Theoretical Research on Quantum Coherence in a Single Quantum Dot and coupled Quantum Dots

近场扫描光学显微镜揭示了半导体纳米结构的许多重要特征。近些年来，NSOM获得了前所未有的高空间分辨率，并报道了单点光谱中一个引人注目的新特征。也就是说，从双激子态到激子态的跃迁对应的发光的空间范围比激子到基态的跃迁的空间范围要小得多。我们用双激子态的质心约束来解释这一现象，即一种死层模型。虽然这个模型给出了定性的解释，但需要一个更定量的理论来估计两种发光图案的空间范围的半高宽之比，并理解该比对量子点大小的依赖关系。我们发展了这样的理论，并与实验取得了很好的一致。近年来，单量子点中的单电子或单施主电子从应用到量子信息处理的兴趣被广泛研究。特别是，电子自旋退相干时间是决定量子信息处理性能优值的重要参数之一。我们发展了一个理论来定量地预测电子自旋退相干时间及其与温度的关系，并阐明了电子-声子相互作用主导的区域和超精细相互作用与核自旋相互作用主导的区域之间发生交叉的温度范围。对从双激子态到基态的级联过程中发射的两个光子进行了光子关联测量，发现两个光子没有纠缠。指出在各向异性形状的量子点中，由于各向异性交换作用引起的激子态分裂是量子点无纠缠的根源。

Near-field scanning optical microscopy revealed many important features of semiconductor nanostructures. The unprecedented high spatial resolution of NSOM was achieved recently and a strikingly new feature in the single-dot spectroscopy was reported. Namely, the spatial extent of the luminescence corresponding to the transition from the biexciton state to the exciton state is remarkably narrower than that of the exciton to the ground state transition. This observation was interpreted in terms of the center-of-mass confinement of the biexciton state, namely a kind of the dead-layer model Although this model gives a qualitative interpretation, a more quantitative theory is necessary to estimate the ratio of the FWHM of the spatial extent of both luminescence patterns and to understand the quantum-dot-size dependence of that ratio. We developed such a theory and obtained a good agreement with experiments.In recent years, a single electron in a single quantum dot or a single donor electron is extensively studied from the interest in the application to the quantum information processing. Especially, the electron spin decoherence time is one of the most important parameters which determine the performance figure of merit of the quantum information processing. We developed a theory to predict quantitatively the electron spin decoherence time and its temperature dependence and to clarify the temperature range in which the cross-over occurs between the regime dominated by the electron-phonon interaction and the regime governed by the hyperfine interaction with nuclear spinsPhoton correlation measurements were done on two photons emitted in the cascade process from a biexciton state to the ground state through an exciton state and the two photons were found not to be entangled. The origin of the absence of entanglement was pointed out to be the exciton state splitting due to the anisotropic exchange interaction in a quantum dot with anisotropic shape.