Pairwise entanglement of a three-qubit Heisenberg XY chain in a nonuniform magnetic field with intrinsic decoherence

Taking into account the intrinsic decoherence, the concurrence of the nearest and the next-to-nearest neighbor qubits in a three-qubit Heisenberg XY chain are investigated when a nonuniform magnetic field is included. We show that the effects of the external magnetic field, including the uniform and inhomogeneous magnetic fields, on the time evolution of entanglement between the nearest and the next-to-nearest neighbor qubits rely deeply on the initial states. We can moderate the destructive effect of intrinsic decoherence by controlling the uniform and inhomogeneous magnetic fields, so that a proper value of uniform and inhomogeneous magnetic fields can, to a great extent enhance the stationary entanglement.     

Physical Realization of Quantum C-Not Gate

We investigate a Heisenberg spin cluster with two particles controlled by a time-dependent magnetic field. The system is controlled by tuning the amplitude, frequency, and interaction time of the three-step time-dependent magnetic field. Then we solve the time-dependent Schrodinger equation of the two-particle system, and obtain the time evolution operator. By the three-timestep interaction, the wavefunction evolves from the initial state to the final state, and the total evolution operator can be expressed as a product of the three evolution operators. By adjusting the physical parameters, the key two-qubit logic gate, the C-Not gate, can be realized physically.     

Nonlinear dynamics of filamentation instability and current filament merging in a high density current-driven plasma

The magnetic field generation due to the filamentation instability (FI) of a high density current-driven plasma is studied through a new nonlinear diffusion equation. This equation is obtained on the basis of quantum hydrodynamic model and numerically solved by applying the Crank–Nicolson method. The spatiotemporal evolution of the magnetic field and the electron density distribution exhibits the current filament merging as a nonlinear phase of the FI which is responsible for the strong magnetic fields in the current-driven plasmas. It is found that the general behaviour of the FI is the same as that of the classical case but the instability growth rate, its magnitude, and the saturation time are affected by the quantum effects. It is eventually concluded that the quantum effects can play a stabilizing role in such situation.     

Conductance distribution in the magnetic field

Using a modification of the Shapiro scaling approach, we derive the distribution of conductance in the magnetic field applicable in the vicinity of the Anderson transition. This distribution is described by the same equations as in the absence of a field. Variation of the magnetic field does not lead to any qualitative effects in the conductance distribution and only changes its quantitative characteristics, moving a position of the system in the three-parameter space. In contrast to the original Shapiro approach, the evolution equation for quasi-1D systems is established from the generalised DMPK equation, and not by a simple analogy with one-dimensional systems; as a result, the whole approach became more rigorous and accurate.     

Radiofrequency magnetic field gradient echoes have reduced sensitivity to susceptibility gradients

The amplitudes of gradient-echoes produced using static field gradients are sensitive to diffusion of tissue water during the echo evolution time. Gradient-echoes have been used to produce MR images in which image intensity is proportional to the self-diffusion coefficient of water. However, such measurements are subject to error due to the presence of background magnetic field gradients caused by variations in local magnetic susceptibility. These local gradients add to the applied gradients. The use of radiofrequency (RF) gradients to produce gradient-echoes may avoid this problem. The RF magnetic field is orthogonal to the offset field produced by local magnetic susceptibility gradients. Thus, the effect of the local gradients on RF gradient-echo amplitude is small if the RF field is strong enough to minimize resonance offset effects. The effects of susceptibility gradients can be further reduced by storing magnetization longitudinally during the echo evolution period. A water phantom was used to evaluate the effects of background gradients on the amplitudes of RF gradient-echoes. A surface coil was used to produce an RF gradient of between 1.3 and 1.6 gauss/cm. Gradient-echoes were detected with and without a 0.16 gauss/cm static magnetic field gradient applied along the same direction as the RF gradient. The background static field gradient had no significant effect on the decay of RF gradient-echo amplitude as a function of echo evolution time. In contrast, the effect of the background gradient on echoes produced using a 1.6 gauss/cm static field gradient is calculated to be significant. This analysis suggests that RF gradient-echoes can produce MR images in which signal intensity is a function of the self-diffusion coefficient of water, but is not significantly affected by background gradients.     

Large parity violating effects in atomic dysprosium with nearly degenerate Floquet eigenvalues

In this article we study effects of parity nonconservation in atomic dysprosium, where one has a pair of nearly degenerate levels of opposite parity. We consider the time evolution of this two-level system within oscillatory electric and magnetic fields. These are chosen to have a periodical structure with the same period, such that a Floquet matrix describes the time evolution of the quantum states. We show that, if the states are unstable, the eigenvalues of the Floquet matrix may have contributions proportional to the square root of the parity violating interaction matrix element while they are almost degenerate in their parity even part. This leads to beat frequencies proportional to which are expected to be larger by several orders of magnitude compared to ordinary P-violating contributions which are of order . However, for the simple field configurations we considered, it still seems to be difficult to observe these P-violating beat effects, since the states decay too fast. On the other hand, we found that, within only a few Floquet cycles, very large parity violating asymmetries with respect to experimental setups of opposite chirality may be obtained. The electric and magnetic fields as well as the time intervals necessary for this are in an experimentally accessible range. For statistically significant effects beyond one standard deviation a number of about 10⁷ atoms is required. Our ideas may be applied directly to other 2-level atomic systems and different field configurations. We hope that these ideas will stimulate experimental work in this direction. Received 16 September 1999 and Received in final form 10 March 2000     

能量输运对柱状等离子体开关磁场穿透过程的影响

 对于高密度、导通时间为μs级的柱状等离子体开关,利用磁流体动力学理论(MHD),对其导通阶段的磁场穿透过程进行了模拟,得到了磁场分布随时间的变化;研究了开关导通过程中能量输运导致的温度不均匀分布对磁场穿透过程的影响。模拟结果表明：对于高密度等离子体开关,磁场以远大于磁扩散速率的速度穿透到等离子体中;在磁压对等离子体产生的压缩效应和欧姆加热效应共同作用下,激波区域的等离子体温度显著升高,这进一步加速了磁场穿透;当考虑能量输运方程时,开关导通时间为0.87 μs,比等温模型的结果0.92 μs短,与实验结果0.87 μs相一致。     

Berry phase in open quantum systems: a quantum Langevin equation approach

The evolution of a two level system with a slowly varying Hamiltonian, modeled as a spin 1/2 in a slowly varying magnetic field, and interacting with a quantum environment, modeled as a bath of harmonic oscillators is analyzed using a quantum Langevin approach. This allows to easily obtain the dissipation time and the correction to the Berry phase in the case of an adiabatic cyclic evolution.     

Exchange of helicity in a knotted electromagnetic field

There are solutions of Maxwell equations in vacuum in which the magnetic and the electric lines have a nontrivial topology. This behaviour has physical consequences since it is related to classical expressions indicating aspects of the photon content of the electromagnetic field. In this work we present for the first time an exact solution of Maxwell equations in vacuum, having non trivial topology, in which there is an exchange of helicity between the electric and magnetic part of such field. We calculate the temporal evolution of the magnetic and electric helicities, and explain the exchange of helicity making use of the Chern‐Simon form. We also have found and explained that, as time goes to infinity, both helicities reach the same value and the exchange between the magnetic and electric part of the field stops.     

Evidence of domain wall scattering in thin films of granular CoFe-AgCu

Thin films of the giant magnetoresistive granular CoFe-AgCu system prepared by rf sputtering displayed a great variety of domain-like microstructures with a net out-of-plane component of the magnetization for ferromagnetic volume concentrations above about 0.25. Therefore, magnetic percolation takes place at ferromagnetic concentrations much lower than the physical percolation threshold. The out-of-plane structure of the as-deposited samples in magnetic virgin state consisted of a distribution of both quasi-circular domains and short stripes depending on the ferromagnetic content. Furthermore, these samples present high metastability and a variety of remanent in-plane and out-of-plane microstructures can be achieved as a function of the magnetic history. Besides, the evolution of the magnetic microstructure yields strong training effects on magnetotransport properties, due to the extra contribution of the electron scattering at the domain walls. All in all, the observed behavior is the result of a subtle correlation between perpendicular anisotropy produced by residual stresses, exchange interparticle interactions due to CoFe alloyed in the matrix, and dipolar interactions. Thus, as high structural evolution occurs through annealing, the features of randomly distributed ferromagnetic particles are recovered and, the out-of-plane domain structures and the training effects disappear. Received 30 September 1999     

飞秒激光与等离子体相互作用中自生磁场的热电机制

采用理论分析和数值模拟研究了考虑相对论效应的自生磁场及其产生机制,给出了自生磁场的解析表达式,得到了温度梯度和密度梯度的非共线性所引起的自生磁场空间分布的时间演化关系。数值结果表明,当峰值强度为1019 W/cm2的激光作用于凹形靶前表面时,在等离子体表面领域观察到的自生磁场最大值为51102 T量级,与实验测量结果相符合。     

Landau electron in a rotating environment: A general factorization of time evolution

For the Landau problem with a rotating magnetic field and a confining potential in the (changing) direction of the field, we derive a general factorization of the time evolution operator that includes the adiabatic factorization as a special case. The confining potential is assumed to be of a general form and it can correspond to nonlinear Heisenberg equations of motion. The rotation operator associated with the solid angle Berry phase is used to transform the problem to a rotating reference frame. In the rotating reference frame, we derive a natural factorization of the time evolution operator by recognizing the crucial role played by a gauge transformation. The major complexity of the problem arises from the coupling between motion in the direction of the magnetic field and motion perpendicular to the field. In the factorization, this complexity is consolidated into a single operator which approaches the identity operator when the potential confines the particle sufficiently close to a rotating plane perpendicular to the magnetic field. The structure of this operator is clarified by deriving an expression for its generating Hamiltonian. The adiabatic limit and non-adiabatic effects follow as consequences of the general factorization which are clarified using the magnetic translation concept.     

Effects of the insulated magnetic field and oblique incidence of electrons on the multipactor in MILO

The theoretical analysis and actual performance of the single-surface multipactor discharge model in the presence of a magnetic field are conducted through simulations. The effects of the magnitude of the insulated magnetic field and the oblique incidence of electrons on the multipactor are analysed. The results show that the multipactor susceptibility region shrinks gradually as the magnetic field increases when the electron cyclotron frequency is close to the RF frequency of the electric field. As a result, the evolution of the multipactor discharge will reach saturation earlier and become saturated at a higher level than the case when the magnetic field is absent, but the change of evolution and saturation as the insulated magnetic field increases is not obvious.     

Simultaneous measurement of D and T2 using the distant dipolar field

The presence of long-range dipolar fields in liquids is known to introduce a non-linear term in the Bloch-Torrey equations which is responsible for many interesting effects in nuclear magnetic resonance as well as in magnetic resonance imaging. We show here, for the first time, that the diffusion coefficient D and the spin-spin relaxation time T2 can be obtained simultaneously from the time evolution profile of the long-range dipolar field refocused signal. In a COSY Revamped by Z-asymmetric Echo Detection sequence, the analytical first-order approximation solution of the Bloch-Torrey equations modified to include the effect of the distant dipolar field is used to demonstrate the technique in an experiment using doped water.     

Magnetically Tunable Terahertz Switch and Band-Pass Filter

A novel design of a tunable terahertz switch and band-pass filter using liquid-crystal-filled photonic crystal waveguide is demonstrated. The effects of magnetic field on the photonic bandgaps and transmitting properties of a THz wave are investigated by using the plane wave expansion method and finite difference time domain method. The efficient photonic bandgap tuning is predicted such that the two-dimensional liquid-crystal-filled photonie crystal waveguide can serve as a switch and continuously tunable band-pass filter with controlling of the magnetic field.     

Cosmology with inhomogeneous magnetic fields

We review spacetime dynamics in the presence of large-scale electromagnetic fields and then consider the effects of the magnetic component on perturbations to a spatially homogeneous and isotropic universe. Using covariant techniques, we refine and extend earlier work and provide the magnetohydrodynamic equations that describe inhomogeneous magnetic cosmologies in full general relativity. Specialising this system to perturbed Friedmann–Robertson–Walker models, we examine the effects of the field on the expansion dynamics and on the growth of density inhomogeneities, including non-adiabatic modes. We look at scalar perturbations and obtain analytic solutions for their linear evolution in the radiation, dust and inflationary eras. In the dust case we also calculate the magnetic analogue of the Jeans length. We then consider the evolution of vector perturbations and find that the magnetic presence generally reduces the decay rate of these distortions. Finally, we examine the implications of magnetic fields for the evolution of cosmological gravitational waves.     

Nonlinear propagation of ultra-low-frequency electromagnetic modes in a magnetized dusty plasma

A theoretical investigation has been made of nonlinear propagation of ultra-low-frequency electromagnetic waves in a magnetized two fluid (negatively charged dust and positively charged ion fluids) dusty plasma. These are modified Alfvén waves for small value of and are modified magnetosonic waves for large , where is the angle between the directions of the external magnetic field and the wave propagation. A nonlinear evolution equation for the wave magnetic field, which is known as Korteweg de Vries (K-dV) equation and which admits a stationary solitary wave solution, is derived by the reductive perturbation method. The effects of external magnetic field and dust characteristics on the amplitude and the width of these solitary structures are examined. The implications of these results to some space and astrophysical plasma systems, especially to planetary ring-systems, are briefly mentioned. Received 8 July 1999 and Received in final form 11 October 1999     

Effect of hyperdiffusivity on turbulent dynamos with helicity

In numerical studies of turbulence, hyperviscosity is often used as a tool to extend the inertial subrange and to reduce the dissipative subrange. By analogy, hyperdiffusivity (or hyperresistivity) is sometimes used in magnetohydrodynamics. The underlying assumption is that only the small scales are affected by this manipulation. In the present paper, possible side effects on the evolution of the large-scale magnetic field are investigated. It is found that for turbulent flows with helicity, hyperdiffusivity causes the dynamo-generated magnetic field to saturate at a higher level than normal diffusivity. This result is successfully interpreted in terms of magnetic helicity conservation, which also predicts that full saturation is reached only after a time comparable to the large-scale magnetic (hyper)diffusion time.     

Changes in structure and magnetic hyperfine fields of Finemet alloys, induced by transition elements substitution

Amorphous as well as two-phase nanocrystalline Finemet-type soft magnetic alloys substituted by transition elements (Ni, Co, V) have been studied by means of Mössbauer spectroscopy. The main subject of interest was the distribution of magnetic hyperfine field (MHF) in Ni substituted alloys. The increase of mean MHF was stated as a result of crystallization. It was found that changes of transition metal concentration cause different kind of hyperfine field evolution, attributed both to the alteration in grain structure and other effects related to crystallization processes.     

Study of low concentrated ionic ferrrofluid stability in magnetic field by ultrasound spectroscopy

Ultrasound spectroscopy has been used to investigate the effect of magnetic field on particle (cluster) size distribution in ionic ferrofluid. The method applied is non-destructive, fast and sensitive to structural changes of a suspension as it is based on measurements of ultrasonic attenuation. Changes in the ultrasound attenuation induced by an external magnetic field have been measured for different frequencies of the acoustic wave. According to the Harker-Temple theory, the aggregation process has been analysed on the basis of the cluster size distribution determined for different magnetic field intensities.