【摘要】：量子力學(xué)一百年來促進(jìn)了科學(xué)技術(shù)的迅猛發(fā)展,隨著量子計(jì)算和量子信息的理論探索和實(shí)驗(yàn)進(jìn)展,量子系統(tǒng)展現(xiàn)出了巨大的潛力。由于相干性帶來的并行性,其能力遠(yuǎn)超過經(jīng)典計(jì)算機(jī)。但是實(shí)現(xiàn)量子計(jì)算和量子信息的要求是非�？量痰�,固態(tài)體系因?yàn)榫哂锌杉尚院涂烧{(diào)節(jié)性的優(yōu)點(diǎn),成為重要的物理平臺(tái),主要包括超導(dǎo)系統(tǒng)、量子點(diǎn)系統(tǒng)等。同時(shí),控制參數(shù)的噪音和系統(tǒng)與外界環(huán)境的耦合,引起了系統(tǒng)的消相干。用于精確量子調(diào)控的Berry相位中幾何退相位的出現(xiàn),也破壞了幾何相位的可觀測性。而衡量量子系統(tǒng)性能最重要的指標(biāo)便是在相干時(shí)間內(nèi)能夠進(jìn)行多少個(gè)量子邏輯門操作。提高相干時(shí)間的方法有量子糾錯(cuò)碼、消相干自由子空間、動(dòng)力學(xué)去耦合等。本論文針對(duì)這些熱點(diǎn),主要圍繞固態(tài)體系的消相干效應(yīng)和量子調(diào)控展開,主要包括以下幾個(gè)方面內(nèi)容：1.簡單介紹了量子計(jì)算和量子信息的基本單元量子比特和相應(yīng)的邏輯門操作�？偨Y(jié)了常見的量子計(jì)算機(jī)的物理實(shí)現(xiàn)平臺(tái),尤其以固態(tài)量子體系為重點(diǎn),介紹了其量子化方法。對(duì)于開放量子系統(tǒng),分別給出了馬爾科夫主方程和非馬爾科夫主方程的推導(dǎo)。本論文中還要用到幾何相位的概念,這里主要闡述了Berry相位以及對(duì)其的推廣。對(duì)于本文中用到的基礎(chǔ)知識(shí)和相關(guān)背景我們進(jìn)行了一定概括。2.固態(tài)體系電荷量子比特會(huì)在簡并點(diǎn)(甜蜜點(diǎn))處表現(xiàn)出最好的相干性質(zhì),但是最近在雙量子點(diǎn)的實(shí)驗(yàn)中除了觀測到甜蜜點(diǎn)之外,還在大偏置區(qū)實(shí)現(xiàn)了比甜蜜點(diǎn)更長的消相干時(shí)間。為了揭示其中的機(jī)制,我們將體系擴(kuò)展到三能級(jí),考慮其中一個(gè)量子點(diǎn)中的更高能級(jí)。利用自旋玻色模型描述體系與環(huán)境的相互作用,發(fā)現(xiàn)偏置較大時(shí)兩個(gè)低能態(tài)位于同一個(gè)量子點(diǎn)中,能級(jí)弛豫和純退相位都會(huì)消失。但是在甜蜜點(diǎn),噪音算符與能量較低的兩個(gè)態(tài)的子空間存在σx型耦合,引起了額外的弛豫過程,從而使此處的相干性質(zhì)不如大偏置區(qū)。通過數(shù)值模擬相干控制過程,得到的計(jì)算結(jié)果和實(shí)驗(yàn)數(shù)據(jù)非常吻合,驗(yàn)證了我們對(duì)消相干物理機(jī)制分析的有效性。絕熱沖擊模型可以把整個(gè)控制過程簡化為光學(xué)干涉設(shè)備,進(jìn)而討論了脈沖形狀對(duì)振蕩可見度的影響和利用帽子型脈沖提高可見度的方法。3.經(jīng)典波動(dòng)場導(dǎo)致動(dòng)力學(xué)相位的隨機(jī)分布,從而引起量子系統(tǒng)的動(dòng)力學(xué)退相位。對(duì)于實(shí)現(xiàn)Berry相位的控制過程,它也會(huì)引起閉合環(huán)路的擾動(dòng),從而產(chǎn)生幾何退相位。傳統(tǒng)的動(dòng)力學(xué)去耦合序列可以抵消動(dòng)力學(xué)相位并且有效抑制動(dòng)力學(xué)退相位,但是對(duì)幾何退相位并沒有效果。我們?cè)O(shè)計(jì)了Berry相位控制過程中的兩種動(dòng)力學(xué)去耦合序列,可以抑制殘留的幾何退相位,其可行性經(jīng)過了數(shù)值計(jì)算的驗(yàn)證。并且隨著幾何退相位的抑制,量子系統(tǒng)的相干性提高,Berry相位也會(huì)得到恢復(fù)。當(dāng)經(jīng)典波動(dòng)場的關(guān)聯(lián)時(shí)間變短時(shí),抑制效率會(huì)降低。我們還說明了π脈沖的寬度比較窄時(shí),實(shí)際控制不會(huì)破壞設(shè)計(jì)序列的抑制效果。4.利用超導(dǎo)傳輸線振子可以構(gòu)成耦合振子陣列,人造二能級(jí)原子與相鄰的兩個(gè)傳輸線振子耦合,由于干涉可以形成控制單光子輸運(yùn)的量子開關(guān),其開關(guān)狀態(tài)由量子比特處于自旋向上還是自旋向下決定。在耦合陣列中,我們預(yù)先調(diào)節(jié)與二能級(jí)原子耦合的傳輸線振子的特征頻率,單光子輸運(yùn)開關(guān)的保真度可以極大提高。如果二能級(jí)原子處在疊加態(tài),量子開關(guān)的作用類似于分束器,單光子反射波包和透射波包,也會(huì)攜帶著量子比特狀態(tài)信息傳播到遠(yuǎn)處。
[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.
【學(xué)位授予單位】：中國科學(xué)技術(shù)大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類號(hào)】：O413.1

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