To make photonic quantum information a reality 1,2 , a number of extraordinary challenges need to be overcome. One of the outstanding challenges is the achievement of large arrays of reproducible "entangled" photon generators, maintaining the compatibility with integration with optical devices and detectors 3,4,5 . Semiconductor quantum dots (QDs) are potentially ideal for this. They allow generating photons on demand 6,7 without relying on probabilistic processes 8,9 . Nevertheless, most QD systems are limited by the intrinsic lack of symmetry, which allows to obtain only a small number (typically 1/100 or worse) of good dots per chip. The recent retraction of Mohan et al. 10 seemed to question the very possibility of matching site-control and high symmetry. Here we show that with a new family of (111) grown pyramidal sitecontrolled InGaAs 1- N  QDs, it is possible to overcome previous difficulties and obtain areas containing as much as 15% of polarization-entangled photon emitters, showing fidelities as high as 0.721±0.043.The idea underlining the principle of entangled photon emission with QDs relies on fundamental quantum physics: particle indistinguishability generates a superposition state when two energetically nearly degenerate quantum levels are populated at the same time. In QDs, entanglement resides in polarization of two photons emitted during the cascaded biexciton-exciton recombination 11 . Here one difficulty arises: when the two excitonic levels are not perfectly degenerate (i.e. there is a fine structure splitting, FSS), the entanglement in the emission persists, but a phase term between the two (linearly) polarized photons proportional to both energy and time is introduced. This results in a relative rotation of the two photon polarizations (not constant in time) making entanglement substantially impossible to be detected in a simple way 12 .All currently reported QD systems allowing entangled photon emission tend to present a large FSS, fundamentally allowing only a few (post-growth selected) QDs on a semiconductor wafer as good sources, while till now no entangled photon emission has been

'A site-controlled quantum dot system offering both high uniformity and spectral purity', Applied Physics Letters, 94(22), pp. 223121.

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Original citationDimastrodonato, V., Pelucchi, E. and Vvedensky, D. D. (2012) 'Selflimiting evolution of seeded quantum wires and dots on patterned substrates', Physical Review Letters, 108(25), 256102 (5pp).

A study of highly symmetric site-controlled Pyramidal In 0.25 Ga 0.75 As 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 biexcitonexciton 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.