固态体系的消相干效应和量子调控

发布时间：2018-08-05 15:29

【摘要】：量子力学一百年来促进了科学技术的迅猛发展,随着量子计算和量子信息的理论探索和实验进展,量子系统展现出了巨大的潜力。由于相干性带来的并行性,其能力远超过经典计算机。但是实现量子计算和量子信息的要求是非常苛刻的,固态体系因为具有可集成性和可调节性的优点,成为重要的物理平台,主要包括超导系统、量子点系统等。同时,控制参数的噪音和系统与外界环境的耦合,引起了系统的消相干。用于精确量子调控的Berry相位中几何退相位的出现,也破坏了几何相位的可观测性。而衡量量子系统性能最重要的指标便是在相干时间内能够进行多少个量子逻辑门操作。提高相干时间的方法有量子纠错码、消相干自由子空间、动力学去耦合等。本论文针对这些热点,主要围绕固态体系的消相干效应和量子调控展开,主要包括以下几个方面内容：1.简单介绍了量子计算和量子信息的基本单元量子比特和相应的逻辑门操作。总结了常见的量子计算机的物理实现平台,尤其以固态量子体系为重点,介绍了其量子化方法。对于开放量子系统,分别给出了马尔科夫主方程和非马尔科夫主方程的推导。本论文中还要用到几何相位的概念,这里主要阐述了Berry相位以及对其的推广。对于本文中用到的基础知识和相关背景我们进行了一定概括。2.固态体系电荷量子比特会在简并点(甜蜜点)处表现出最好的相干性质,但是最近在双量子点的实验中除了观测到甜蜜点之外,还在大偏置区实现了比甜蜜点更长的消相干时间。为了揭示其中的机制,我们将体系扩展到三能级,考虑其中一个量子点中的更高能级。利用自旋玻色模型描述体系与环境的相互作用,发现偏置较大时两个低能态位于同一个量子点中,能级弛豫和纯退相位都会消失。但是在甜蜜点,噪音算符与能量较低的两个态的子空间存在σx型耦合,引起了额外的弛豫过程,从而使此处的相干性质不如大偏置区。通过数值模拟相干控制过程,得到的计算结果和实验数据非常吻合,验证了我们对消相干物理机制分析的有效性。绝热冲击模型可以把整个控制过程简化为光学干涉设备,进而讨论了脉冲形状对振荡可见度的影响和利用帽子型脉冲提高可见度的方法。3.经典波动场导致动力学相位的随机分布,从而引起量子系统的动力学退相位。对于实现Berry相位的控制过程,它也会引起闭合环路的扰动,从而产生几何退相位。传统的动力学去耦合序列可以抵消动力学相位并且有效抑制动力学退相位,但是对几何退相位并没有效果。我们设计了Berry相位控制过程中的两种动力学去耦合序列,可以抑制残留的几何退相位,其可行性经过了数值计算的验证。并且随着几何退相位的抑制,量子系统的相干性提高,Berry相位也会得到恢复。当经典波动场的关联时间变短时,抑制效率会降低。我们还说明了π脉冲的宽度比较窄时,实际控制不会破坏设计序列的抑制效果。4.利用超导传输线振子可以构成耦合振子阵列,人造二能级原子与相邻的两个传输线振子耦合,由于干涉可以形成控制单光子输运的量子开关,其开关状态由量子比特处于自旋向上还是自旋向下决定。在耦合阵列中,我们预先调节与二能级原子耦合的传输线振子的特征频率,单光子输运开关的保真度可以极大提高。如果二能级原子处在叠加态,量子开关的作用类似于分束器,单光子反射波包和透射波包,也会携带着量子比特状态信息传播到远处。
[Abstract]:Quantum mechanics has promoted the rapid development of science and technology in the past one hundred years. With the theoretical exploration and experimental progress of quantum computing and quantum information, quantum systems have shown great potential. Because of the parallelism brought by coherence, their ability is far beyond the classical computer. However, the requirements of quantum computation and quantum information are very demanding. The solid-state system has become an important physical platform because of its integrated and adjustable advantages. It mainly includes superconducting system, quantum dot system and so on. At the same time, the noise of control parameters and the coupling of the system and the external environment cause the system's decoherence. The appearance of the geometric dephase in the Berry phase for precise quantum control is also destroyed. The most important measure of quantum system performance is how many quantum logic gates can be performed in the coherent time. The methods of improving the coherence time are quantum error correction code, decoherence free subspace, dynamic decoupling and so on. This paper focuses on the elimination of phase in the solid state system. The development of dry and quantum control mainly includes the following aspects: 1. the basic unit qubits and corresponding logic gate operations of quantum computing and quantum information are briefly introduced. The physical realization platform of the common quantum computer is summarized, especially in the solid state quantum system. The quantization method is introduced. The deduction of the Markoff's principal equation and the non Markoff principal equation is given. The concept of the geometric phase is also used in this paper. The Berry phase and the extension of the phase are also discussed in this paper. The basic knowledge and the related background used in this paper are given a certain generalization of the charge quantum ratio of the.2. solid system. It will show the best coherence properties at the degenerate point (sweet point), but recently in the experiment of double quantum dots, in addition to the observation of the sweet point, the decoherence time is longer than the sweet point in the large offset region. In order to reveal the mechanism, we extend the system to three level, considering the higher of one of the quantum dots. The energy level. Using the spin Bose model to describe the interaction between the system and the environment, it is found that two low-energy states are located in the same quantum dot when the bias is large, and the energy level relaxation and the pure retrograde phase will disappear. But in the sweet point, the noise operator and the subspace of the two states with lower energy have the sigma x coupling, resulting in the extra relaxation process. The coherent property is not as good as the large offset region. The results obtained by numerical simulation are in good agreement with the experimental data, which proves the effectiveness of our analysis of the decoherence mechanism. The adiabatic impact model can simplify the whole control process as an optical device, and then discuss the oscillation of the pulse shape to the oscillation. The influence of the visibility and the method of using the hat pulse to improve the visibility, the classical wave field of.3. leads to the random distribution of the dynamic phase, thus causing the kinetic dephase of the quantum system. For the control process of the Berry phase, it will also cause the disturbance of the closed loop, thus producing the geometric dephase. The traditional dynamic decoupling order The column can offset the dynamic phase and effectively suppress the kinetic dephase, but it has no effect on the geometric dephase. We designed two dynamic decoupling sequences in the Berry phase control process, which can suppress the residual geometric dephase. The feasibility is verified by the numerical calculation. And with the geometric dephase suppression, When the coherence time of the quantum system is improved, the Berry phase will be restored. When the correlation time of the classical wave field becomes shorter, the suppression efficiency will be reduced. We also show that when the width of the pion pulse is narrow, the actual control will not destroy the suppression effect of the design sequence..4. can make up the coupling oscillator array by the superconducting transmission line oscillator, and the artificial two energy can be used. The atom is coupled with two adjacent transmission lines. Because interference can form a quantum switch that controls the single photon transport, the switch state is determined by the spin up or spin down of the qubits. In the coupled array, we adjust the characteristic frequency of the transmission line oscillator coupled with the two level atom, and the single photon transport switch. The fidelity can be greatly improved. If the two level atom is in the superposition state, the function of the quantum switch is similar to the beam splitter, and the single photon reflected wave packet and the transmission wave packet will also carry the qubit state information to the distance.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O413.1

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