Self-assembled InAs/GaAs single quantum dots with suppressed InGaAs wetting layer states and low excitonic fine structure splitting for quantum memory

Epitaxial semiconductor quantum dots (QDs) have been demonstrated as on-demand entangled photon sources through biexciton-exciton (XX-X) cascaded radiative processes. However, perfect entangled photon emitters at the specific wavelengths of 880 nm or 980 nm, that are important for heralded entanglement distribution by absorptive quantum memories, remain a significant challenge. We successfully extend the QD emission wavelength to 880 nm via capping Stranski-Krastanow grown In(Ga)As/GaAs QDs with an ultra-thin Al x Ga1-xAs layer. After carefully investigating the mechanisms governing the vanishing of wetting-layer (WL) states and the anisotropy of QDs, we optimize the growth conditions and achieve a strong suppression of the WL emission as well as a measured minor fine structure splitting of only similar to(3.2 +/- 0.25) mu eV for the exciton line. We further extend this method to fabricate In(Ga)As QDs emitted at 980 nm via introducing InGaAs capping layer, and demonstrate a two-photon resonant excitation of the biexciton without any additional optical or electrical stabilized source. These QDs with high symmetry and stability represent a highly promising platform for the generation of polarization entanglement and experiments on the interaction of photons from dissimilar sources, such as rare-earth-ion-doped crystals for solid quantum memory.