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arXiv:0803.1849 · Physical Review B

Theory of two-photon processes in quantum dots: Coherent evolution and phonon-induced dephasing

Paweł Machnikowski

Published Nov 19, 2008• 30 citations
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Abstract

The paper discusses coherent control of charge and spin states of a biexciton system in a quantum dot via coherent two-photon transitions. Rabi oscillations between the ground state of a quantum dot and the biexciton state, as well as oscillations between the two single-exciton states induced by laser pulses with different circular or linear polarizations are studied. The effect of phonon-induced decoherence on these processes is described. System properties and driving conditions that lead to optimal coherent control are identified. It is shown that proper optimization allows one to control the two-qubit biexciton system via two-photon transitions with a high fidelity.

DOI
10.1103/physrevb.78.195320

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We report on Rabi oscillations of the biexciton in a single ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}∕\mathrm{Ga}\mathrm{As}$ quantum dot, induced via a coherent two-photon process. In contrast to single-exciton Rabi oscillations the measured population oscillation is not purely sinusoidal in excitation pulse area. The experimental results are well reproduced by a theoretical calculation that relies only on the pulse shape and biexciton binding energy. Dephasing times of the biexciton are similar to those of the single exciton under the applied measurement conditions reaching up to 220ps220\phantom{\rule{0.3em}{0ex}}\mathrm{ps}220ps.
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We report on coherent control of the fundamental exciton state in a single quantum dot by means of two phase-locked picosecond laser pulses. By analyzing the experiments performed in the Rabi regime and comparing them to numerical simulations, we show that we can address the exciton coherence and get an insight into the involved dephasing processes. The total decoherence time T2{T}_{2}T2​ (170 ps) is comparable to the effective exciton lifetime T1{T}_{1}T1​ (200 ps), although not reaching the upper theoretical limit of 2T12{T}_{1}2T1​. We conclude that energy relaxation and pure dephasing processes due to virtual scattering with phonons both contribute on an equal footing to the loss of coherence in this kind of monolayer-step-induced GaAs quantum dots.