Using coherent nonlinear laser spectroscopy, we show spectral features that indicate contributions from an optically induced nonradiative quantum coherence between the two excitonic states of the Zeeman doublet in a single gallium arsenide quantum dot. The decay rate of this coherence is comparable to the radiative coherence decay rate of the individual excitonic states. The experiments are explained using a theory based on a simple model taking into consideration the exciton-exciton Coulomb interaction. (C) 2001 Elsevier Science Ltd. All rights reserved. References: 35
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