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Physical Review B

Raman coherence beats from the entangled state involving polarized excitons in single quantum dots

Xiaoqin Li, Yanwen Wu, Duncan G. Steel, D. Gammon, Lu Jeu Sham

Published Nov 19, 2004• 20 citations
Publisher

Abstract

The optically induced and detected entangled state involving an exciton Zeeman doublet with entanglement entropy as high as $\ensuremath{\sim}0.7$ was created using picosecond lasers in single $\mathrm{GaAs}$ quantum dots. The temporal evolution of the nonradiative Raman coherence between two exciton states was directly resolved in quantum beats measured in the homodyne detected differential transmission experiment. The Raman coherence time, $66\ifmmode\pm\else\textpm\fi{}15\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$, was determined from the decay of the envelope of the quantum beats, and was found to be limited by the lifetimes of the exciton transitions, $50\ifmmode\pm\else\textpm\fi{}3\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$.

DOI
10.1103/physrevb.70.195330

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The optically induced and detected entangled state involving an exciton Zeeman doublet with entanglement entropy as high as \ensuremath∼0.7\ensuremath{\sim}0.7\ensuremath∼0.7 was created using picosecond lasers in single GaAs\mathrm{GaAs}GaAs quantum dots. The temporal evolution of the nonradiative Raman coherence between two exciton states was directly resolved in quantum beats measured in the homodyne detected differential transmission experiment. The Raman coherence time, 66\ifmmode±\else\textpm\fi15ps66\ifmmode\pm\else\textpm\fi{}15\phantom{\rule{0.3em}{0ex}}\mathrm{ps}66\ifmmode±\else\textpm\fi15ps, was determined from the decay of the envelope of the quantum beats, and was found to be limited by the lifetimes of the exciton transitions, 50\ifmmode±\else\textpm\fi3ps50\ifmmode\pm\else\textpm\fi{}3\phantom{\rule{0.3em}{0ex}}\mathrm{ps}50\ifmmode±\else\textpm\fi3ps.
The homodyne-detected transient four-wave-mixing response of InAs/GaAs self-assembled quantum dots shows temporal oscillations of the optically induced Raman coherence arising from two entangled polarization eigenstates of the exciton. The phase sensitive nature of the homodyne detection enables us to follow the time evolution of the nonradiative quantum coherence between the polarization states, providing a measurement of the fine-structure splitting in the dots, which is much less than the inhomogeneous broadening, and the corresponding decoherence rate of the entangled state.
Quantum beats involving the light-hole and heavy-hole excitons in GaAs quantum wells are studied using both conventional and novel temporally gated differential detection techniques. The contributions of the exciton-exciton correlations are separated from those of the intervalence-band Raman coherence and the interband (population) coherence by monitoring the signal in the probe direction and by taking advantage of the polarization selection rules for the excitonic transitions. The results are shown to be in agreement with a theoretical analysis based on the dynamics-controlled truncation formalism.
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Quantum beats involving the light hole (lh) and heavy hole (hh) excitons in GaAs are observed using homodyne detection of the transient four-wave mixing response. Unlike the beats of earlier studies, these beats are detected in a geometry for which the effect would be forbidden in the absence of exciton interactions and arise from coherence induced between the lh and hh excitons. The beats demonstrate the existence of the Raman coherence due to interaction, and the decay of the coherence shows the correlation of exciton dephasing rates to be relatively unaffected by the interaction.