Mar 2011, Volume 6 Issue 1

Cover illustration

  • The experimental achievement of ultracold atomic Bose-Einstein condensates (BEC) has led to a broad of exciting advances in the past decade. As a spectacular example, the emerging field of quantum superchemistry (QS) or collective non-Arrhenius reaction of matter waves has attracted great interests in recent years, from the simplest atom-dimer conversion, to the assembly of complex molecules and even to the bimolecular reaction A2+B→AB+A (experimentally observed in 2010) [Detail] ...

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    Kuang-ta CHAO
    Wen YANG, Zhen-Yu WANG, Ren-Bao LIU

    In quantum information processing, it is vital to protect the coherence of qubits in noisy environments. Dynamical decoupling (DD), which applies a sequence of flips on qubits and averages the qubit-environment coupling to zero, is a promising strategy compatible with other desired functionalities, such as quantum gates. Here, we review the recent progresses in theories of dynamical decoupling and experimental demonstrations. We give both semiclassical and quantum descriptions of the qubit decoherence due to coupling to noisy environments. Based on the quantum picture, a geometrical interpretation of DD is presented. The periodic Carr-Purcell-Meiboom-Gill DD and the concatenated DD are reviewed, followed by a detailed exploration of the recently developed Uhrig DD, which employs the least number of pulses in an unequally spaced sequence to suppress the qubit-environment coupling to a given order of the evolution time. Some new developments and perspectives are also discussed.

    Hui JING (景辉), Ya-jing JIANG (蒋亚静), Yuan-gang DENG (邓元刚)

    The experimental realization of atomic Bose–Einstein condensation at ultracold temperature has led to rapid advances in creating and manipulating cold molecules, and which has given birth to a new research field of quantum matter-wave superchemistry. Contrary to the classical Arrhenius law, the tunnelingdominated ultracold reactions can be realized through the highly-controlled magneto–optical technique. Novel quantum effects have been identified in these cold reactions, such as the super-selectivity rule in dissociating triatomic molecules, and the quantum size (vessel-shape) effect. In this review, we focus on a variety of new achievements in this fascinating matter-wave wonderland, including the quantum finitenumber effect and double-slit interference in assembling cold molecules, the quantum noise in triggering collective abstraction reaction, and the magnetic phase transition in a laser-catalyzed quantum spin-mixing gas. The practical applications of matter-wave superchemistry are also introduced, such as the optical information storage via quantum photo-association, and the laser-enhanced creation of spinor or even chiral molecules.

    Xiao-fei ZHANG (张晓斐), Xing-hua HU (胡兴华), Deng-shan WANG (王灯山), Xun-xu LIU (刘循序), Wu-ming LIU (刘伍明)

    We review our recent theoretical advances in the dynamics of Bose–Einstein condensates with tunable interactions using Feshbach resonance and external potential. A set of analytic and numerical methods for Gross–Pitaevskii equations are developed to study the nonlinear dynamics of Bose– Einstein condensates. Analytically, we present the integrable conditions for the Gross–Pitaevskii equations with tunable interactions and external potential, and obtain a family of exact analytical solutions for one- and two-component Bose–Einstein condensates in one and two-dimensional cases. Then we apply these models to investigate the dynamics of solitons and collisions between two solitons. Numerically, the stability of the analytic exact solutions are checked and the phenomena, such as the dynamics and modulation of the ring dark soliton and vector-soliton, soliton conversion via Feshbach resonance, quantized soliton and vortex in quasi-two-dimensional are also investigated. Both the exact and numerical solutions show that the dynamics of Bose–Einstein condensates can be effectively controlled by the Feshbach resonance and external potential, which offer a good opportunity for manipulation of atomic matter waves and nonlinear excitations in Bose–Einstein condensates.

    Ya-dong XU(徐亚东), Lei GAO(高雷), Huan-yang CHEN(陈焕阳)

    An illusion counter-part displays the same scattering far-field pattern as that of a real object. In this paper, for the first time, we demonstrate that such an unreal illusion can be cloaked by an external cloak. This phenomenon shall be called “cloak an illusion”. Numerical simulations were performed to demonstrate such a phenomenon.

    Qiang SU (苏强), Bin LIU (刘斌), Ji-ping HUANG (黄吉平)

    Due to the correspondence of the acoustic equations to Maxwell’s equations of one polarization in two dimensions, we exploit theoretically the acoustic counterpart of the recently proposed remote invisibility cloak. The cloak consists of a circular cylindrical core with designed bulk moduli, and an “anti-object” embedded inside a shell with anisotropic mass densities. The material parameters of the cloaking shells are obtained by using the coordinate transformation method. The essence of the new design of cloaks relies on the ability that the cloaked object is no longer deafened by the cloaking shell, which is verified by both the far-field and near-field full-wave finite-element imulations in two dimensions.

    Guan-xia YU(余观夏), Yang-fan LIN(林扬帆), Gui-qing ZHANG(张贵清), Zi YU(喻孜), Li-li YU(于莉莉), Jun SU(苏峻)

    A square-shaped heat flux cloak and a square-shaped heat flux concentrator have been designed theoretically according to the invariance symmetry of steady state thermal conductive equation. The direction of heat flux in these devices can be modulated as desired. Using the method of coordinate transformation, the inhomogeneous and anisotropic thermal conductivity in the transformation region have been acquired. Two-dimensional finite element simulations were performed to confirm the theoretical results.

    Xun-ya JIANG(蒋寻涯), Zheng LIU(刘征), Zi-xian LIANG(梁子贤), Pei-jun YAO(姚培军), Xu-lin LIN(林旭林)

    We investigate the dynamic characteristics of metamaterial systems, such as the temporal coherence gain of the superlens, the causality limitation on the ideal cloaking systems, the relaxation process and essential elements in the dispersive cloaking systems, and the extending of the working frequency range of cloaking systems. The key point of our study is the physical dispersive properties of metamaterials, which are well-known to be intrinsically strongly dispersive. With physical dispersion, new physical pictures can be obtained for the waves propagating inside metamaterial, such as the “group retarded time” for waves inside the superlens and cloak, the causality limitation on real metamaterial systems, and the essential elements for design optimization. Therefore, we believe the dynamic study of metamaterials will be an important direction for further research. All theoretical derivations and conclusions are demonstrated by powerful finite-difference time-domain simulations.

    Fang WU (吴芳), Er-jun KAN (阚二军), Zhen-yu LI (李震宇)

    The magnetic properties of oxide PbMn(SO4)2 consisted of MnO6 octahedra which connected with each other through SO4 tetrahedra, are well studied in experiments. In this paper, we explored its interesting electronic and magnetic properties with first-principle calculations. Our results show that all Mn ions have high spin states, namely, S = 5/2, and the magnetic couplings between NN and NNN are antiferromagnetic, which agree well with the experimental results. Besides, the surprising results of spin exchange interactions between the NN and NNN are excellently explained with extended Hüuckel tight-binding calculations.

    Wolfgang FISCHER

    The diffusive scaling is studied based on pomeron loop equations in the fixed coupling case. At Y?YDS, the gluon number fluctuations become important, the geometric scaling is replaced by the diffusive scaling. In the diffusive scaling regime, the deep inelastic scattering (DIS) total scattering cross-section is a function of single variable ln?[1/(r2Qs2(X))]/DY. We show that the deep inelastic scattering experimental data lie on a single curve, which seems to indicate the existence of the diffusive scaling phenomenology in the deep inelastic scattering.

    Han DING (丁翰), Wen-biao LIU (刘文彪)

    Using the Hamilton–Jacobi method, Hawking radiation from the apparent horizon of a dynamical Vaidya black hole is calculated. The black hole thermodynamics can be built successfully on the apparent horizon. If a relativistic perturbation is given to the apparent horizon, a similar calculation can also lead to a purely thermal spectrum, which corresponds to a modified temperature from the former. The first law of thermodynamics can also be constructed successfully at a new supersurface which has a small deviation from the apparent horizon. When the event horizon is thought as such a deviation from the apparent horizon, the expressions of the characteristic position and temperature are consistent with the previous result that asserts that thermodynamics should be built on the event horizon. It is concluded that the thermodynamics should be constructed on the apparent horizon exactly while the event horizon thermodynamics is just one of the perturbations near the apparent horizon.

    Ning XU(徐宁)

    Amorphous systems undergo the jamming transition when the density increases, temperature drops, or external shear stress decreases, as described by the jamming phase diagram which was proposed to unify different processes such as the glass transition, random close packing, and yielding under shear stress. At zero temperature and shear stress, the jamming transition occurs at a critical density at Point J. In this paper, we review recent studies of the material properties of marginally jammed solids and the glassy dynamics in the vicinity of Point J. As the only singular point in the jamming phase diagram, Point J exhibits special criticality in both mechanical and vibrational quantities. Dynamics approaching the glass transition in the vicinity of Point J show critical scalings, suggesting that the molecular glass transition and the colloidal glass transition are equivalent in the hard sphere limit. All these studies shed light on the long-standing puzzles of the glass transition and unusual properties of amorphous solids.

    Xu-hong LIAO (廖旭红), Yu QIAN (钱郁), Yuan-yuan MI (弭元元), Qin-zhi XIA (夏勤智), Xiao-qing HUANG (黄晓清), Gang HU (胡岗)

    Self-sustained oscillations in complex networks consisting of nonoscillatory nodes have attracted long-standing interest in diverse natural and social systems. We study the self-sustained periodic oscillations in random networks consisting of excitable nodes. We reveal the underlying dynamic structure by applying a dominant phase-advanced driving method. The oscillation sources and wave propagation paths can be illustrated clearly via the dynamic structure revealed. Then we are able to control the oscillations with surprisingly high efficiency based on our understanding.

    Liao-fu LUO (罗辽复)

    Assuming that the main variables in the life processes at the molecular level are the conformation of biological macromolecules and their frontier electrons a formalism of quantum theory on conformation-electron system is proposed. Based on the quantum theory of conformation-electron system, the protein folding is regarded as a quantum transition between torsion states on polypeptide chain, and the folding rate is calculated by nonadiabatic operator method. The rate calculation is generalized to the case of frequency variation in folding. An analytical form of protein folding rate formula is obtained, which can be served as a useful tool for further studying protein folding. The application of the rate theory to explain the protein folding experiments is briefly summarized. It includes the inertial moment dependence of folding rate, the unified description of two-state and multistate protein folding, the relationship of folding and unfolding rates versus denaturant concentration, the distinction between exergonic and endergonic foldings, the ultrafast and the downhill folding viewed from quantum folding theory, and, finally, the temperature dependence of folding rate and the interpretation of its non-Arrhenius behaviors. All these studies support the view that the protein folding is essentially a quantum transition between conformational states.