Point defects in SiC: Coupling of Light, Spin, and Matter

The Collaborative Research Centre-Transregio Quantum Cooperativity of Light and Matter (TRR306) aims to characterize, control, and design cooperativity in systems at the quantum level. In TRR306 the Friedrich-Alexander-University Erlangen-Nürnberg, Johannes-Gutenberg- University Mainz, and University of the Saarland along with teams from DESY, Hamburg, TU Kaiserslautern, and Johannes Kepler University Linz have joined forces to accomplish a great goal. Our team at the Johannes Kepler University Linz addresses quantum cooperativity in the coupling of spins located in defects in semiconductors, so called color centers, with light and mechanical modes of the semiconductor. Color centers in semiconductors such as silicon carbide (SiC) enable the implementation of solid- state quantum bits and single photon light sources. Such solid-state quantum bits offer quantum sensing applications for magnetic and electric fields or temperature with high precision and may be used for quantum computing applications. Single photon light sources are relevant for quantum cryptography applications. Exploring coherent coupling among individual color centers in a single device and the achievement of cooperative effects such as spin-spin coupling and superradiance is still challenging. Not only technological complexity has to be controlled, but also competing physical mechanisms have to be unraveled. In our project, we investigate promising color centers in SiC as parts of a novel, versatile and monolithic quantum technology platform. We aim for coherent coupling of two, or eventually more centers via optical and/or mechanical resonators. To reach the final ambitious goal of a monolithic quantum technology platform, the complementary competences of our group and our partners in nanotechnology and SiC physics, as well as photo physics of defects in experiment and theory will be combined. Here, the role of the theory team at the Johannes Kepler University is to investigate atomistic models for promising novel defect centers in collaboration with experiment. Our modeling of the coupling of individual defects to resonators and between two defects in the resonator will be based on a predictive first-principles description of the spin and photo physics of the defects. It will unravel key physical mechanism for achieving the desired coupling and thereby aid the design of the envisaged monolithic quantum technology platform.

The Collaborative Research Centre-Transregio "Quantum Cooperativity of Light and Matter" (TRR306) focused on the characterization, control, and design of cooperativity in systems at the quantum level. Fitting to this general and timely research topic, the project "Point defects in SiC: Coupling of Light, Spin, and Matter" investigated the interaction of color centers embedded in the electronically mature semiconductor silicon carbide with photons and mechanical resonator modes. Prototypical color centers with electron spins are promising candidates for realizing qubits for quantum technologies. Our groups at the Friedrich-Alexander University Erlangen-Nuremberg, Rheinland-Pfälzische Technische Universität Kaiserslautern-Landau, and Johannes Kepler Universität Linz joined our expertise in nano- and SiC technology, as well as experimental and theoretical defect physics to design and realize a monolithic SiC-platform for the coupling of color centers with resonators and to achieve the mutual coupling of two spin-qubits a long-term goal. Here the focus is on the achievements of the theory group at the Johannes Kepler University. The coupling of spin-qubits via resonators as a long-term goal posed several technological and methodological challenges also for the theory group. Optical resonator modes drive the optical cycle of color centers including spin-flipping, non-radiative relaxation or their adversary optical ionisation into optically inactive chargestates, while mechanical modes enable spin-transitions via strain coupling. With the implementation of spin-orbit and spin-spin interactions in our quantum embedding method for color centers (CI-cRPA), that features the description of highly correlated states, a key theoretical challenge was tackled concerning the long-term goal. Based on the implementation of an advanced embedding method regarding the calculation of vibrational lineshapes, the prediction of intersystem crossing rates in the optical cycle of color centers was enabled. For the complex case of the silicon vacancy with a multitude of intermediate states that could not be resolved in recent experimental analysis, our insight confirms the experimental model and provides further understanding enabling future engineering of the optical drive e.g for high spin contrast. At the same time engineering concerns optical ionization and chargestate control. Based on our calculated optical ionization crosssections, we identified the prevalent ionization mechanism of two prototypical qubit centers in SiC. A collaboration with the University Stuttgart (Wrachtrup group) confirmed these findings experimentally for the V2 center (silicon vacancy) in a SiC-Schottky device integrated with optical microstructures for quantum applications. In addition to these direct results the project laid ground for ongoing research towards our long-term goal.