【作者】 朱飞云；

【导师】 郭光灿； 郭国平；

【作者基本信息】 中国科学技术大学 ， 光学， 2009， 硕士

【摘要】 半导体量子点是最有希望实现量子计算机的固态器件之一,电子自旋又是很有应用潜力的量子比特载体。本文主要研究了量子点材料中,量子点接触(QPC)测量对系统相干性的影响。论文主要内容包括:文章首先介绍了研究的背景和动机、并介绍了半导体量子点以及量子比特的基本知识。随后,我们介绍了GaAs半导体量子点的退相干实验,描述了量子点接触(QPC)测量引起的理想双量子点模型中单电荷态的退相干机制,同时还介绍了量子Zeno效应。论文中,我们具体研究了QPC测量对单量子点自旋态相干性的影响。推导了系统的有效哈密顿量以及主方程、计算了测量过程中电子自旋态的演化行为;随后通过计算分析,我们发现QPC测量引起的退相干时间T2Q PC约为100ns以及高频重复的QPC测量会减缓电子自旋态之间的演化、局域系统在其初态,表现出量子Zeno效应。同时,论文中我们还研究了测量过程中QPC对双量子点系统中两电子自旋态退相干的影响。写出了系统的有效哈密顿量和主方程、计算了实际实验系统中两电子自旋态的时间演化情况,发现QPC导致的退相干时间T2≈1μs,同样我们也在双量子点系统中,发现到量子Zeno效应的现象,并提出了一些延长退相干时间的方法。论文中,我们还研究了在施加微波场的条件下,双量子点系统中两电子态的演化情况,发现双量子点系统可以实现单个量子比特的全部操作,同时微波场可以延长退相干时间。最后,我们还介绍了一些环境引起的量子点中退相干机制的相关知识。更多还原

【Abstract】 Semiconductor quantum dot is one of the most promising candidates for quantum computation, electron spins in quantum dot represent a potential qubit. This dissertation introduces the spin-dephasing induced by quantum point contact (QPC) in quantum dot system. The main content of this dissertation are listed as follows:First of all, we introduce the background and motivation of our research, describe the basic knowledge of quantum dot and qubit.Then, this thesis introduces the experiment about dephasing in GaAs semiconductor quantum dot, reviews the mechanism of charge-dephasing induced by QPC in an ideal double quantum system, and we also introduce the quantum Zeno effect.In this dissertation, we study the evolution behaviors of an election spin on a quantum dot due to coupling to a nearby QPC as a measurement. Chapter 3 is devoted to the derivation of the master equation for the system. We find that the depasing time T2Q PCis about 100 ns when the spin decoherence is only induced by the QPC, and the repeated QPC measurement will slow down the transition rate between spin states which can be interpreted in terms of quantum Zeno effect.Meanwhile, we also study the effect of QPC measurement on two-electron spin state in double quantum dot system. The thesis gives an effective Hamiltonian, derives the master equations of the whole system and calculates the time evolution of spin states. We also find the QPC measurement induced dephasing time T2≈1μs, and provide a simple and transparent description of the enhanced QPC measurement which could trap the system for small t and be interpreted in terms of quantum Zeno effect. During the discussion we propose some methods to extend the dephasing time.Furthermore, we take a microwave field into account and discuss evolution of electrons in this situation. We find that the two electrons in double-dot system could perform all the operation of single qubit, and microwave field would extend the dephasing time.Finally, we introduce the relevant knowledge of environment-induced decoherence in quantum dot.更多还原