Optical pumping and initialization of a hole spin in site-controlled InGaAs pyramidal quantum dots

We characterize site-controlled In_0.25Ga_0.75As 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-_ (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.