We characterize site-controlled In0.25Ga0.75As quantum dots in (111)B GaAs pyramidal recesses as a platform for spin qubit implementation. By combining scanning confocal cryomicroscopy, magnetophotoluminescence studies, and resonant excitation, we identify and isolate a positively charged exciton with a hole spin in its ground state. Application of a strong 5-T magnetic field parallel to the growth axis induces a fourfold splitting of the energy levels of the positively charged exciton, creating an optically addressable double-Λ (Lambda) system. We combine weak above-band and resonant excitation to demonstrate spin-pumping and high-fidelity spin initialization through all four optical transitions and study the system behavior as a function of the resonant driving strength, showing the existence of a robust spin that can be optically pumped and initialized. These results demonstrate the potential of these quantum dots for precise spin manipulation and their relevance for future quantum hardware.
Barcan et al. (Tue,) studied this question.