Great attention is being devoted to epitaxial semiconductor quantum dots (QDs) as polarization-entangled photon sources . In order for the biexciton-exciton radiative cascade to exhibit high fidelity values, a high QD structural symmetry and a proper choice of materials are crucial to tackle the main sources of entanglement degradation, namely the presence of a fine structure energy splitting (FSS) between the two bright exciton states  and fluctuating nuclear magnetic fields  due to the hyperfine interaction.
In this work we present GaAs/AlGaAs QDs grown on a (111)A substrate by a novel approach based on droplet epitaxy, where the fundamental crystallization step is performed at a temperature which is significantly higher than in previous reports [4,5]. Thanks to the specific substrate orientation, we overcome a long-standing limitation hampering standard droplet epitaxy, which must rely upon a high As flux and a low substrate temperature because of the elevated Ga diffusivity on the AlGaAs (100) surface. The increase in growth temperature improves the crystalline quality of the QDs and strongly reduces the impact of interdiffusion, as determined by morphological characterization combined with ensemble optical spectroscopy. We demonstrate as well that the (111)A orientation results in nanostructures with high in-plane symmetry, which is an essential requirement to achieve vanishing FSS. The control over the growth dynamics leads to the fabrication of QDs with different aspect ratios and to the reproducible design of the emission wavelength. We demonstrate also the possibility of operation in the 780 nm range, which allows frequency-matching with Rb-based quantum memories, an important target for the realization of quantum repeaters and long distance qubit teleportation .
Polarization-resolved single dot photoluminescence reveals a remarkable improvement in optical quality, evaluated in terms of neutral exciton linewidth, and a very low average FSS of (4 ± 2) μeV in the spectral region of interest. Time-resolved measurements under resonant excitation unveil a short exciton lifetime of about 230 ps, so that a high fraction of emitters meets the basic requirements for generating an entangled photon pair. Fidelity measurements were successfully performed under two-photon resonant excitation and yielded a value of 0.8, which already reaches the state-of-the-art for the more studied In(Ga)As/GaAs QDs in absence of postselection or external FSS tuning, thus confirming the intriguing potentiality of this materials system.
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