The spin of a single electron confined in a semiconductor quantum dot is a possible candidate for a qubit to realize quantum information processing. Here we discuss the initialization of the electron spins in single quantum dots using a semi-magnetic spin aligner layer. We have succeeded in the concurrent initialization of five quantum dots with near-unity fidelity The electron spin is read-out by observing the polarization of the emitted photons. These originate from excitonic recombination of the electron with an unpolarized hole in the dot. We find dots emitting highly polarized photons over the whole inhomogeneously broadened quantum-dot ensemble emission spectrum. However, a pronounced dependency between the statistical average of the polarization degree and the quantum-dot ground-state energy is found. The influence of charged-exciton recombination due to donors and defects is discussed.

We investigate the dynamics of electrons injected into InAs/GaAs quantum dots by initializing and further observing the spin state of the electrons. For this purpose, we use spin polarized light-emitting diodes where the electron spin is set in a semimagnetic ZnMnSe layer. We find that the degree of optical polarization depends strongly on the ground state energy of the quantum dot. A dependence of polarization on dopant concentration in the spin aligner suggests an influence of residual electrons in the quantum dots.

Optical micropillar Bragg cavities of different diameters and coupled by a small bridge have been realized experimentally by means of a focused ion beam system. The resonator modes in these coupled microcavities are either localized in one pillar or delocalized over the whole photonic structure, a fact that could be exploited to control the coupling between two spatially separated quantum dots, i.e. placed in different pillars, via the enhanced electromagnetic field in such a coupled microcavity. A simplified two dimensional simulation has been used to predict the resonant wavelengths and design the optical modes in these coupled Bragg cavities.

We discuss two different types of GaAs-based microcavities: Single and coupled pillar-type resonators with AlAs/GaAs distributed Bragg reflectors (DBRs) were fabricated by means of molecular-beam epitaxy (MBE) and focussed-ion-beam (FIB) milling. The dependence of the observed cavity modes on the pillar diameter and on the coupling bridge were investigated. Additionally, we present an alternative cavity design consisting of a pyramidal GaAs resonator placed on top of an AlAs/GaAs DBR. Single or even coupled pyramids were achieved by combining electron-beam lithography and wet chemical etching. In(Ga)As quantum dots (QDs) served as a broad-band light source at around 950nm.

Electrical spin injection from an n-type ZWnSe spin aligner into III-V p-i-n diode structures with InGaAs quantum dots (QDs) in the active layer is investigated. Analysis of the circular polarization degree (CPD) of the device emission indicates the spin polarization of the injected electrons. Values > 70% are obtained for the electroluminescence (EL) from the wetting layer and QDs with high ground-state energy. Towards the low-energy end of the emission spectrum, the CPD drops strongly. Temperature-dependent measurements suggest, that this is due to spin relaxation taking place at a stage, when the electrons are not yet finally captured in the dots, i.e. in the GaAs spacer or the wetting layer. Furthermore, we demonstrate electrical spin injection into single InGaAs QDs, a prerequisite for future single spin manipulation experiments within the context of quantum information processing.

The influence of the growth conditions during molecular-beam epitaxy on the properties of InAs/GaAs quantum dot structures were systematically investigated by low temperature photoluminescence spectroscopy and transmission electron microscopy. The circular polarization degree (CPD) of the electroluminescence was compared for two quantum-dot spin-injection light-emitting diodes. The CPD depends on the position of the emission energy in the luminescence band. This correlation is similar for both samples despite the strongly different quantum dot morphologies. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.