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

Picosecond and femtosecond spectroscopy allow the detailed study of carrier dynamics in nanostructured materials. In such experiments, a laser pulse normally excites several nanostructures at once. However, spectroscopic information may also be acquired using pulses from an electron beam in a modern electron microscope, exploiting a phenomenon called cathodoluminescence. This approach offers several advantages. The multimode imaging capabilities of the electron microscope enable the correlation of optical properties (via cathodoluminescence) with surface morphology (secondary electron mode) at the nanometre scale. The broad energy range of the electrons can excite wide-bandgap materials, such as diamond- or gallium-nitride-based structures that are not easily excited by conventional optical means. But perhaps most intriguingly, the small beam can probe a single selected nanostructure. Here we apply an original time-resolved cathodoluminescence set-up to describe carrier dynamics within single gallium-arsenide-based pyramidal nanostructures with a time resolution of 10 picoseconds and a spatial resolution of 50 nanometres. The behaviour of such charge carriers could be useful for evaluating elementary components in quantum computers, optical quantum gates or single photon sources for quantum cryptography.

Alloing, B.; Zinoni, C.; Zwiller, V.; Li, L.; Monat, C.; Gobet, M.; Buchs, G.; Fiore, A.; Pelucchi, E.; Kapon, E.

No abstract

Quantum dots ͑QDs͒ of high symmetry ͑e.g., C 3v ͒ have degenerate bright exciton states, unlike QDs of C 2v symmetry, making them intrinsically suitable for the generation of entangled photon pairs. Deviations from C 3v symmetry are detected in real QDs by polarization-resolved photoluminescence spectroscopy in side-view geometry of InGaAs/AlGaAs dots formed in tetrahedral pyramids. The theoretical analysis reveals both an additional symmetry plane and weak symmetry breaking, as well as the interplay with electron-hole and hole-hole exchange interactions manifested by the excitonic fine structure. DOI: 10.1103/PhysRevB.81.161307 PACS number͑s͒: 78.67.Hc, 71.70.Gm, 73.21.La, 78.55.Ϫm Semiconductor quantum dots ͑QDs͒ exhibit atomiclike energy spectra potentially useful in the area of quantuminformation processing. The indistinguishable radiation paths of the biexciton cascade decay have been proposed as the source of polarization-entangled photons.1 In the conventional QD fabrication process the nucleation of strained InAs QDs occurs spontaneously on the ͑001͒ plane of Zincblende crystals. The symmetry of these QDs is thus limited by the crystal to C 2v .2 The resulting anisotropy of the confined exciton breaks the degeneracy of its bright states, which prohibits entanglement and produces a fine structure splitting ͑FSS͒ characterized by the emission of two linearly polarized photons of unequal energies. Nevertheless, entangled photon pairs from such QDs have been detected by means of careful preselection of particular QDs, 3,4 by spectral postselection, 5 at the price of losing photons, or by the heavy use of external magnetic fields to restore the intermediate level degeneracy. 6 In the quest of more efficient QD sources of entangled photons, it was recently predicted that replacing the conventional GaAs barriers by InP significantly reduces the exciton FSS in such InAs self-assembled QDs. 7 Until now, however, studies of the FSS of neutral and charged exciton complexes have been limited to QDs of C 2v or lower symmetry. [2][3][4][5][6][7][8][9][10][11] In this Rapid Communication, we experimentally and theoretically investigate the FSS in QDs with high symmetry. Zincblende QDs of C 3v symmetry can ideally be achieved by choosing ͓111͔ as the crystallographic direction of crystal growth instead of the conventional ͓001͔ direction. For this growth geometry, including the lack of inversion symmetry in the crystal and the effects of strain and piezoelectric fields, the minimal symmetry is C 3v as long as the QD heterostructure has symmetrical shape. Here we utilize InGaAs/AlGaAs QDs that allow the simultaneous study of the FSS of dominating heavy-hole ͑hh͒ and light-hole ͑lh͒ excitons as well as a hybrid hh-lh trion by side-view polarization-resolved photoluminescence ͑PL͒ spectroscopy. We show how these trion states can probe a small symmetry breaking in otherwise ideal C 3v QDs due to exchange interactions.The polarization properties of the exciton fine structure depend on the symmetries of the initial and fi...