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Coherent properties of a two-level system based on a quantum-dot photodiode


Present-day information technology is based mainly on incoherent processes in conventional semiconductor devices1. To realize concepts for future quantum information technologies, which are based on coherent phenomena, a new type of ‘hardware’ is required2. 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 fields3,4,5,6,7. 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 π-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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Figure 1: The two-level system in a quantum dot, and the single-quantum-dot photodiode.
Figure 2: Photocurrent spectrum of a single QD in the region of the excitonic ground state energy EX.
Figure 3: Rabi oscillations of the photocurrent at resonance for increasing excitation amplitude Aexc.


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This work was supported by the BMBF and by the Deutsche Forschungsgemeinschaft.

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Correspondence to A. Zrenner.

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Zrenner, A., Beham, E., Stufler, S. et al. Coherent properties of a two-level system based on a quantum-dot photodiode. Nature 418, 612–614 (2002).

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