24,781 research outputs found
Inhomogeneous substructures hidden in random networks
We study the structure of the load-based spanning tree (LST) that carries the
maximum weight of the Erdos-Renyi (ER) random network. The weight of an edge is
given by the edge-betweenness centrality, the effective number of shortest
paths through the edge. We find that the LSTs present very inhomogeneous
structures in contrast to the homogeneous structures of the original networks.
Moreover, it turns out that the structure of the LST changes dramatically as
the edge density of an ER network increases, from scale free with a cutoff,
scale free, to a starlike topology. These would not be possible if the weights
are randomly distributed, which implies that topology of the shortest path is
correlated in spite of the homogeneous topology of the random network.Comment: 4 pages, 4 figure
Entangled coherent states versus entangled photon pairs for practical quantum information processing
We compare effects of decoherence and detection inefficiency on entangled
coherent states (ECSs) and entangled photon pairs (EPPs), both of which are
known to be particularly useful for quantum information processing (QIP). When
decoherence effects caused by photon losses are heavy, the ECSs outperform the
EPPs as quantum channels for teleportation both in fidelities and in success
probabilities. On the other hand, when inefficient detectors are used, the
teleportation scheme using the ECSs suffers undetected errors that result in
the degradation of fidelity, while this is not the case for the teleportation
scheme using the EPPs. Our study reveals the merits and demerits of the two
types of entangled states in realizing practical QIP under realistic
conditions.Comment: 9 pages, 6 figures, substantially revised version, to be published in
Phys. Rev.
Near-deterministic quantum teleportation and resource-efficient quantum computation using linear optics and hybrid qubits
We propose a scheme to realize deterministic quantum teleportation using
linear optics and hybrid qubits. It enables one to efficiently perform
teleportation and universal linear-optical gate operations in a simple and
near-deterministic manner using all-optical hybrid entanglement as off-line
resources. Our analysis shows that our new approach can outperforms major
previous ones when considering both the resource requirements and fault
tolerance limits.Comment: 10 pages, 5 figures; extended version, title, abstract and figures
changed, details added, to be published in Phys. Rev.
Scale-free trees: the skeletons of complex networks
We investigate the properties of the spanning trees of various real-world and
model networks. The spanning tree representing the communication kernel of the
original network is determined by maximizing total weight of edges, whose
weights are given by the edge betweenness centralities. We find that a
scale-free tree and shortcuts organize a complex network. The spanning tree
shows robust betweenness centrality distribution that was observed in
scale-free tree models. It turns out that the shortcut distribution
characterizes the properties of original network, such as the clustering
coefficient and the classification of networks by the betweenness centrality
distribution
Importance sampling schemes for evidence approximation in mixture models
The marginal likelihood is a central tool for drawing Bayesian inference
about the number of components in mixture models. It is often approximated
since the exact form is unavailable. A bias in the approximation may be due to
an incomplete exploration by a simulated Markov chain (e.g., a Gibbs sequence)
of the collection of posterior modes, a phenomenon also known as lack of label
switching, as all possible label permutations must be simulated by a chain in
order to converge and hence overcome the bias. In an importance sampling
approach, imposing label switching to the importance function results in an
exponential increase of the computational cost with the number of components.
In this paper, two importance sampling schemes are proposed through choices for
the importance function; a MLE proposal and a Rao-Blackwellised importance
function. The second scheme is called dual importance sampling. We demonstrate
that this dual importance sampling is a valid estimator of the evidence and
moreover show that the statistical efficiency of estimates increases. To reduce
the induced high demand in computation, the original importance function is
approximated but a suitable approximation can produce an estimate with the same
precision and with reduced computational workload.Comment: 24 pages, 5 figure
Bifurcations and bistability in cavity assisted photoassociation of Bose-Einstein condensed molecules
We study the photo-association of Bose-Einstein condensed atoms into
molecules using an optical cavity field. The driven cavity field introduces a
new dynamical degree of freedom into the photoassociation process, whose role
in determining the stationary behavior has not previously been considered. The
semiclassical stationary solutions for the atom and molecules as well as the
intracavity field are found and their stability and scaling properties are
determined in terms of experimentally controllable parameters including driving
amplitude of the cavity and the nonlinear interactions between atoms and
molecules. For weak cavity driving, we find a bifurcation in the atom and
molecule number occurs that signals a transition from a stable steady state to
nonlinear Rabi oscillations. For a strongly driven cavity, there exists
bistability in the atom and molecule number
Rules for Computing Symmetry, Density and Stoichiometry in a Quasi-Unit-Cell Model of Quasicrystals
The quasi-unit cell picture describes the atomic structure of quasicrystals
in terms of a single, repeating cluster which overlaps neighbors according to
specific overlap rules. In this paper, we discuss the precise relationship
between a general atomic decoration in the quasi-unit cell picture atomic
decorations in the Penrose tiling and in related tiling pictures. Using these
relations, we obtain a simple, practical method for determining the density,
stoichiometry and symmetry of a quasicrystal based on the atomic decoration of
the quasi-unit cell taking proper account of the sharing of atoms between
clusters.Comment: 14 pages, 8 figure
Fundamental Structural Constraint of Random Scale-Free Networks
We study the structural constraint of random scale-free networks that
determines possible combinations of the degree exponent and the upper
cutoff in the thermodynamic limit. We employ the framework of
graphicality transitions proposed by [Del Genio and co-workers, Phys. Rev.
Lett. {\bf 107}, 178701 (2011)], while making it more rigorous and applicable
to general values of kc. Using the graphicality criterion, we show that the
upper cutoff must be lower than for , whereas
any upper cutoff is allowed for . This result is also numerically
verified by both the random and deterministic sampling of degree sequences.Comment: 5 pages, 4 figures (7 eps files), 2 tables; published versio
Conditional Production of Superpositions of Coherent States with Inefficient Photon Detection
It is shown that a linear superposition of two macroscopically
distinguishable optical coherent states can be generated using a single photon
source and simple all-optical operations. Weak squeezing on a single photon,
beam mixing with an auxiliary coherent state, and photon detecting with
imperfect threshold detectors are enough to generate a coherent state
superposition in a free propagating optical field with a large coherent
amplitude () and high fidelity (). In contrast to all
previous schemes to generate such a state, our scheme does not need photon
number resolving measurements nor Kerr-type nonlinear interactions.
Furthermore, it is robust to detection inefficiency and exhibits some
resilience to photon production inefficiency.Comment: Some important new results added, to appear in Phys.Rev.A (Rapid
Communication
- …