非绝热、非周期演化量子相位特征与测量研究

以二能级原子系统为模型, 细致地研究了非绝热演化量子相位特征. 基于量子干涉原理, 提出了一种测量非绝热演化量子相位的新方法. 且阐述了这种测量方法的物理机理, 利用这种方法在原子态布居几率的干涉条纹中可直接获得量子相位的分布信息.     

广义双模光学系统的几何相位

在广义双模系统中利用正规有序法得到了由双模相干态演化而来的的解，并讨论了该解的周期条件，基于得到的解我们求得了系统的周期几何相位(即从相位)和更广义的Pancharatnam相位，该广义相位不局限于绝热、周期和幺正等条件。     

绝热条件下二能级系统相干控制动态分析

在量子力学的缀饰态表象中，运用布洛赫矢量及其球形图，研究了正弦相位调制的整形脉冲作用到二能级系统的跃迁过程，对其随时间演化过程进行形象化动态分析．结果表明，合适的正弦函数位相整形脉冲作用到二能级系统，无论整形脉冲选择何种位相，都可使粒子布居数反转，并使粒子维持在该态上．在绝热条件下，光场与系统作用过程中粒子的绝热状态随态和位相走势的不同而变化．     

哈密顿不依赖时间的系统的AA几何位相

讨论哈密顿不依赖于时间的系统的非绝热几何位相，结果证明：对于这样的系统，除了定态是几何位相为零的循环态之外，循环态条件仅在如下情况满足：①两能级系统或只有两个本征态的几率幅不为０；②系统的激发能是可公度的．如果循环态存在，系统的非绝热几何位相与初态｜ψ（０）＞有关     

量子力学相位问题研究进展

综述量子力学相位问题的背景、产生及进展.最初由Berry提出的适用于线性系统绝热演化的Berry相位受到了局限,在实际应用中,Berry相位被推广到非绝热演化(AA相)、非循环过程(非对角几何相)及非线性系统(一般量子态的几何相位).指出对于非线性系统非对角几何相的研究是个新的研究课题,并对其研究前景进行展望.     

囚禁离子简并混合态的几何相位

讨论了离子阱中简并混合态的几何相位及相干度.计算得出混合态几何相位由演化时间,初态确定以及在离子的作用.另外,我们还计算出在特殊的作用下,几何相位不随离子的初态变化而变化.囚禁离子的高效检测性意味着本文所研究的混合态几何相位能够被验证.     

在旋转磁场中海森伯自旋链的几何相位

用代数动力学方法,研究旋转磁场中海森伯自旋链的几何相位.选用一个有部分各向异性海森伯耦合的3自旋1/2粒子环链系统.发现系统的哈密顿量具有su(2)代数结构.通过选择一个最佳规范变换,运用代数动力学方法得到系统的精确解.计算了系统的非绝热和绝热.几何相位.把部分各向异性海森伯耦合推广到一般情况下的海森伯耦合,发现:在绝热近似下,海森伯耦合强度不影响系统的几何相位.     

能量算符与广义含时谐振子循回演化的位相

本文证明，应用能量算符而不是Ｈａｍｉｌｔｏｎｉａｎ来定义量子态的位相，Ｂｅｒｒｙ位相和动力学位相都是规范不变的。作为例子，我们重新考查了广义含时谐振子绝热循回演化的量子态位相。结果表明，规范不变的Ｂｅｒｒｙ位相为零，而系统演化的几何特性体现在其动力学位相中。     

静态规范变换和Berry相因数

静态规范变换既不是么正变换也不是一种严格的规范变换，互为静态规范变换的两个动力学系统是规范不等价的，但它们绝热循回演化的Ｂｅｒｒｙ位相却存在一个非常简单的关系．     

原子与双光场耦合系统量子信息保真度研究

讨论了在相位阻尼作用下，一个二能级原子与两个不同光场相互作用时系统的量子信息保真度随时间演化的过程，并对一个任意纯态量子比特通过光场与二能级原子耦合系统进行量子传输的保真度进行了研究，分析了系统作为传输信道对信息的支持程度（即系统的传真度）；着重讨论了相位阻尼和失谐对量子传输保真度的影响，并且获得了通过该信道进行传输的最大保真度。     

Designer Dirac fermions and topological phases in molecular graphene

The observation of massless Dirac fermions in monolayer graphene has generated a new area of science and technology seeking to harness charge carriers that behave relativistically within solid-state materials. Both massless and massive Dirac fermions have been studied and proposed in a growing class of Dirac materials that includes bilayer graphene, surface states of topological insulators and iron-based high-temperature superconductors. Because the accessibility of this physics is predicated on the synthesis of new materials, the quest for Dirac quasi-particles has expanded to artificial systems such as lattices comprising ultracold atoms. Here we report the emergence of Dirac fermions in a fully tunable condensed-matter system-molecular graphene-assembled by atomic manipulation of carbon monoxide molecules over a conventional two-dimensional electron system at a copper surface. Using low-temperature scanning tunnelling microscopy and spectroscopy, we embed the symmetries underlying the two-dimensional Dirac equation into electron lattices, and then visualize and shape the resulting ground states. These experiments show the existence within the system of linearly dispersing, massless quasi-particles accompanied by a density of states characteristic of graphene. We then tune the quantum tunnelling between lattice sites locally to adjust the phase accrual of propagating electrons. Spatial texturing of lattice distortions produces atomically sharp p-n and p-n-p junction devices with two-dimensional control of Dirac fermion density and the power to endow Dirac particles with mass. Moreover, we apply scalar and vector potentials locally and globally to engender topologically distinct ground states and, ultimately, embedded gauge fields, wherein Dirac electrons react to 'pseudo' electric and magnetic fields present in their reference frame but absent from the laboratory frame. We demonstrate that Landau levels created by these gauge fields can be taken to the relativistic magnetic quantum limit, which has so far been inaccessible in natural graphene. Molecular graphene provides a versatile means of synthesizing exotic topological electronic phases in condensed matter using tailored nanostructures.     

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.     

光子石墨烯中光传输特性的研究

光子石墨烯与电子石墨烯有相同的微结构,是具有三角晶格的二维光子晶体,其能带结构中存在中心奇异点,称为狄拉克点。这种狄拉克准粒子的存在导致了一种新的传输状态"赝扩散"。利用共面接地波导实现了单方向性窄的波束通过二维光子晶体。利用数值仿真的方法验证了"赝扩散"的传输特性。实验结果表明,对于二维光子晶体有些能带是不能被激发的。     

Bipolar supercurrent in graphene

Graphene--a recently discovered form of graphite only one atomic layer thick--constitutes a new model system in condensed matter physics, because it is the first material in which charge carriers behave as massless chiral relativistic particles. The anomalous quantization of the Hall conductance, which is now understood theoretically, is one of the experimental signatures of the peculiar transport properties of relativistic electrons in graphene. Other unusual phenomena, like the finite conductivity of order 4e(2)/h (where e is the electron charge and h is Planck's constant) at the charge neutrality (or Dirac) point, have come as a surprise and remain to be explained. Here we experimentally study the Josephson effect in mesoscopic junctions consisting of a graphene layer contacted by two closely spaced superconducting electrodes. The charge density in the graphene layer can be controlled by means of a gate electrode. We observe a supercurrent that, depending on the gate voltage, is carried by either electrons in the conduction band or by holes in the valence band. More importantly, we find that not only the normal state conductance of graphene is finite, but also a finite supercurrent can flow at zero charge density. Our observations shed light on the special role of time reversal symmetry in graphene, and demonstrate phase coherent electronic transport at the Dirac point.     

可调谐石墨烯光电子器件

凭借着良好的光电、力学和热学性能,石墨烯是目前凝聚态物理领域一个重要的研究热点.而且由于石墨烯的色散关系在狄拉克点附近呈现线性特性,石墨烯的光电特性可以通过外加电场、磁场和温度来加以调节.因此,石墨烯是研究可调谐器件的良好平台.基于石墨烯的电导率随费米能级有明显改变的特点,在分析石墨烯器件的调制机制的基础上,对石墨烯可调谐器件在太赫兹、中红外和近红外的应用发展进行了综述研究.     

Detection of geometric phases in superconducting nanocircuits

When a quantum-mechanical system undergoes an adiabatic cyclic evolution, it acquires a geometrical phase factor' in addition to the dynamical one; this effect has been demonstrated in a variety of microscopic systems. Advances in nanotechnology should enable the laws of quantum dynamics to be tested at the macroscopic level, by providing controllable artificial two-level systems (for example, in quantum dots and superconducting devices). Here we propose an experimental method to detect geometric phases in a superconducting device. The setup is a Josephson junction nanocircuit consisting of a superconducting electron box. We discuss how interferometry based on geometrical phases may be realized, and show how the effect may be applied to the design of gates for quantum computation.     

六角晶格中Dirac准粒子的动力学模拟与操控

本文基于装载在六角可调光晶格中的超冷原子气体，研究了无质量狄拉克准粒子的动力学行为. 此系统与石墨烯具有类似的能带结构，即在简并点附近满足线性色散关系. 我们分别运用解析求解海森堡运动方程和数值波包动力学的方法，得到了准粒子质心位置. 我们可视化地展示了准粒子在不同宽度能隙处的演化行为. 结果发现准粒子的运动并没有出现相对论Zitterbewegung现象. 此外，我们还进一步研究了有效光速、Dirac旋量对系统动力学演化的影响. 结果表明波包的演化方式取决于初态Dirac旋子的形式，而运动速度只依赖于有效光速. 由于光晶格超冷原子系统具有纯净和高可操控性等优点，我们相信我们理论给出的有趣结果不久即可在超冷原子体系实验中得到验证.     

Electronic spin transport and spin precession in single graphene layers at room temperature

Electronic transport in single or a few layers of graphene is the subject of intense interest at present. The specific band structure of graphene, with its unique valley structure and Dirac neutrality point separating hole states from electron states, has led to the observation of new electronic transport phenomena such as anomalously quantized Hall effects, absence of weak localization and the existence of a minimum conductivity. In addition to dissipative transport, supercurrent transport has also been observed. Graphene might also be a promising material for spintronics and related applications, such as the realization of spin qubits, owing to the low intrinsic spin orbit interaction, as well as the low hyperfine interaction of the electron spins with the carbon nuclei. Here we report the observation of spin transport, as well as Larmor spin precession, over micrometre-scale distances in single graphene layers. The 'non-local' spin valve geometry was used in these experiments, employing four-terminal contact geometries with ferromagnetic cobalt electrodes making contact with the graphene sheet through a thin oxide layer. We observe clear bipolar (changing from positive to negative sign) spin signals that reflect the magnetization direction of all four electrodes, indicating that spin coherence extends underneath all of the contacts. No significant changes in the spin signals occur between 4.2 K, 77 K and room temperature. We extract a spin relaxation length between 1.5 and 2 mum at room temperature, only weakly dependent on charge density. The spin polarization of the ferromagnetic contacts is calculated from the measurements to be around ten per cent.