Engineering Electron Couplings with Phonons and Spins in Solid-State Color Centers

Abstract

Solid-state color centers, such as NV centers in diamond and silicon carbide defects, are promising platforms for advancing quantum applications. These point defects operate as single photon sources at room temperature and optically addressable spin qubits, making them versatile tools for nanoscale quantum sensors, coherent spin memories and quantum communication. Moreover, spin-photon and multi-spin entanglement facilitates quantum applications such as quantum registers, quantum nodes and quantum repeaters, underscoring their potential in quantum information processing and quantum networks. However, the inherent constraints of the solid-state crystal environment pose significant challenges in achieving high-fidelity and high-efficiency quantum systems. Color centers often encounter unintentional interactions with the surrounding spin bath, including nuclear spins of isotopes in the host material and dopant-induced impurities. These unavoidable interactions induce decoherence, which degrades the fidelity of defect spin initialization and manipulation and ultimately disrupts essential quantum operations. To address this, dynamical decoupling techniques were employed to suppress such interactions. This approach utilizes optimal pulse sequences to perform phase-transition operations and repeated phase refocusing control, effectively decoupling the defect spin from interactions with surrounding nuclear spin bath. Using the CPMG pulse sequence, we achieved a 6.27-fold enhancement in the spin coherence time of the NV center in diamond. This technique also enabled selective coupling with individual nuclear spins decoupled from spin bath, facilitating indirect control of the nuclear spins through manipulation of the defect spin. Leveraging this capability, we demonstrated two-qubit gate operations and successfully initialized weakly coupled 13C nuclear spins with fidelity of 93.30±3.63 %. Furthermore, by utilizing multiple surrounding 13C nuclear spins, we established the potential to form a high-fidelity multi-spin register, providing a robust foundation for scalable quantum information processing technologies. In solid-state quantum systems, another inevitable intrinsic challenge is electron-phonon coupling. While this coupling can facilitate beneficial effects, such as phonon-assisted absorption, it typically results in broad emissions that impede quantum interference with high visibility and consequently limit scalability. Additionally, the high refractive index of bulk crystals causes low photon extraction efficiency due to total internal reflection. To address these limitations, we investigate the point defects in silicon carbide nanowires as an alternative to bulk crystals. SiC nanowires are inherently low-dimensional nanostructures that naturally undergo mixed polytype stacking during growth. This process results in the formation of stacking faults, which function as two-dimensional planar defects within the crystal lattice. The presence of stacking faults further induces crystal symmetry breaking, lattice distortions, and modifications to phonon modes, leading to alternation in electron-phonon interactions. Consequently, the interplay between these planar defects and existing point defects in silicon carbide establishes a novel platform with transformative effects, most notably the electron-phonon decoupling. This decoupling enables silicon carbide nanowires to exhibit a strong zero-phonon transition. To quantify this effect, we calculated the Debye-Waller factor, which represents the partition of zero-phonon line emission to total emissions. The Debye-Waller factor for the silicon carbide nanowire was calculated to be 50.5 % at 4K and 18.2% at room temperature, a remarkable observation such as strong zero-phonon transitions at room temperature have not yet been reported in studies of bulk SiC. Furthermore, the defect complexes system demonstrated a record-high brightness of 360 kcounts/s (33 times higher than bulk), a fast recombination time of 0.92 ns (6 times faster than bulk), and high-purity single-photon emission with g^((2) ) (0) ~ 0.03. These advancements in optical performance highlight the potential for achieving quantum interference with high visibility even at room temperature, presenting significant opportunities for scalable quantum applications and further developments in quantum technologies.