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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.

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Figure 1: Level diagram and measured PL spectra.
Figure 2: Schematic description of the DE’s state preparation, its precession and readout.
Figure 3: Measured correlation functions and polarization degree.
Figure 4: Measured ‘background-free’ polarization degree of the DE.


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The authors thank T. Rudolph, O. Kenneth and J. E. Avron for fruitful discussions. The support of the US-Israel Binational Science Foundation (BSF), the Israeli Science Foundation (ISF), the Ministry of Science and Technology (MOST) and that of Technion’s RBNI are gratefully acknowledged.

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The experiments were conceived and designed by D.G. and carried out by Y.K., E.P., D.G. and C.T. The data were analysed by E.P., Y.K. and N.H.L. Additional material was contributed by C.T., B.D.G. and P.M.P. The manuscript was written by E.P., N.H.L. and D.G.

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Correspondence to D. Gershoni.

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The authors declare no competing financial interests.

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Poem, E., Kodriano, Y., Tradonsky, C. et al. Accessing the dark exciton with light. Nature Phys 6, 993–997 (2010).

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