Present-day information technology is based mainly on incoherent processes in conventional semiconductor devices. To realize concepts for future quantum information technologies, which are based on coherent phenomena, a new type of 'hardware' is required. Semiconductor quantum dots are promising candidates for the basic device units for quantum information processing. One approach is to exploit optical excitations (excitons) in quantum dots. It has already been demonstrated that coherent manipulation between two excitonic energy levels--via so-called Rabi oscillations--can be achieved in single quantum dots by applying electromagnetic fields. Here we make use of this effect by placing an InGaAs quantum dot in a photodiode, which essentially connects it to an electric circuit. We demonstrate that coherent optical excitations in the quantum-dot two-level system can be converted into deterministic photocurrents. For optical excitation with so-called pi-pulses, which completely invert the two-level system, the current is given by I = fe, where f is the repetition frequency of the experiment and e is the elementary charge. We find that this device can function as an optically triggered single-electron turnstile.

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The photoluminescence (PL) of excitons confined in an electric field tunable coupled AlAs/GaAs quantum well has been investigated at D350 mK and magnetic field H<14 T. In the indirect regime when electrons and holes a r e separated both in real-and in k-space, magnetic field was found t o result in (i) a strong change of both the PL intensity and decay time which i s attributed t o the anomalies in the exciton transport and (ii) an appearance of a huge broad band noise in the PL intensity which i s an evidence of exciton condensation.The electron-hole (e-h) interaction in the neutral e-h system has been predicted t o

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We report a conceptionally new approach to achieve electrostatically induced transport and confinement for spatially indirect excitons. Experimentally, exciton transport is demonstrated in an electric-field-tunable GaAs/AlAs coupled quantum well structure, which is configured as a three-terminal device. In spatially resolved photoluminescence experiments, it is shown that indirect excitons experience a drift field, which is given by an electrostatically induced band-gap gradient in the plane of the coupled quantum well structure.

A novel structure containing self-assembled, unstrained GaAs quantum dots is obtained by combining solid-source molecular beam epitaxy and atomic-layer precise in situ etching. Photo-luminescence (PL) spectroscopy reveals light emission with very narrow inhomogeneous broadening and clearly resolved excited states at high excitation intensity. The dot morphology is determined by scanning probe microscopy and, combined with single band and eight-band k.p theory calculations, is used to interpret PL and single-dot spectra with no adjustable structural parameter.

Spectrally resolved photoresistance investigations of charge storage effects in self-assembled InAs quantum dots (QDs) are reported. Resonant optical excitation of the QDs produces a strong increase of the lateral resistance of a spatially separated electron channel (ΔR) which reflects the stored charge density. This photoresponse is persistent for many hours at 145 K and can be controllably reversed electrically. Pronounced oscillations observed in the spectral variation ΔR are shown to reflect the excitation spectrum of the QD ensemble showing resonances that arise from both direct and phonon-assisted absorption processes.

Quantum dots, often referred to as artificial atoms, open the field of quantum resolved spectroscopy to semiconductor physics. The current article is designed to review the field of interband optical spectroscopy on single semiconductor quantum dots.