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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We report level bleaching in the ground state of a single In0.5Ga0.5As quantum dot. This behavior arises from the nonlinear absorption of a single quantum state. The level bleaching is observed in terms of a saturation of the photocurrent with increasing excitation power under the condition of resonant excitation in the quantum dot ground state. Furthermore, the photocurrent saturation is put down to a fundamental rate equation model. The steady-state solutions are in good agreement with the experimentally observed power dependence of the photocurrent.