8,928 research outputs found
Realizing quantum controlled phase-flip gate through quantum dot in silicon slow-light photonic crystal waveguide
We propose a scheme to realize controlled phase gate between two single
photons through a single quantum dot in slow-light silicon photonic crystal
waveguide. Enhanced Purcell factor and beta factor lead to high gate fidelity
over broadband frequencies compared to cavity-assisted system. The excellent
physical integration of this silicon photonic crystal waveguide system provides
tremendous potential for large-scale quantum information processing.Comment: 9 pages, 3 figure
Target-searching on the percolation
We study target-searching processes on a percolation, on which a hunter
tracks a target by smelling odors it emits. The odor intensity is supposed to
be inversely proportional to the distance it propagates. The Monte Carlo
simulation is performed on a 2-dimensional bond-percolation above the
threshold. Having no idea of the location of the target, the hunter determines
its moves only by random attempts in each direction. For lager percolation
connectivity , it reveals a scaling law for the searching time
versus the distance to the position of the target. The scaling exponent is
dependent on the sensitivity of the hunter. For smaller , the scaling law is
broken and the probability of finding out the target significantly reduces. The
hunter seems trapped in the cluster of the percolation and can hardly reach the
goal.Comment: 5 figure
Measuring Significance of Community Structure in Complex Networks
Many complex systems can be represented as networks and separating a network
into communities could simplify the functional analysis considerably. Recently,
many approaches have been proposed for finding communities, but none of them
can evaluate the communities found are significant or trivial definitely. In
this paper, we propose an index to evaluate the significance of communities in
networks. The index is based on comparing the similarity between the original
community structure in network and the community structure of the network after
perturbed, and is defined by integrating all the similarities. Many artificial
networks and real-world networks are tested. The results show that the index is
independent from the size of network and the number of communities. Moreover,
we find the clear communities always exist in social networks, but don't find
significative communities in proteins interaction networks and metabolic
networks.Comment: 6 pages, 4 figures, 1 tabl
Variational ground states of 2D antiferromagnets in the valence bond basis
We study a variational wave function for the ground state of the
two-dimensional S=1/2 Heisenberg antiferromagnet in the valence bond basis. The
expansion coefficients are products of amplitudes h(x,y) for valence bonds
connecting spins separated by (x,y) lattice spacings. In contrast to previous
studies, in which a functional form for h(x,y) was assumed, we here optimize
all the amplitudes for lattices with up to 32*32 spins. We use two different
schemes for optimizing the amplitudes; a Newton/conjugate-gradient method and a
stochastic method which requires only the signs of the first derivatives of the
energy. The latter method performs significantly better. The energy for large
systems deviates by only approx. 0.06% from its exact value (calculated using
unbiased quantum Monte Carlo simulations). The spin correlations are also well
reproduced, falling approx. 2% below the exact ones at long distances. The
amplitudes h(r) for valence bonds of long length r decay as 1/r^3. We also
discuss some results for small frustrated lattices.Comment: v2: 8 pages, 5 figures, significantly expanded, new optimization
method, improved result
Electronic Structure in Gapped Graphene with Coulomb Potential
In this paper, we numerically study the bound electron states induced by long
range Coulomb impurity in gapped graphene and the quasi-bound states in
supercritical region based on the lattice model. We present a detailed
comparison between our numerical simulations and the prediction of the
continuum model which is described by the Dirac equation in (2+1)-dimensional
Quantum Electrodynamics (QED). We also use the Fano's formalism to investigate
the quasi-bound state development and design an accessible experiments to test
the decay of the supercritical vacuum in the gapped graphene.Comment: 5 page, 4 figure
Quantum Chaos of Bogoliubov Waves for a Bose-Einstein Condensate in Stadium Billiards
We investigate the possibility of quantum (or wave) chaos for the Bogoliubov
excitations of a Bose-Einstein condensate in billiards. Because of the mean
field interaction in the condensate, the Bogoliubov excitations are very
different from the single particle excitations in a non-interacting system.
Nevertheless, we predict that the statistical distribution of level spacings is
unchanged by mapping the non-Hermitian Bogoliubov operator to a real symmetric
matrix. We numerically test our prediction by using a phase shift method for
calculating the excitation energies.Comment: minor change, 4 pages, 4 figures, to appear in Phys. Rev. Let
The Absorption Signatures of Dwarf Galaxies: The z=1.04 Multicloud Weak MgII Absorber toward PG 1634+706
We analyze high resolution spectra of a multi--cloud weak [defined as
W_r(MgII) < 0.3 A] absorbing system along the line of sight to PG 1634+706.
This system gives rise to a partial Lyman limit break and absorption in MgII,
SiII, CII, SiIII, SiIV, CIV, and OVI. The lower ionization transitions arise in
two kinematic subsystems with a separation of ~150 km/s. Each subsystem is
resolved into several narrow components, having Doppler widths of 3-10 kms. For
both subsystems, the OVI absorption arises in a separate higher ionization
phase, in regions dominated by bulk motions in the range of 30-40 km/s. The two
OVI absorption profiles are kinematically offset by ~50 km/s with respect to
each of the two lower ionization subsystem. In the stronger subsystem, the
SiIII absorption is strong with a distinctive, smooth profile shape and may
partially arise in shock heated gas. Moreover, the kinematic substructure of
SiIV traces that of the lower ionization MgII, but may be offset by ~3 km/s.
Based upon photoionization models, constrained by the partial Lyman limit
break, we infer a low metallicity of ~0.03 solar for the low ionization gas in
both subsystems. The broader OVI phases have a somewhat higher metallicity, and
they are consistent with photoionization; the profiles are not broad enough to
imply production of OVI through collisional ionization. Various models,
including outer disks, dwarf galaxies, and superwinds, are discussed to account
for the phase structure, metallicity, and kinematics of this absorption system.
We favor an interpretation in which the two subsystems are produced by
condensed clouds far out in the opposite extremes of a multi-layer dwarf galaxy
superwind
Antibunching photons in a cavity coupled to an optomechanical system
We study the photon statistics of a cavity linearly coupled to an
optomechanical system via second order correlation functions. Our calculations
show that the cavity can exhibit strong photon antibunching even when
optomechanical interaction in the optomechanical system is weak. The
cooperation between the weak optomechanical interaction and the destructive
interference between different paths for two-photon excitation leads to the
efficient antibunching effect. Compared with the standard optomechanical
system, the coupling between a cavity and an optomechanical system provides a
method to relax the constraints to obtain single photon by optomechanical
interaction.Comment: 7 papes, 5 figure
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