| Budget Amount *help |
¥17,400,000 (Direct Cost: ¥17,400,000)
Fiscal Year 2001: ¥1,900,000 (Direct Cost: ¥1,900,000)
Fiscal Year 2000: ¥15,500,000 (Direct Cost: ¥15,500,000)
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| Research Abstract |
In semiconductor quantum dots, the electrons are confined three-dimensionally, and their energies become completely discrete, just as in the case of atoms. The collision probability of electrons and phonons decreases remarkably in quantum dots (phonon bottleneck). As a result, the dephasing of the polarization induced by coherent light is limited at low temperatures by the radiation process. Thus the dephasing time becomes ultralong in quantum dots, resulting in an extremely narrow homogeneous linewidth (inversely proportional to the dephasing time). We investigated the spectral width of optical transitions, especially of exciton transitions, in GaAs quantum dots. We also measured the exciton dephasing time in the quantum dots.
For measurements of the spectral linewidth in single quantum dot, we used the microPL technique. Through this technique, the luminescence from a single quantum dot can be observed. The spectral width of an individual line is below the resolution (0.2 meV) of the
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measurement system. Thus it can be confirmed that the spectral width of the exciton line in the GaAs quantum dot is very narrow as predicted in the theory.
To study the coherent phenomena directly in the time-domain, we applied the photon echo method to GaAs quantum dots. It can be seen that the dephasing time of the exciton is extremely long (〜1 ns) in the quantum dots. From the dephasing time, the spectral width of the exciton line in a single quantum dot can be estimated as 〜1 peV. When changing the excitation pulse power, the echo intensity is found to oscillate. It can be understood that this oscillation is analogous to the Rabi oscillation, which is observed in resonantly driven two level systems. It is therefore considered that we observed the Rabi oscillation of the exciton in the quantum dot, Because the Rabi oscillation is a fundamental phenomenon to one-qubit rotatation in quantum logic gates, it can be concluded that the quantum dot is promising as the logical circuit material of the quantum computing. Less
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