The spin of a single electron subject to a static magnetic field provides a natural two-level system that is suitable for use as a quantum bit, the fundamental logical unit in a quantum computer. Semiconductor quantum dots fabricated by strain driven self-assembly are particularly attractive for the realization of spin quantum bits, as they can be controllably positioned, electronically coupled and embedded into active devices. It has been predicted that the atomic-like electronic structure of such quantum dots suppresses coupling of the spin to the solid-state quantum dot environment, thus protecting the 'spin' quantum information against decoherence. Here we demonstrate a single electron spin memory device in which the electron spin can be programmed by frequency selective optical excitation. We use the device to prepare single electron spins in semiconductor quantum dots with a well defined orientation, and directly measure the intrinsic spin flip time and its dependence on magnetic field. A very long spin lifetime is obtained, with a lower limit of about 20 milliseconds at a magnetic field of 4 tesla and at 1 kelvin.

We investigate experimentally and theoretically few-particle effects in the optical spectra of single quantum dots (QDs). Photo-depletion of the QD together with the slow hopping transport of impurity-bound electrons back to the QD are employed to efficiently control the number of electrons present in the QD. By investigating structurally identical QDs, we show that the spectral evolutions observed can be attributed to intrinsic, multi-particle-related effects, as opposed to extrinsic QDimpurity environment-related interactions. From our theoretical calculations we identify the distinct transitions related to excitons and excitons charged with up to five additional electrons, as well as neutral and charged biexcitons. 73.20.Dx, 78.66.Fd, 78.55.Cr Quantum confinement in low-dimensional semiconductors has been shown to profoundly affect Coulomb correlations among charge carriers. In two-dimensional (2D) quantum wells (QWs), enhanced electron-hole correlations yield stable excitons that dominate the optical absorption and emission spectra near the band edge [1]. In 1D quantum wires (QWRs), excitons play an even more important role due to the reduced Sommerfeld factor [2], and their dominance in optical spectra was observed across many interband transitions [3]. In quantum dot (QD) systems, the importance of Coulomb correlations varies considerably as a function of the dot size L due to the difference between the 1/L dependence of the Coulomb potential versus the ∼ 1/L 2 dependence of the confinement energy. Many-particle states can dramatically change the electronic spectra of QDs compared to the simple-minded single particle picture of these fully confined states.Experimentally, the role of Coulomb correlation and many-body effects in quantum nanostructures has been extensively studied using different techniques. Evidence for formation of few-electron states in QDs was provided by capacitance and by far-infrared spectroscopies [4]. Multi-exciton states were observed in the luminescence spectra of QDs formed in QWs and QWRs due to interface disorder [5] and of QDs produced by Stranski-Krastanow island growth [6]. The formation of charged excitons in doped QWs was also reported [7]. In the present Letter, we report the observation of multicharged exciton states in the photoluminescence (PL) spectra of QDs with controlled structure and composition. The binding energies and PL-fine-structure of the multi-particle states incorporating up to six electrons are found to be in good agreement with a theoretical model. Our QDs are fabricated by epitaxial growth on (111)Boriented GaAs substrates patterned with an array of micron-sized tetrahedral recesses [8,9]. Deposition of AlGaAs/GaAs/AlGaAs QW-layers results in the selfformation of a GaAs QD exactly at the sharp tip of each tetrahedral recess. Thus, each QD's position is precisely controlled by the placement of the recess-patterns while its size is controlled by the growth parameters [10]. Based on atomic force microscopy studies [11], we estimate the thickness and di...

We present a method for fabricating quantum dots using seeded self-organized growth of GaAs/AlGaAs heterostructures on substrates patterned with inverted pyramids. This method produces, at the tip of each inverted pyramid, highly uniform quantum dots whose size and position can be accurately controlled. In addition, a system of connected GaAs and AlGaAs two- and one-dimensional nanostructures is identified in the inverted pyramids using cross-sectional atomic force microscopy. A substrate removal technique is used to optimally prepare our samples for optical studies, allowing the increase of the luminescence efficiency of the quantum dots by up to three orders of magnitude. Micro-photoluminescence and cathodo-luminescence spectroscopy are used to study in detail the bandgap structure of the connected nanostructures identified in the pyramids, which constitute a complex, but controlled, barrier environment for the quantum dots.

Arrays of single GaAs/AlGaAs quantum dot (QD) heterostructures grown by organometallic chemical vapor deposition in inverted tetrahedral pyramids on {111}B GaAs substrates are investigated. Cross-sectional atomic force microscopy images evidence a pronounced thickening of the GaAs quantum well layer at the tip of the pyramid, giving rise to a lens-like QD structure. Low-temperature photoluminescence and cathodoluminescence spectra show distinct luminescence from the dots, exhibiting filling of QD states separated by 33 meV at increased carrier densities. Luminescence linewidths of 15 meV and line energy variations of less than 5 meV are obtained across mm2 sample areas.