PCCs) can augment the optical emission of the${\mathrm{V}}_{\text{Si}}^{-}$, yet fine-tuning the defect–cavity interaction remains challenging. We report on two postfabrication processes that result in enhancement of the$\mathrm{V}1\prime$optical emission from our PCCs, an indication of improved coupling between the cavity and ensemble of silicon vacancies. Below-bandgap irradiation at 785-nm and 532-nm wavelengths carried out at times ranging from a few minutes to several hours results in stable enhancement of emission, believed to result from changing the relative ratio of${\mathrm{V}}_{\text{Si}}^0$(“dark state”) to${\mathrm{V}}_{\text{Si}}^{-}$(“bright state”). The much faster change effected by 532-nm irradiation may result from cooperative charge-state conversion due to proximal defects. Thermal annealing at 100 °C, carried out over 20 min, also results in emission enhancements and may be explained by the relatively low-activation energy diffusion of carbon interstitials${\mathrm{C}}_{\mathrm{i}}$, subsequently recombining with other defects to create additional${\mathrm{V}}_{\text{Si}}^{-}$s. These PCC-enabled experiments reveal insights into defect modifications and interactions within a controlled, designated volume and indicate pathways to improved defect–cavity interactions.

    

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3. Optical parametric oscillation in silicon carbide nanophotonics

   https://doi.org/10.1364/OPTICA.394138  

   Guidry, Melissa A. ; Yang, Ki Youl ; Lukin, Daniil M. ; Markosyan, Ashot ; Yang, Joshua ; Fejer, Martin M. ; Vučković, Jelena ( September 2020 , Optica)

   Silicon carbide (SiC) is rapidly emerging as a leading platform for the implementation of nonlinear and quantum photonics. Here, we find that commercial SiC, which hosts a variety of spin qubits, possesses low optical absorption that can enable SiC integrated photonics with quality factors exceeding${10}^7$. We fabricate multimode microring resonators with quality factors as high as 1.1 million, and observe low-threshold ($8.5\pm <\#comment/>0.5\mathrm{m}\mathrm{W}$) optical parametric oscillation using the fundamental mode as well as optical microcombs spanning 200 nm using a higher-order mode. Our demonstration is an essential milestone in the development of photonic devices that harness the unique optical properties of SiC, paving the way toward the monolithic integration of nonlinear photonics with spin-based quantum technologies.

    

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4. Parallel single-shot measurement and coherent control of solid-state spins below the diffraction limit

   https://doi.org/10.1126/science.abc7821  

   Chen, Songtao ; Raha, Mouktik ; Phenicie, Christopher M. ; Ourari, Salim ; Thompson, Jeff D. ( October 2020 , Science)

   Solid-state spin defects are a promising platform for quantum science and technology. The realization of larger-scale quantum systems with solid-state defects will require high-fidelity control over multiple defects with nanoscale separations, with strong spin-spin interactions for multi-qubit logic operations and the creation of entangled states. We demonstrate an optical frequency-domain multiplexing technique, allowing high-fidelity initialization and single-shot spin measurement of six rare-earth (Er³⁺) ions, within the subwavelength volume of a single, silicon photonic crystal cavity. We also demonstrate subwavelength control over coherent spin rotations by using an optical AC Stark shift. Our approach may be scaled to large numbers of ions with arbitrarily small separation and is a step toward realizing strongly interacting atomic defect ensembles with applications to quantum information processing and fundamental studies of many-body dynamics.

    

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5. Coherent acoustic control of a single silicon vacancy spin in diamond

   https://doi.org/10.1038/s41467-019-13822-x  

   Maity, Smarak ; Shao, Linbo ; Bogdanović, Stefan ; Meesala, Srujan ; Sohn, Young-Ik ; Sinclair, Neil ; Pingault, Benjamin ; Chalupnik, Michelle ; Chia, Cleaven ; Zheng, Lu ; et al ( January 2020 , Nature Communications)

   Phonons are considered to be universal quantum transducers due to their ability to couple to a wide variety of quantum systems. Among these systems, solid-state point defect spins are known for being long-lived optically accessible quantum memories. Recently, it has been shown that inversion-symmetric defects in diamond, such as the negatively charged silicon vacancy center (SiV), feature spin qubits that are highly susceptible to strain. Here, we leverage this strain response to achieve coherent and low-power acoustic control of a single SiV spin, and perform acoustically driven Ramsey interferometry of a single spin. Our results demonstrate an efficient method of spin control for these systems, offering a path towards strong spin-phonon coupling and phonon-mediated hybrid quantum systems.

    

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