Silicon vacancies in silicon carbide towards scalable quantum applications

Abstract

Quantum technologies harness the principles of quantum mechanics to solve complex problems and are expected to realize practical applications that are impossible with classical technologies. In this innovative field, spin defects in solid-state systems offer unique advantages for quantum computing, sensing and communication, due to their long coherence times and compatibility with existing photonic and electronic devices. This thesis focuses on the silicon vacancy (VSi) centers in 4H-SiC, an emerging and promising platform for scalable quantum technologies. 4H-SiC is a wide-bandgap semiconductor, known for its wafer-scale availability and compatibility with complementary metal-oxide semiconductor fabrication techniques, making it an ideal host material for spin defects and enabling scalable quantum technologies. VSi centers in 4H-SiC exhibit long spin coherence times and narrow zero-phonon line (ZPL) emission linewidths at near-infrared wavelengths, making them favorable for low-loss transmission in fiber networks. These excellent spin and optical properties are preserved when VSi centers are integrated into nanostructures. The research presented in this thesis begins with a comprehensive theoretical and experimental investigation of the internal spin-optical dynamics of VSi centers. Notable achievements include establishing the complete electronic fine structure of VSi centers with the involved intersystem-crossing and deshelving mechanisms, and obtaining the previously unknown radiative and non-radiative transition rates with carefully designed spin-dependent measurements. These findings provide crucial parameters, such as spin initialization fidelity, quantum efficiency, and setup collection efficiency, which guide the nanophotonic optimization of VSi centers. Additionally, the thesis demonstrates the viability of VSi centers for scalable quantum applications by showcasing the high indistinguishability of emitted ZPL photons and advanced spectral tuning based on the piezoelectric effect, thereby fully revealing the device compatibility of 4H-SiC. To further enhance the optical properties and collection efficiency of VSi centers, the thesis implements integration techniques such as the fabrication of Fabry-Pérot nanocavities and tapered-fiber waveguide interfaces under cryogenic conditions. A hybrid photonic cavity system is proposed, combining 4H-SiC nanocavities with lithium niobate (LiNb) substrates for realizing individual cavity tuning exploiting the large electro-optic coefficient of LiNb, further pushing the boundaries of scalable on-chip quantum technological implementations.