Coherent spectroscopy of single semiconductor quantum dots

Semiconductor quantum dots (QDs) are a promising candidate for the realization of qubits for quantum computation [1]. With coherence times of below 1 ns, writing, manipulation and read-out of a qubit requires ultrafast laser pulses interacting coherently with the system [2]. A key experiment in this context is the observation of Rabi oscillations, where the population of a two-level system can be driven coherently back and forth between the ground state and the excited state. The GaAs/AlGaAs material system is favourable for spectroscopy, because the emission energy of the exciton of around 1.7 eV falls into a region where Si-detectors have still a high quantum efficiency. As the GaAs substrate is absorbing at the exciton transition energy, we have to employ a shot-noise limited pump-probe technique operating in reflection geometry. We accomplished to observe Rabi oscillations in one of the fine-structure split ground state excitonic states of our GaAs QDs, monitoring its population by the bleaching it imposes on the second ground state exciton transition (see Fig.1 left). The first period of these population oscillations yields a dipole moment of about 15 D for the s-shell exciton. A second period is still visible, but stretched and shifted to higher pulse areas (see Fig.1). This behavior can be explained by a phenomenological model that takes into account the interaction with hot, delocalized carriers which are excited by the pump pulse in the GaAs substrate.