Ultrafast Coherent Dynamics in Semiconductor Quantum Dots
In this chapter we will review the studies on ultrafast coherence phenomena of self-assembled semiconductor quantum dots (SAQDs). These studies provide the understanding required for the utilization of quantum dots as the fundamental building block of a solid-state quantum computer. We performed extensive studies on quantum-decoherence processes of excitons trapped in the various excited states of SAQDs. Energy-level structure and dephasing times of excited states were first determined by conducting photoluminescence-excitation spectroscopy and wavepacket interferometry on a large number of individual SAQDs. Major mechanisms responsible for the dephasing of various excited quantum states are determined through the systematic analysis of the correlation between dephasing times and energy-level structure of the QDs. The studies revealed that the dephasing in some of the energetically isolated excited states was strongly suppressed due to the ‘‘phonon bottleneck’’ effect. These states with long dephasing times further provide opportunities to explore other fundamental quantum-coherent phenomena. We observed the direct experimental evidence of Rabi oscillation in these types of excited states. Furthermore, wavepacket-interferometry experiments performed on these states in the strong-excitation regime revealed a new type of quantum-interference phenomenon that emerged from the interplay between quantum interference and the nonlinear effect of Rabi oscillation. This phenomenon can be utilized as a coherent control mechanism where both phase and amplitude of a wavefunction can be manipulated simultaneously.
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