【作者】 张辉；

【导师】 郭光灿； 郭国平；

【作者基本信息】 中国科学技术大学 ， 光信息科学与技术， 2008， 博士

【摘要】 本文研究了半导体量子点中的若干重要的量子信息过程。半导体量子点是最有可能实现量子计算机的固态器件之一。它具有容易操作和扩展,同时有较长的相干时间的特点。通过把量子比特编码在量子点中囚禁电子的自旋自由度上,比如单电子的自旋上和下,或者双电子的单重态和三重态等,人们可以研究各种量子信息过程。本文中,我们的工作主要集中在用双量子点结构来实现量子计算,主要内容包括:1.文章首先介绍了半导体量子点的研究背景和研究动机,并具体介绍了半导体量子点的结构以及如何利用量子点中电子自旋进行编码实现量子计算。2.介绍了一种无需两比特相互作用的双量子点结构,可以用来实现基于测量的量子计算。基于这种结构,我们提出了非破坏性量子Bell态测量的方案,并且进一步地可以实现n粒子的GHZ态,从而实现量子计算的方案。3.我们具体讨论了耦合双量子点结构(即双量子点分子),并基于双量子点分子结构提出了两种方案来实现量子计算。一种方案是通过在双量子点分子链构造出cluster态,结合单比特操作和读出,可以实现量子计算:另一种方法是通过用双量子点分子构造普适量子门的方法来实现量子计算。4.为了提高双量子点分子之间的耦合强度,我们提出了用超导传输线腔来耦合多个双量子点分子。并将量子比特置于腔的大失谐区进行操作,这样可以有效地压制腔模损耗引起的退相干机制。5.结合目前的实验条件和理论,我们对基于双量子点结构的量子计算做了具体讨论,并对将来的工作做了实际的展望。本文的主要创新点:1.在无需两比特相互作用的双量子点结构上,提出了非破坏性测量Bell态的方案,使得整个体系支持在非绝热条件下运行。这种非破坏性测量的想法,可以用来制各n粒子GHZ态,从而可以用来做量子计算。2.我们提出了一种双量子点分子链的排布方案:双量子点分子中两个量子点的排布方向垂直于双量子点分子排布方向的结构。这种结构大大简化了近邻量子比特之间的相互作用(为Ising相互作用),可以一步实现cluster态制备以及近邻比特之间的两比特门和Bell态测量。进一步地,这种排布具有很好的扩展性,可以将逻辑比特扩展到二维空间中。3.我们设计的双量子点分子中两个量子点的排布方向垂直于双量子点分子排布方向的结构,经过计算,可以有效地消除掉非近邻量子比特带来的相互作用影响。4.我们提出了用传输线腔(TLR)来耦合多个双量子点分子的模型,大大加强双量子点分子之间的耦合强度(直接的库仑相互作用强度较小)。这是一个多比特模型,通过腔耦合,我们可以选择任意两个双量子点分子进行两比特操作。此外,我们提出了利用传输线腔的大失谐区作为操作区,这样可以大大降低由于腔模损耗引起的退相干。更多还原

【Abstract】 This dissertation conducts several important quantum information processes in semiconductor quantum dots. Semiconductor quantum dots is one of the solid-state devices which are most capable to build quantum computer. It can be accessed and scaled more easily, and has a long coherence time. By encoding qubits on electron spins in quantum dots, for example single electron spin up and down, or two-electron spin singlet state and triplet state, we can research many kinds of quantum information processes. In this dissertation, our work focuses on using quantum double-dot systems to realize quantum computation. The main content and characteristics of this dissertation are listed as follows:1. First, this thesis introduces the background and motivation of the research about semiconductor quantum dots. We concretely introduce the structure of quantum dots, and how to encode qubits on electron spins in quantum dots for quantum computation.2. We introduce a double quantum dot structure without two-qubit interactions, which can be used to realize the measurement-based quantum computation. Based on this structure, we propose to complete the quantum non-demolition Bell-state measurement, and further prepare n-party GHZ state which can be used to realize quantum computation.3. We detailedly discuss the coupled double-dot structure (quantum double dot molecular), and table two proposal for quantum computation based on it. One way is using double quantum dot molecule chain to generate cluster state. Combining cluster state with single qubit operation and readout, we can realize quantum computation. In another way, we use double quantum dot molecules to construct universal gate, and then quantum computation can also be performed.4. In order to increase the interaction strength between two double quantum dot molecules, we propose to use a superconducting transmission line resonator (TLR) to connect many double quantum dot molecules. In our scheme, qubits are biased at the large detuning region, and thus the decoherence mechanism of the cavity loss can be effectively suppressed.5. Combining theory with present experimental conditions, we discuss quantum computation based on double quantum dot structure in detail, and give aview about future work.The main innovation results of this dissertation:1. We give a proposal about non-demolition Bell-state measurement based ondouble quantum dot structure without two-qubit interaction, and the double quantum dot molecular system can be run under the nonadiabatic condition. This idea about non-demolition Bell-state measurement can be used to prepare n-party GHZ state, and then quantum computation can be realized.2. We propose to arrange many double quantum dots as a chain: the two dots inside each molecule are perpendicular to the qubit scaling line. This structure greatly simplify the interaction between two nearest-neighbor qubits (as a Ising model), and can generate the cluster state by one step. This simplified interaction can create two-qubit and realize Bell-state measurement. Further, this arrangement is good for spread and we can expand it into two- dimensional logic qubit space.3. In our scheme of double quantum dot molecular chain, we can effectively eliminate the influence of the interaction between non-nearest-neighbor qubits through calculating.4. We propose to couple double quantum dot molecules with a transmission line resonator (TLR), and thus the coupling strength between two double quantum dot molecules can be much enlarged (the direct coulomb interaction is not large enough). This model is a multi-qubit model, and through TLR we can choose any two double quantum dot molecules to perform two-qubit operation. Futhermore, we propose to put qubit operation in the dispersive coupling regime, and in this way we can decrease the decoherence induced by cavity loss.更多还原