The microscopic nature of localization in the quantum Hall effect

The quantum Hall effect arises from the interplay between localized and extended states that form when electrons, confined to two dimensions, are subject to a perpendicular magnetic field. The effect involves exact quantization of all the electronic transport properties owing to particle localization. In the conventional theory of the quantum Hall effect, strong-field localization is associated with a single-particle drift motion of electrons along contours of constant disorder potential. Transport experiments that probe the extended states in the transition regions between quantum Hall phases have been used to test both the theory and its implications for quantum Hall phase transitions. Although several experiments on highly disordered samples have affirmed the validity of the single-particle picture, other experiments and some recent theories have found deviations from the predicted universal behaviour. Here we use a scanning single-electron transistor to probe the individual localized states, which we find to be strikingly different from the predictions of single-particle theory. The states are mainly determined by Coulomb interactions, and appear only when quantization of kinetic energy limits the screening ability of electrons. We conclude that the quantum Hall effect has a greater diversity of regimes and phase transitions than predicted by the single-particle framework. Our experiments suggest a unified picture of localization in which the single-particle model is valid only in the limit of strong disorder.     

Imaging of localized electronic states in the quantum Hall regime

The concept of electron localization has long been accepted to be essential to the physics of the quantum Hall effect in a two-dimensional electron gas. The exact quantization of the Hall resistance and the zero of the diagonal resistance over a range of filling factors close to integral are attributed to the localization of electronic states at the Fermi level in the interior of the gas. As the electron density is changed, charging of the individual localized states may occur by single-electron jumps, causing associated oscillations in the local electrostatic potential. Here we search for such a manifestation of localized states in the quantum Hall regime, using a scanning electrometer probe. We observe localized potential signals, at numerous locations, that oscillate with changing electron density. In general, the corresponding spatial patterns are complex, but well-defined objects are often seen which evidently arise from individual localized states. These objects interact, and at times form a lattice-like arrangement.     

Experimental observation of the quantum Hall effect and Berry's phase in graphene

When electrons are confined in two-dimensional materials, quantum-mechanically enhanced transport phenomena such as the quantum Hall effect can be observed. Graphene, consisting of an isolated single atomic layer of graphite, is an ideal realization of such a two-dimensional system. However, its behaviour is expected to differ markedly from the well-studied case of quantum wells in conventional semiconductor interfaces. This difference arises from the unique electronic properties of graphene, which exhibits electron-hole degeneracy and vanishing carrier mass near the point of charge neutrality. Indeed, a distinctive half-integer quantum Hall effect has been predicted theoretically, as has the existence of a non-zero Berry's phase (a geometric quantum phase) of the electron wavefunction--a consequence of the exceptional topology of the graphene band structure. Recent advances in micromechanical extraction and fabrication techniques for graphite structures now permit such exotic two-dimensional electron systems to be probed experimentally. Here we report an experimental investigation of magneto-transport in a high-mobility single layer of graphene. Adjusting the chemical potential with the use of the electric field effect, we observe an unusual half-integer quantum Hall effect for both electron and hole carriers in graphene. The relevance of Berry's phase to these experiments is confirmed by magneto-oscillations. In addition to their purely scientific interest, these unusual quantum transport phenomena may lead to new applications in carbon-based electronic and magneto-electronic devices.     

Slow light between two absorbing resonance in asymmetry double quantum dots

We demonstrate the slow light in double quantum dots (QDs) resonance dispersion material in theory. The slow factor, absorption, and bandwidth are greatly influenced by the energy difference of the two resonance energy. The bandwidth of this system is up to 60 GHz. The 20 ps input signal pulse is delayed by 180 ps (group index of approximately 55) relative to free-space propagation with little broadening in 1 mm dispersion material for the optical communication wavelength. The signal pulse delay can be tuned by the pump pulse.     

Trajectory tracking control of quantum systems

For quantum state trajectory tracking of density matrix in Liouville equation of quantum systems,with the help of concept in quantum system control,one can apply unitary transformation both to controlled system and free-evolutionary target system such as to change the time-variant and non-stationary target system into a stationary state.Therefore,the quantum state trajectory tracking problem becomes a steering one.State steering control law of the system transformed is designed by means of the Lyapunov stability theorem.Finally,numerical simulation experiments are given for a five-level energy quantum system.The comparison analysis of original system’s trajectory tracking with other method illustrates the advantage in control time of the method proposed.     

尺寸和边界形貌对三角锯齿型石墨烯量子点的磁性影响

采用约束路径量子蒙特卡罗计算方法和基于密度泛函理论的第一性原理数值计算方法,研究了不同尺寸的规则三角锯齿型石墨烯量子点（ZZ）和边界重构后的三角锯齿型石墨烯量子点（ZZST）的磁学特性和电子结构特征．2种方法的计算结果均表明：在所有尺寸下的ZZ的基态均处于铁磁态;当边界重构后,除了尺寸很小的外,其他尺寸的基态均处于反铁磁态．通过Drool^3软件包计算费米能级附近的上下自旋电子态密度分布,发现ZZ在费米能级附近出现明显自旋向上和自旋向下的态密度分布的劈裂,边界重构后费米能级附近的自旋极化被弱化,表明不规则的边界对石墨烯量子点的磁性具有抑制作用．     

对边简支梁式夹层板的非线性振动

夹层板壳结构由于其优异的力学特性在工程中被广泛使用,但有关其非线性振动特性的研究还不够完善,其精确解答一般很难得到。本文对具有软夹心和极薄表层夹层矩形板的非线性自由振动方程进行简化,并将振型设成时间和空间函数的分离形式,时间函数取谐函数,空间函数未知。将假定的振型函数带入微分方程,得到对边简支梁式夹层板无量纲化的空间模态控制方程。采用修正迭代法和伽辽金法对其进行求解,得到了梁式夹层板振型的一个解析解,以及梁式夹层板非线性振动的振幅和振频的解析关系式,并进一步分析了夹层板剪切参数对非线性振动特性的影响。     

Coherent quantum state storage and transfer between two phase qubits via a resonant cavity

As with classical information processing, a quantum information processor requires bits (qubits) that can be independently addressed and read out, long-term memory elements to store arbitrary quantum states, and the ability to transfer quantum information through a coherent communication bus accessible to a large number of qubits. Superconducting qubits made with scalable microfabrication techniques are a promising candidate for the realization of a large-scale quantum information processor. Although these systems have successfully passed tests of coherent coupling for up to four qubits, communication of individual quantum states between superconducting qubits via a quantum bus has not yet been realized. Here, we perform an experiment demonstrating the ability to coherently transfer quantum states between two superconducting Josephson phase qubits through a quantum bus. This quantum bus is a resonant cavity formed by an open-ended superconducting transmission line of length 7 mm. After preparing an initial quantum state with the first qubit, this quantum information is transferred and stored as a nonclassical photon state of the resonant cavity, then retrieved later by the second qubit connected to the opposite end of the cavity. Beyond simple state transfer, these results suggest that a high-quality-factor superconducting cavity could also function as a useful short-term memory element. The basic architecture presented here can be expanded, offering the possibility for the coherent interaction of a large number of superconducting qubits.     

论第二次国共合作与"一国两制"的相似性

第二次国共合作与"一国两制"之间有相似性,表现在它们都是我党顺应时代潮流,不断调整自己政策而提出来的;都是在"一个中国"的前提下,在两种不同的社会制度之间进行的;都需要同出卖民族利益的可耻行径做坚决斗争,坚决捍卫民族利益.这种相似性给我们以下启示应坚持合作,应求同存异,应坚持一个中国的原则.     

量子体系中外场对体系熵的影响

讨论并计算了量子情况下磁场中二维电子气体系的熵，发现存在磁场时二维电子气体系的熵有可能大于无磁场时的熵，从而证明了对于在外场下的量子体系Ｂａｉｅｒｌｅｉｎ的观点不成立。     

Observation of a quarter of an electron charge at the nu = 5/2 quantum Hall state

The fractional quantum Hall effect, where plateaus in the Hall resistance at values of h/nue2 coexist with zeros in the longitudinal resistance, results from electron correlations in two dimensions under a strong magnetic field. (Here h is Planck's constant, nu the filling factor and e the electron charge.) Current flows along the sample edges and is carried by charged excitations (quasiparticles) whose charge is a fraction of the electron charge. Although earlier research concentrated on odd denominator fractional values of nu, the observation of the even denominator nu = 5/2 state sparked much interest. This state is conjectured to be characterized by quasiparticles of charge e/4, whose statistics are 'non-abelian'-in other words, interchanging two quasiparticles may modify the state of the system into a different one, rather than just adding a phase as is the case for fermions or bosons. As such, these quasiparticles may be useful for the construction of a topological quantum computer. Here we report data on shot noise generated by partitioning edge currents in the nu = 5/2 state, consistent with the charge of the quasiparticle being e/4, and inconsistent with other possible values, such as e/2 and e. Although this finding does not prove the non-abelian nature of the nu = 5/2 state, it is the first step towards a full understanding of these new fractional charges.     

两类非线性系统的周期解研究

本文研究了两类非线性系统,首先对第一个系统作变换,使其约化为熟知的广义Li啨nard系统.然后,利用构造的一个广义旋转向量场得到另一系统的周期解的存在条件.     

量子点体系中自旋过滤、自旋流产生和自旋注入

自旋电子学是一门新兴的交叉学科,其中心主题就是对固体电子系统中电子的自旋自由度进行有效地操作和控制.量子点体系中的自旋效应近期受到了理论和实验较多的关注.本文着重介绍了自旋轨道耦合效应对量子点体系输运性质的影响,探讨了怎样利用自旋轨道耦合效应来实现对自旋的有效过滤和纯自旋流产生.基于四铁磁端双量子点体系中电子的交换相互作用机制,指出了一种可以显著提高从铁磁金属到半导体量子点自旋注入效率的新方法.     

Multi-step evolution and measurement control of finite-dimensional quantum systems

For finite-dimensional quantum systems,we propose a quantum control scheme based on a multi-step unitary evolution and quantum projective measurements.The objective is to design a control law to steer the system to a target eigenstate of the measurement operator in the least number of steps.Within each control step,unitary evolution and quantum projective measurement are performed in turn until the system reaches the target state.The control process can be modeled as a finite-state Markov chain with an absorbing state.We prove that the controlled system will converge to the target eigenstate with probability one after a finite number of control steps and find a minimal-step-number condition that would steer the system to the target eigenstate in the least number of steps.     

能量转换量子系统的热力学特性及其研究进展

量子物理是理解微观粒子运动规律的现代物理学理论.它与信息、生命以及化学等学科相互联系日益紧密,引发了许多新技术革命,深刻影响着人类的生活,已成为社会经济发展的原动力之一.当前,量子物理和能源科学结合,正在产生新的学科生长点.本文主要概述多种类型能量转换量子系统的发展;结合国内外的研究现状,系统地阐明如何应用热力学理论研究量子体系的能量操控以及微观器件的优化设计;总结量子热力学循环、能量选择量子电子器件、量子点热管理器件等在理论和实验方面研究的代表性成果和进展;展望量子物理与能源科学交叉领域发展的新方向.     

不同系统之间的混沌同步

基于稳定性理论,针对SQCF系统和SCCF系统提出了两种不同系统之间的混沌同步控制策略：一是选取一正定的李亚普诺夫函数,基于李亚普诺夫直接法求出混沌同步的控制量,从而使两个不同的系统达到混沌同步;二是运用激活控制法实现两个不同系统的混沌同步．这两种方法均适用于一般的混沌系统,并用Mathematica软件给出了数值模拟．理论分析及仿真结果都表明该方法可以较快地实现不同系统之间的混沌同步．     

非自治异结构混沌系统同步及其在保密通信中的应用

根据非自治控制思想、异结构广义投影同步和Lyapunov稳定性定理,构造控制器使得2个异结构的混沌系统在短时间内实现同步,证明了混沌同步的稳定性,并将此同步系统应用到掩盖保密通信中。MATLAB数值仿真结果表明,该混沌系统能够实现稳定地同步,且在保密通信中,有用信号均能从接收端有效地恢复出来。     

Optimal quantum measurement of finite-dimensional systems and coherent anti-Stokes Raman spectroscopy

Quantum measurement is a fundamental problem in quantum control theory and experiments.It can obtain unknown information of quantum systems,and can also change state of the systems inevitably.Both the outcome and back action could be used to control quantum systems.This paper presents recent research progress about optimal control of state transformation in finite-dimensional quantum systems by back action of non-selective quantum measurement,and optimal control of signal and background of CARS (coherent anti-Stokes Raman spectroscopy) by phase shaping technique.In measurement sequence control of finite-dimensional quantum systems,the necessary condition for critical points of the underlying state transformation objective is found to be a highly symmetric form as a chain of equalities,and analytical and numerical solutions in several cases are explored.In the CARS control,it is found that the maximal resonant signal and minimal background at a specific frequency can be achieved by shaping the probe pulse only while keeping pump and Stokes pulses in transform limited forms (TLFs).An arctan-type phase function is obtained for the probe pulse to simultaneously enhance the resonant signal and suppress the background.For broadband background elimination,we find that the optimal phase shaping scheme of probe pulse is quasi-time-delay while keeping the pump and Stokes pulses in TLFs.These conclusions could help design control strategies of quantum devices.     

Real-time compensation of phase drift for phase-encoded quantum key distribution systems

Phase drift is an inherent problem in phase-encoded quantum key distribution(QKD) systems.The current active phase trackingand compensation solutions cannot satisfy the requirements of a system with nonlinearity in phase modulation.This paper presents a four-phase scanning method,which is based on the quantitative analysis of the quantum bit error rate(QBER) from phasedrift and the performance requirements of phase compensation.By obtaining the four interference fringes and adjusting the codingmatrix of the system,this method automatically calculates the accurate driving voltages for the phase modulator.The implementation and experimental tests show that the proposed method can compensate phase drift caused by environmental changes and thesystem’s nonlinearity,and is applicable to large-scale QKD networks.