  A study of highly symmetric site-controlled Pyramidal In0.25Ga0.75As quantum
dots (QDs) is presented. It is discussed that polarization-entangled photons
can be also obtained from Pyramidal QDs of different designs from the one
already reported in Juska et al. (Nat. Phot. 7, 527, 2013). Moreover, some of
the limitations for a higher density of entangled photon emitters are
addressed. Among these issues are (1) a remaining small fine-structure
splitting and (2) an effective QD charging under non-resonant excitation
conditions, which strongly reduce the number of useful biexciton-exciton
recombination events. A possible solution of the charging problem is
investigated exploiting a dual-wavelength excitation technique, which allows a
gradual QD charge tuning from strongly negative to positive and, eventually,
efficient detection of entangled photons from QDs, which would be otherwise
ineffective under a single-wavelength (non-resonant) excitation.
