Accessing the dark exciton with light

The fundamental optical excitation in semiconductors is an electron-hole pair with antiparallel spins: the 'bright' exciton. Bright excitons in optically active, direct-bandgap semiconductors and their nanostructures have been thoroughly studied. In quantum dots, bright excitons provide an essential interface between light and the spins of interacting confined charge carriers. Recently, complete control of the spin state of single electrons and holes in these nanostructures has been demonstrated, a necessary step towards quantum information processing with these two-level systems. In principle, the bright exciton's spin could also be used directly as a two-level system. However, because of its short radiative lifetime, its usefulness is limited. An electron-hole pair with parallel spins forms a long-lived, optically inactive 'dark exciton', and has received less attention as it is mostly regarded as an inaccessible excitation. In this work we demonstrate that the dark exciton forms a coherent two-level system that can fairly easily be accessed by external light. We demonstrate: optical preparation of its spin state as a coherent superposition of two eigenstates, coherent precession of its spin state at a frequency defined by the energy difference between its eigenstates, and readout of the spin by charge addition and subsequent polarized photon detection.