Phase separation and flux quantization in the doped quantum dimer model on square and triangular lattices

The doped two-dimensional quantum dimer model is investigated by numerical techniques on the square and triangular lattices, with significantly different results. On the square lattice, at small enough doping, there is always a phase separation between an insulating valence-bond solid and a uniform superfluid phase, whereas on the triangular lattice, doping leads directly to a uniform superfluid in a large portion of the resonating-valence-bond (RVB) phase. Under an applied Aharonov-Bohm flux, the superfluid exhibits quantization in terms of half-flux quanta, consistent with Q=2e elementary charge quanta in transport properties.     

Modulation response of dispersion cumulative medium to nonstationary quantum flux

Dynamics of cumulative medium with energy dispersion properties under the influence of quanta flux from nonstationary source is investigated. The intended purpose was to estimate the opportunity for the releasing phase in the spatial-temporal profile of the energy being accumulated by the medium. The examination showed that this opportunity does exist at the interaction of the fluxes characterized by rapid temporal variations of intensity with the weak-dispersion mediums. The latter provides favorable conditions for the modulation regime regarding to the dissipating energy.     

Nanoscale superconducting quantum bits

Various physical systems were proposed for quantum information processing. Among those nanoscale devices appear most promising for integration in electronic circuits and large-scale applications. We discuss Josephson junction circuits in two regimes where they can be used for quantum computing. These systems combine intrinsic coherence of the superconducting state with control possibilities of single-charge circuits. In the regime where the typical charging energy dominates over the Josephson coupling, the low-temperature dynamics is limited to two states differing by a Cooper-pair charge on a superconducting island. In the opposite regime of prevailing Josephson energy, the phase (or flux) degree of freedom can be used to store and process quantum information. Under suitable conditions the system reduces to two states with different flux configurations. Several qubits can be joined together into a register. The quantum state of a qubit register can be manipulated by voltage and magnetic field pulses. The qubits are inevitably coupled to the environment. However, estimates of the phase coherence time show that many elementary quantum logic operations can be performed before the phase coherence is lost. In addition to manipulations, the final state of the qubits has to be read out. This quantum measurement process can be accomplished using a single-electron transistor for charge Josephson qubits, and a d.c.-SQUID for flux qubits. Recent successful experiments with superconducting qubits demonstrate for the first time quantum coherence in macroscopic systems.     

Direkte Beobachtung einzelner magnetischer Flußquanten in supraleitenden Hohlzylindern. I

By use of electron interferometry in connection with the Aharonov-Bohm effect it was possible to observe directly single magnetic flux quanta, trapped in superconducting tubes with an inner diameter ranging from 0.5 μm to 1 μm. The flux coincided with the fluxoid quantumhc/2e within the experimental error of 4% for thick walled cylinders. The flux distribution along the cylinders was found to be not constant in time and space. Flux quanta down to 8 μm length have been observed. The method permits the observation of flux creep of single flux quanta.     

Some electromagnetic consequences of a geometric unified theory of gravitation and electromagnetism

The geometrical unified theory of gravitation and electromagnetism discussed in previous publications requires that electromagnetism be represented by an SU(2) subgroup. Electric matter should correspond to irreducible representations of the structure group, SL(4,), induced from its compact subgroup, rotation SU(2)x electromagnetic SU(2). Results predict the quantization of electric charge, magnetic flux and angular momentum without requiring magnetic monopoles. Unexpectedly, the necessary quanta of charge and flux imply fractional quantization of transverse resistance, under certain conditions (Fractional Quantum Hall Effect).     

Time evolution of initially localized states in two-dimensional quantum dot arrays in a magnetic field

The evolution of non-stationary localized states |Ψ(t=0) is investigated in two-dimensional tight binding systems of N potential wells with and without a homogeneous field perpendicular to the plane. Most results are presented in analytical form, what is almost imperative if the patterns are as complex as for rings in a magnetic field, where the qualitatively different features arise depending on rational or irrational numbers. The systems considered comprise finite linear chains (N=2,3), finite rings (N=3–6), infinite chains, finite rings (N=3–6) in a magnetic field, and rings with leads attached to each ring site. The position of the particle at time t is described by the projection of the wave function P_m(t)=|m|Ψ(t)|² onto the localized basis function at site m. For finite chains and rings with N=3,4,6 the time evolution is periodic, whereas it is non-periodic for N=5 and N greater then 6. Rings in a magnetic field show a rich spectrum of different features depending on N and the number of flux quanta through the ring, including periodic oscillation and rotation of the charge as well as non-periodic charge fluctuations.     

Self-assembled nanocrystalline CdSe thin films

The topic of this contribution is the investigation of quantum states and quantum Hall effect in electron gas subjected to a periodic potential of the lateral lattice. The potential is formed by triangular quantum antidots located on the sites of the square lattice. In such a system the inversion center and the four-fold rotation symmetry are absent. The topological invariants which characterize different magnetic subbands and their Hall conductances are calculated. It is shown that the details of the antidot geometry are crucial for the Hall conductance quantization rule. The critical values of lattice parameters defining the shape of triangular antidots at which the Hall conductance is changed drastically are determined. We demonstrate that the quantum states and Hall conductance quantization law for the triangular antidot lattice differ from the case of the square lattice with cylindrical antidots. As an example, the Hall conductances of magnetic subbands for different antidot geometries are calculated for the case when the number of magnetic flux quanta per unit cell is equal to three.     

Electric charge as a quantum of flux

Ratio of electron charge radius and Compton wavelength of electron is known to be equal to the dimensionless electromagnetic coupling constant e²/? c. It is pointed out that the coupling constant has two alternative interpretations: as a ratio of two angular momenta since Planck constant has the dimension of angular momentum, and two flux quanta e and hc/e. We argue that it has deep physical significance such that the electronic charge becomes flux itself and at a fundamental level fractional spin of quantized vortex. A unified perspective of the three interpretations of the coupling constant is presented invoking the new interpretation of the magnetic moment of the electron comprising three terms. A critical discussion on the magnetism and flux quantum is given and the implication on the spintronics is pointed out.     

Magnetoresistance oscillations of superconducting Al-film cylinders covering InAs nanowires below the quantum critical point

When odd multiples of half flux quanta thread a cylindrical superconducting shell with a diameter d shorter than the zero temperature coherence length ξ(0), superconductivity is predicted to be destroyed. We show here that as d is reduced in comparison to ξ(0) the resistance attains the normal state value, which seems to be temperature independent in the vicinity of half flux quanta. The data are in agreement with recent theoretical results.     

Dark states and Aharonov-Bohm oscillations in multi-quantum-dot systems

We study the formation of dark states and the Aharonov-Bohm effect in symmetrically/asymmetrically coupled three-and four-quantum-dot systems.It is found that without a transverse magnetic field,destructive interference can trap an electron in a dark state.However,the introduction of a transverse magnetic field can disrupt the dark state,giving rise to oscillation in current.For symmetrically structured quantum-dot systems,the oscillation has a period of one flux quanta.But for asymmetrically structured dot systems,the period of oscillation is halved.In addition,the dephasing due to charge noise also blocks the formation of dark states,while it does not change the period of oscillation.     

Direct observation of geometrical phase transitions in mesoscopic superconductors by scanning tunneling microscopy

Using scanning tunneling microscopy, we mapped the distribution of the local density of states in a single crystal superconductor heterostructure with an array of submicron normal metal islands. We observe the coexistence of strongly interacting multiquanta vortex lattice with interstitial Abrikosov vortices. The newly formed composite magnetic flux structure undergoes a series of phase transitions between different topological configuration states. The vortex configuration states are strongly dependent on the number of flux quanta and the nanoscale confinement architecture of the mesoscopic superconductor. Here, we present images of vortex phase transitions due to confinement effects when the number of magnetic flux quanta in the system changes. The vortex dynamics in these systems could serve as a model for behavior of confined many-body systems when the number of particles changes.     

电荷离散化时相干态下介观金属环中电荷与电流以及能量的量子涨落

基于电荷量子化的事实,运用最小平移算符Q∧的性质等,计算对应的相干态下介观金属环中电荷、电流及能量的量子涨落,研究影响量子涨落的因素.结果表明:计及电荷的离散性,在相干态下介观金属环中电荷、能量的量子涨落不为零,分别与电荷量子、相干态参量等因素有关;此外,能量的量子涨落还决定于金属环的电感、外磁通及其时间变化率的大小.     

Nanoscale electrostatic manipulation of magnetic flux quanta in ferroelectric/superconductor BiFeO3/YBa2Cu3O(7-δ) heterostructures

Using heterostructures that combine a large-polarization ferroelectric (BiFeO3) and a high-temperature superconductor (YBa2Cu3O(7-δ)), we demonstrate the modulation of the superconducting condensate at the nanoscale via ferroelectric field effects. Through this mechanism, a nanoscale pattern of normal regions that mimics the ferroelectric domain structure can be created in the superconductor. This yields an energy landscape for magnetic flux quanta and, in turn, couples the local ferroelectric polarization to the local magnetic induction. We show that this form of magnetoelectric coupling, together with the possibility to reversibly design the ferroelectric domain structure, allows the electrostatic manipulation of magnetic flux quanta.     

On Abrikosov Lattice Solutions of the Ginzburg-Landau Equations

We prove existence of Abrikosov vortex lattice solutions of the Ginzburg-Landau equations of superconductivity, with multiple magnetic flux quanta per fundamental cell. We also revisit the existence proof for the Abrikosov vortex lattices, streamlining some arguments and providing some essential details missing in earlier proofs for a single magnetic flux quantum per a fundamental cell.     

Quantum spin and charge pumping through double quantum dots with ferromagnetic leads

The pumping of electrons through double quantum dots (DQDs) attached to ferromagnetic leads have been theoretically investigated by using the nonequilibrium Green?s function method. It is found that an oscillating electric field applied to the quantum dot may give rise to the pumped charge and spin currents. In the case that both leads are ferromagnet, a pure spin current can be generated in the antiparallel magnetization configuration, where no net charge current exists. The possibility of manipulating the pumped spin current is explored by tuning the dot level and the ac field. By making use of various tunings, the magnitude and direction of the pumped spin current can be well controlled. For the case that only one lead is ferromagnetic, both of the charge and spin currents can be pumped and flow in opposite directions on the average. The control of the magnitude and direction of the pumped charge and spin currents is also discussed by means of the magnetic flux threading through the DQD ring.     

Vortex clusters in quantum dots

We study electronic structures of two-dimensional quantum dots in strong magnetic fields using mean-field density-functional theory and exact diagonalization. Our numerically accurate mean-field solutions show a reconstruction of the uniform-density electron droplet when the magnetic field flux quanta enter one by one the dot in stronger fields. These quanta correspond to repelling vortices forming polygonal clusters inside the dot. We find similar structures in the exact treatment of the problem by constructing a conditional operator for the analysis. We discuss important differences and limitations of the methods used.     

Classical vacuum versus quantum vacuum

We discuss the concept of quantum vacuum in comparison with the classical concept of vacuum in the context of light-wave propagation. The role played by quantum fluctuations is examined with respect to the behaviour of photons as the quanta of electromagnetic field and also in relation to the fact that photons can be regarded as non-zero mass particles. In this context, photon mass is defined from Klein–Gordon equation.     

陆地量子移动通信最优纠缠多址中继方案

为了解决EPR纠缠通过大气空间在通信终端之间的分发问题,使量子移动用户之间及时建立纠缠,提出了一种新的陆地量子移动通信网络的量子纠缠多址中继方案和分区服务模型.分析比较了基于量子受控非门和极化分束器的两种纠缠纯化方法.结果表明:即使在量子移动终端之间没有共享EPR纠缠对的情况下,通过纠缠纯化和量子多址中继,仍然可以完成量子态的无线传输,并且其传输时延与所经过的链路距离和基站数目无关|因此,从数据传输时延的观点来看,该方案是最优的.本研究对于构建大规模量子移动通信网具有一定的奠基作用.     

AC susceptibility of thin Pb films in intermediate and mixed state

Thin films of type I superconductors of a thickness comparable or less than a flux penetration length behave like type II superconductors in a mixed state. With decreasing film thickness normal domains carrying a magnetic flux get smaller with smaller number of flux quanta per domain and finally transform into single quantum flux lines, i.e. quantum vortices similar to those found in type II superconductors. We give an evidence of this behavior from the measurements of the nonlinear response of a total magnetic moment to an applied AC magnetic field, directly from the temperature dependence of an AC susceptibility.     

The Quantum Fluctuations of Two Coupled Josephson Junctions with the Discreteness of Electric Charge

The quantization of two Josephson junctions coupled via inductor with the discreteness of electric charges is proposed. The finite-difference Schrodinger equation of the circuit system has been obtained in charge representation, and the Schrodinger equation is turned to be Mathieu equation in flux representation. The wavefunction and energy spectrum can be solved by adopting the canonical transformation and unitary transformation method. The results indicate that the quantum fluctuations of the flux in the ground states of each mesh exist and are interrelated.