Coherent and incoherent properties of single quantum dot photodiodes

Quantum mechanical systems, like atoms, molecules, ions, spin-systems and recently also semiconductor quantum dots can be described as two-level systems. Under the influence of strong electromagnetic driving fields and in the absence of decoherence such systems exhibit Rabi oscillations. The Rabi flop of a two-level system by means of an optical π-pulse corresponds to an inversion of the system with respect to its initial state, which is equivalent to a qubit rotation in the context of quantum computing. On the basis of a single semiconductor quantum dot incorporated into a photodiode we have succeeded in preparing a two-level system with electric contacts, a setup which was previously not attained on the basis of other two-level systems, such as atoms. On such a single quantum dot photodiode we perform a photocurrent technique that enables us to monitor the occupation probability of the ground state exciton in a single quantum dot. In a first experiment we show that under the condition of resonant ground state excitation the tunneling-photocurrent saturates for high excitation densities. This photocurrent saturation reflects the incoherent saturation limit of a resonantly driven two-level system (excitonic occupancy =0.5). In a second experiment, we demonstrate the transfer of coherent optical excitations into a deterministic photocurrent. Rabi oscillations are shown to be directly reflected in the photocurrent. For the application of π-pulses we observe a quantitative photocurrent which is given by I=f·e, with the repetition frequency of the experiment f and the elementary charge e.