International Journal of Modern Physics C

A computational system for lattice QCD with overlap Dirac quarks is described. The platform is a home-made Linux PC cluster, built with off-the-shelf components. At present the system constitutes of 64 nodes, with each node consisting of one Pentium 4 processor (1.6/2.0/2.5 GHz), one Gbyte of PC800/1066 RDRAM, one 40/80/120 Gbyte hard disk, and a network card. The computationally intensive parts of our program are written in SSE2 codes. The speed of our system is estimated to be 70 Gflops, and its price/performance ratio is better than $1.0/Mflops for 64-bit (double precision) computations in quenched QCD. We discuss how to optimize its hardware and software for computing propagators of overlap Dirac quarks.

We study the interaction of two entangled two-energy-level atoms interacts with a single-mode and two-photon cavity field. The atom state is detected after exiting the cavity, and it is found that the nonclassical properties of the field states depend strongly on the entanglement degree between the two atoms. Meanwhile it is shown that squeezing of field and photon antibunching can be greatly enhanced via selective atomic measurements.

In this paper we provide a quantum key distribution (QKD) scheme based on the correlations of Einstein–Podolsky–Rosen (EPR) pairs. The scheme uses an auxiliary qubit to interact with the EPR pair and does the Bell state measurement to get the key. It is proved to be secure. All EPR pairs are used in distributing the key except some error-checking bits. So it is efficient. On the other hand there are less classical communications needed in the scheme.

We have simulated the evolution of diploid, sexually reproducing populations using the Penna model of aging. We have noted that diminishing the recombination frequency during the gamete production generates a specific diversity of genomes in the populations. When two populations independently evolving for some time were mixed in one environmental niche of the limited size and crossbreeding between them was allowed, the average lifespan of hybrids was significantly shorter than the lifespan of the individuals of parental lines. Another effect of higher hybrid mortality is the faster elimination of one parental line from the shared environment. The two populations living in one environment co-exist much longer if they are genetically separated — they compete as two species instead of crossbreeding. This effect can be considered as the first step to speciation — any barrier eliminating crossbreeding between these populations, leading to speciation, would favor the populations.

In the present work, genetic algorithm method (GA) is applied to the problem of impurity at the center of a spherical quantum dot for infinite confining potential case. For this purpose, any trial variational wave function is considered for the ground state and energy values are calculated. In applying the GA to the problem under investigation, two different approaches were followed. Furthermore, a standard variational procedure is also performed to determine the energy eigenvalues. The results obtained by all methods are found in satisfactory agreement with each other and also with the exact values in literature. But, it is found that the values obtained by genetic algorithm based upon wavefunction optimization are closer to the exact values than standard variational and also than genetic algorithm based on parameter optimization methods.

Recently, Halliday et al. presented an idea by inserting the "source" terms into the two-dimensional (2D) lattice Boltzmann equation (LBE) so that the emergent dynamics of the lattice fluid can be transformed into the cylindrical polar system. This paper further extends the idea of Halliday et al. to include the effect of azimuthal rotation. The terms related to the azimuthal effect are considered as inertia forces. By using our recently developed Taylor-series-expansion and least-square-based lattice Boltzmann method (TLLBM) for the transformed LBE and a second order explicit finite difference method for the azimuthal moment equation, Taylor–Couette flows between two concentric cylinders with the inner cylinder rotating were simulated. To show the performance of the proposed model, the same problem was also simulated by the three-dimensional (3D) LBM. Numerical results showed that the present axisymmetric model is much more efficient than the 3D model for an axisymmetric flow problem.

Magnetostatic interaction between elements of nanomagnetic arrays leads to flips of magnetic moments in the magnetic posts. Such flips are equivalent to errors of information stored in an array. We prove that antiferromagnetic-like modulation strongly reduces the probability of errors, with relatively small reduction in the density of the stored information. We also show that some magnetic configurations of the array generate errors more likely than the others. Elimination of these configurations leads to a substantial improvement of the storage quality.

The short-time and long-time dynamics of the Bak–Sneppen model of biological evolution are investigated using the damage spreading technique. By defining a proper Hamming distance measure, we are able to make it exhibit an initial power-law growth which, for finite size systems, is followed by a decay towards equilibrium. In this sense, the dynamics of self-organized critical states is shown to be similar to the one observed at the usual critical point of continuous phase transitions and at the onset of chaos of nonlinear low-dimensional dynamical maps. The transient, pre-asymptotic and asymptotic exponential relaxation of the Hamming distance between two initially uncorrelated equilibrium configurations is also shown to be fit within a single mathematical framework.

We employ the Monte Carlo method to study a constrained optimization problem — packing hard spheres with unequal radii (r₂ > r₁) into a 3D bounded region and discuss its connection with the Gamma Knife radiosurgery treatment planning. Selection of the best fit solution is based on the Boltzmann factor, e^-ΔE/T, which allows us to search for the global optimal solution. As an illustration we determined the least number (≤15) of packed spheres that will occupy the largest volume for three different hypothetical tumor sizes (4115, 10 000 and 36 000 voxels). For the bounded regions and the sizes of the packed spheres that we studied here, the optimal volume packing ratio ranges from 41.3 to 48.7%. From our study, using a lower r₂/r₁ ratio is more desirable due to the ≤15 radiation shots constraint. The optimal volume packing ratio can be obtained within a relative short CPU computing time and could provide a good starting point for the radiosurgery treatment planning.

Some modified versions of susceptible-infected-recovered-susceptible (SIRS) model are defined on small-world networks. Latency, incubation and variable susceptibility are separately included. Phase transitions in these models are studied. Then inhomogeneous models are introduced. In some cases, the application of the models to small-world networks is shown to increase the epidemic region.

We present an adaptation of the lattice BGK method for fast convergence of simulations of laminar time-dependent flows. The technique is an extension to the recent accelerated procedures for steady flow computations. Being based on Mach number annealing, the present technique substantially improves the accuracy and computational efficiency of the standard lattice BGK method for such unsteady flows.

Long-term stability studies of nonlinear Hamiltonian systems require symplectic integration algorithms which are both fast and accurate. In this paper, we study a symplectic integration method wherein the symplectic map representing the Hamiltonian system is refactorized using polynomial symplectic maps. This method is analyzed for the three degree of freedom case. Finally, we apply this algorithm to study a large particle storage ring.