An ideal emitter of entangled photon pairs combines the perfect symmetry of an atom with the convenient electrical trigger of light sources based on semiconductor quantum dots. Our source consists of strain-free GaAs dots self-assembled on a triangular symmetric (111)A surface. The emitted photons reveal a fidelity to the Bell state as high as 86(±2)% without postselection. We show a violation of Bell's inequality by more than five times the standard deviation, a prerequisite to test a quantum cryptography channel for eavesdropping. Due to the strict nonlocal nature the source can be used for real quantum processing without any postprocessing. The remaining decoherence channel of the photon source is ascribed to random charge and nuclear spin fluctuations in and near the dot.

A stable wurtzite phase of ZnO is commonly observed. In this letter, we report the growth and characterization of zinc-blende ZnO on GaAs(001) substrates. The ZnO films grown on GaAs(001) substrates using microwave-plasma-assisted metalorganic molecular-beam epitaxy were characterized by reflection high-energy electron diffraction, x-ray diffraction, transmission electron microscope, and atomic force microscope measurements. The use of a ZnS buffer layer was found to lead to the growth of the zinc-blende ZnO films. Although the zinc-blende ZnO films were polycrystalline with columnar structures, they showed bright band-edge luminescence at room temperature.

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The emission cascade of a single quantum dot is a promising source of entangled photons. A prerequisite for this source is the use of a symmetric dot analogous to an atom in a vacuum, but the simultaneous achievement of structural symmetry and emission in a telecom band poses a challenge. Here we report the growth and characterization of highly symmetric InAs/InAlAs quantum dots self-assembled on C3v symmetric InP(111)A. The broad emission spectra cover the O (λ ∼ 1.3 µm), C (λ ∼ 1.55 µmm), and L (λ ∼ 1.6 µm) telecom bands. The distribution of the fine-structure splittings is considerably smaller than those reported in previous works on dots at similar wavelengths. The presence of dots with degenerate exciton lines is further confirmed by the optical orientation technique. Thus, our dot systems are expected to serve as efficient entangled photon emitters for long-distance fiber-based quantum key distribution.Semiconductor quantum dots (QD) are expected to play a central role in quantum information networks. A noteworthy device based on dots is the solid-state single photon source, which ensures absolute security in quantum key distribution (QKD)1 . Since QDs can confine charged carriers in nanometer-sized regions, recombination enables single photons to appear on demand, i.e., synchronously with a master clock shared in networks 2 . QKD over a 50 km commercial fiber has already been demonstrated with QD photon sources, which emitted at a wavelength of 1.5 µm 3 . The transmission distance in that work was limited purely by the absorption loss of silicate fibers. Exceeding this fundamental limit requires the development of quantum link protocols, which exploit the nonlocality inherent in quantum theory. An efficient source of entangled photon pairs is a key element in the realization of such protocols, examples of which include quantum teleportation 4 and entanglement swapping 5 . The generation of entangled photons with semiconductor QDs is directly linked to the singlet configuration of two excitons (X), which form a biexciton (XX). Eventually, two photons associated with the XX-X cascade show polarization correlations independent of the choice of measurement basis, yielding quantum entanglement in the polarization state. However, a common class of QDs exhibits considerable fine-structure splittings (FSS) 6-10 , which exclude entanglement in emitted photons 11 . Numerous attempts have been made to suppress FSS and recover the symmetry of QDs grown on conventional (001) oriented substrates [12][13][14][15][16][17][18][19] . However, from a practical point of view, the reproducible growth of symmetric dots with (at least) near-zero FSS is highly desirable.A noteworthy strategy for achieving such high QD symmetry is the application of C 3v symmetric (111) makes it possible to grow QDs on (111) substrates. Hence, a great reduction in FSS was observed in these QDs 23,24 , which led to the demonstration of entangled photon emission in pyramidal QDs on patterned (111)B substrates 25 , and the filtering-free violation ...

We report the experimental demonstration of single-photon and cascaded photon pair emission in the infrared, originating from a single InAsP quantum dot embedded in a standing InP nanowire. A regular array of nanowires is fabricated by epitaxial growth on an electron-beam patterned substrate. Photoluminescence spectra taken on single quantum dots show narrow emission lines. Superconducting single photon detectors, which have a higher sensitivity than avalanche photodiodes in the infrared, enable us to measure auto and cross correlations. Clear antibunching is observed ͓g ͑2͒ ͑0͒ = 0.12͔ and we show a biexciton-exciton cascade, which can be used to create entangled photon pairs. © 2010 American Institute of Physics. ͓doi:10.1063/1.3506499͔Semiconductor quantum dot ͑QD͒ structures are attractive candidates for solid-state single photon and/or entangled-photon pair generation. 1-3 Nanowire QDs ͑NW-QDs͒ are promising candidates for such sources because of the controllability of doping, shape, and material freedom. 4,5 Fine structure splitting is predicted to be absent, which makes NW-QDs ideal for the creation of entangled photon pairs. 6 Single photon emission from a NW-QD has been shown at wavelengths shorter than 1000 nm. 7 However, a single photon NW-QD emitter at telecommunication wavelengths and a detailed study of its emission lines has not been reported, because until recently a single photon detector ͑SPD͒, with a high enough signal to noise ratio at infrared wavelengths and an adequate timing resolution was lacking. In this letter, we report on the fabrication and characterization of a regular array of InAsP QD embedded in an InP NW, emitting around 1.3 m and characterization of the QD photoluminescence ͑PL͒ using superconducting SPDs ͑SSPDs͒. We demonstrate controlled positioning of the NWs by growing them in a regular array. Control of the position is important for uniform growth, which is necessary for uniform QDs. SSPDs offer single photon detection with low dark counts, excellent timing resolution, and decent efficiency in the infrared, without the need for gating. In addition, SSPDs have very short dead times ͑10 ns͒ and no after pulsing. These characteristics enable us to perform auto and cross correlation experiments.Arrays of InAsP QDs embedded in InP NWs are synthesized by selective area metal organic vapor phase epitaxy ͑SA-MOVPE͒. 8 A metal catalyst is usually used ͑i.e., Au͒ to grow NW structures, however with SA-MOVPE a catalyst is not needed, preventing diffusion of the metal into the NW. A ͑111͒ InP wafer is covered by 30 nm of SiO 2 . By electron beam lithography and wet-etching, 40-60 nm diameter openings are created to form NW nucleation-sites. At a growth rate of 3 nm/s, first a 1 m long segment of InP is grown by adding trimethylindium and tertiarybutylphosphine ͑TBP͒ to the MOVPE reactor at 640°C. Subsequently the temperature is lowered to 580°C and arsine ͑AsH 3 ͒ is added to the reactor ͑V/III ratio 340, partial pressure TBP: AsH 3 3:1͒ to grow 8 to 10 nm InAsP to form the QDs. The ...