摘要
半导体自组织量子点由于能级结构和单个原子的类似性,有很多和原子相近的光学性质和量子效应。而且由于它的稳定性和与现代半导体器件设备的兼容,使得它的制备和操作变得相对简单。因而半导体自组织量子点成为一个在量子信息中非常有应用前景的固体体系。研究量子点的光学性质,一方面可以探索量子点内部能级结构和载流子弛豫的信息,从而了解如何把量子点构造成为量子计算中的单个量子比特。另一方面,利用量子点发出的单光子或纠缠光子对,可以用来传输单个比特的量子信息。因此本人将半导体自组织量子点的光学性质和其在量子信息中的应用作为博士论文的研究课题。同时还涉及了半导体自组织量子点一些生长知识和固体物理性质。本论文所取得的成果主要有：
1.从实验上发现了量子点系综中的声子-激子强耦合现象,并且给出了进一步的实验验证和理论解释。我们发现在某些特定的温度下,量子点系综的载流子驰豫会和声子相互结合起来,产生一个新的弛豫窗口,从荧光谱上看会产生一个低于原先s壳层发光峰一个声子能量的新发光峰。并且两个峰之间的相对强度会随着温度改变,直到最后,几乎所有的载流子都通过这个新窗口弛豫。我们提出了一种全新测量量子点能级寿命的方法,利用这种方法从能级寿命的角度验证了声子-激子之间的强耦合的存在。
2.我们从量子点的单光子性质出发,研究了其作为单光子源的可行性,并提出了利用量子点单光子源实现量子随机行走算法的实验方案。单光子由于可携带相干的量子信息,并在空间中能基本无消相干的进行传输,成为了传输和构造量子态的重要工具。由于激光激发量子点发射单光子在效率和时间可控性上都非常优秀,我们提出用其来作为量子随机行走中的行走比特载体,实现了可扩展的量子随机行走算法。
3.利用量子点双激子级联发射的路径相干性和精细结构的存在,我们提出了产生多维纠缠光子对的实验方案。这个方案把在其他利用量子点产生纠缠的方案中致命的精细结构劈裂加以应用,除了偏振维度外,让光子在能量维度也纠缠起来,从而实现了在量子信息中性能更强大的多维纠缠态。
4.实验上验证了载流子弛豫过程中俄歇效应的存在,并考量了其对量子点发光谱的影响。我们在实验上观测了不同密度量子点系综发光峰随激发功率的移动过程,证明了在俄歇效应作用下,不同密度的量子点样品会产生不同的发光峰移动趋势。并给出了完美的理论模型解释。
5.研究了量子点系综中的轨道角动量信息,从而为在单量子点实现轨道角动量纠缠提供了必要的实验探索。量子点发出的光子模式并不是我们之前所设想的为单一的高斯光,而是高斯模式和高阶LG模的叠加。
Quantum dots, the so called "artificial atoms", possess many quantum effects and optical properties similar with atoms. Besides, because of the stability and compati-bility with modern semiconductor technologies and devices, it is relatively easy to be manufactured and integrated. Therefore, it is a prospective solid state system in quan-tum information and computation. To study the optical properties of quantum dots, on the one hand we can investigate the intrinsic energy level structure making it as a qubit in quantum computation. On the other hand, the emitted single photon and entangled photon pairs can be tools for quantum communications. So I selected the optical prop-erties as the main topic of this thesis. We also make discussions on some fundamental growth techniques and solid state theory on self-assembled semiconductor quantum dots.
1. We experimentally discovered the exciton-phonon interaction in quantum dots ensemble, and gave further experimental proofs and theoretical explanation. We find under certain temperatures, a exciton in quantum dots ensemble can be coupled to a phonon, thus provide a new relaxation window for the carriers. Form the photo-luminescence picture, a new peak appears separating from the origin s-shell peak with one phonon energy. The relative intensity changes with the temperature. Finally, al-most all the carriers decays through the new window. We put forward a new technique to measure the lifetime of excitons in quantum dots, and the results show that under certain temperatures, the exciton indeed combines with phonon resulting in longer life-time.
2. We discuss the probability of single quantum dots to be single photon source, and propose a scheme to realize quantum random walk algorithm. Single photon can carry coherent state and propagate decoherence-freely in free space, so it can be an important tool for quantum state engineering and for transporting. Single quantum dot excited by a pulse laser can provide single photons effectively on demand, thus can be used as the carrier of walk qubit to realize scalable quantum random walk.
3. We propose a scheme to generate hyper-entangled photon pairs taking use of the cascaded photon emitting from biexciton state. In this scheme, exciton fine-structure splitting, which was previously deemed undesirable in similar schemes, is used here to produce photon pairs entangled in both frequency and polarization degrees of freedom.
4. We study the orbital angular momentum distribution of the photon emitted by quantum dots, to explore the probability of single quantum dot as entanglement source in orbital angular momentum dimension. The photons from quantum dots are not in single Gaussian mode but superposition of Gaussian mode and high order LG modes.
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