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

In this Letter, we present a new class of near-infrared photodetectors comprising Au nanorods-ZnO nanowire hybrid systems. Fabricated hybrid FET devices showed a large photoresponse under radiation wavelengths between 650 and 850 nm, accompanied by an "ultrafast" transient with a time scale of 250 ms, more than 1 order of magnitude faster than the ZnO response under radiation above band gap. The generated photocurrent is ascribed to plasmonic-mediated generation of hot electrons at the metal-semiconductor Schottky barrier. In the presented architecture, Au-nanorod-localized surface plasmons were used as active elements for generating and injecting hot electrons into the wide band gap ZnO nanowire, functioning as a passive component for charge collection. A detailed investigation of the hot electron generation and injection processes is discussed to explain the improved and extended performance of the hybrid device. The quantum efficiency measured at 650 nm was calculated to be approximately 3%, more than 30 times larger than values reported for equivalent metal/semiconductor planar photodetectors. The presented work is extremely promising for further development of novel miniaturized, tunable photodetectors and for highly efficient plasmonic energy conversion devices.

Scalability and foundry compatibility (as for example in conventional silicon based integrated computer processors) in developing quantum technologies are exceptional challenges facing current research. Here we introduce a quantum photonic technology potentially enabling large scale fabrication of semiconductor-based, site-controlled, scalable arrays of electrically driven sources of polarization-entangled photons, with the potential to encode quantum information. The design of the sources is based on quantum dots grown in micron-sized pyramidal recesses along the crystallographic direction (111)B theoretically ensuring high symmetry of the quantum dotsthe condition for actual bright entangled photon emission. A selective electric injection scheme in these non-planar structures allows obtaining a high density of light-emitting diodes, with some producing entangled photon pairs also violating Bell's inequality. Compatibility with semiconductor fabrication technology, good reproducibility and control of the position make these devices attractive candidates for integrated photonic circuits for quantum information processing.To develop quantum technologies, the scientific community is looking into several alternative practical routes, varying as much as superconducting qubits, atoms on-chips, photonic integrated circuits and others 1,2,3,4 . All the explored technologies have to solve the scalability and reproducibility problem if they are to deliver successful real-life applications. In the case of the photonic quantum technologies, scalability requires moving from discrete optical elements to integrated photonic circuits and to on-chip solid-state sources, allowing, for example, thousands of units operating in unisona condition which is very hard to fulfil at the moment.Semiconductor quantum dot technology is fundamentally compatible with modern fab/foundry processes, and on-demand identical, single and entangled photons have been all demonstrated by optical pumping 5,6,7,8,9,10,11,12,13,14 . Nevertheless, while the development of electrically pumped (EP) quantum light sources has advanced in general 15 , the development of a particular resource, EP entangled photon sources, has proven more challenging. After the first report in Nature 16 , the community had to wait several years before a similar result could be obtained by other groups 17 . Importantly, the few devices reported to date, utilized epitaxial selfassembled QD structures, i.e. with no control on the source location, nor on the number of sources in a single device (typically hundreds or more, and not just one or, in the best case scenario, a few): a critical aspect for photonic integration scaling.

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.

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