58,200 research outputs found
On the road to prosperity? The economic geography of China's national expressway network
Over the past two decades, China has embarked on an ambitious program of expressway network expansion. By facilitating market integration, this program aims both to promote efficiency at the national level and to contribute to the catch-up of lagging inland regions with prosperous Eastern ones. This paper evaluates the aggregate and spatial economic impacts of China's newly constructed National Expressway Network, focussing, in particular, on its short-run impacts. To achieve this aim, the authors adopt a counterfactual approach based on the estimation and simulation of a structural "new economic geography" model. Overall, they find that aggregate Chinese real income was approximately 6 percent higher than it would have been in 2007 had the expressway network not been built. Although there is considerable heterogeneity in the results, the authors do not find evidence of a significant reduction in disparities across prefectural level regions or of a reduction in urban-rural disparities. If anything, the expressway network appears to have reinforced existing patterns of spatial inequality, although, over time, these will likely be reduced by enhanced migration
Simulating quantum computation by contracting tensor networks
The treewidth of a graph is a useful combinatorial measure of how close the
graph is to a tree. We prove that a quantum circuit with gates whose
underlying graph has treewidth can be simulated deterministically in
time, which, in particular, is polynomial in if
. Among many implications, we show efficient simulations for
log-depth circuits whose gates apply to nearby qubits only, a natural
constraint satisfied by most physical implementations. We also show that
one-way quantum computation of Raussendorf and Briegel (Physical Review
Letters, 86:5188--5191, 2001), a universal quantum computation scheme with
promising physical implementations, can be efficiently simulated by a
randomized algorithm if its quantum resource is derived from a small-treewidth
graph.Comment: 7 figure
Quantum temporal imaging: application of a time lens to quantum optics
We consider application of a temporal imaging system, based on the
sum-frequency generation, to a nonclassical, in particular, squeezed optical
temporal waveform. We analyze the restrictions on the pump and the phase
matching condition in the summing crystal, necessary for preserving the quantum
features of the initial waveform. We show that modification of the notion of
the field of view in the quantum case is necessary, and that the quantum field
of view is much narrower than the classical one for the same temporal imaging
system. These results are important for temporal stretching and compressing of
squeezed fields, used in quantum-enhanced metrology and quantum communications.Comment: 9 pages, 3 figure
The reduction of the closest disentangled states
We study the closest disentangled state to a given entangled state in any
system (multi-party with any dimension). We obtain the set of equations the
closest disentangled state must satisfy, and show that its reduction is
strongly related to the extremal condition of the local filtering on each
party. Although the equations we obtain are not still tractable, we find some
sufficient conditions for which the closest disentangled state has the same
reduction as the given entangled state. Further, we suggest a prescription to
obtain a tight upper bound of the relative entropy of entanglement in two-qubit
systems.Comment: a crucial error was correcte
Classical simulation of quantum many-body systems with a tree tensor network
We show how to efficiently simulate a quantum many-body system with tree
structure when its entanglement is bounded for any bipartite split along an
edge of the tree. This is achieved by expanding the {\em time-evolving block
decimation} simulation algorithm for time evolution from a one dimensional
lattice to a tree graph, while replacing a {\em matrix product state} with a
{\em tree tensor network}. As an application, we show that any one-way quantum
computation on a tree graph can be efficiently simulated with a classical
computer.Comment: 4 pages,7 figure
Superspace Formulation in a Three-Algebra Approach to D=3, N=4,5 Superconformal Chern-Simons Matter Theories
We present a superspace formulation of the D=3, N=4,5 superconformal
Chern-Simons Matter theories, with matter supermultiplets valued in a
symplectic 3-algebra. We first construct an N=1 superconformal action, and then
generalize a method used by Gaitto and Witten to enhance the supersymmetry from
N=1 to N=5. By decomposing the N=5 supermultiplets and the symplectic 3-algebra
properly and proposing a new super-potential term, we construct the N=4
superconformal Chern-Simons matter theories in terms of two sets of generators
of a (quaternion) symplectic 3-algebra. The N=4 theories can also be derived by
requiring that the supersymmetry transformations are closed on-shell. The
relationship between the 3-algebras, Lie superalgebras, Lie algebras and
embedding tensors (proposed in [E. A. Bergshoeff, O. Hohm, D. Roest, H.
Samtleben, and E. Sezgin, J. High Energy Phys. 09 (2008) 101.]) is also
clarified. The general N=4,5 superconformal Chern-Simons matter theories in
terms of ordinary Lie algebras can be rederived in our 3-algebra approach. All
known N=4,5 superconformal Chern-Simons matter theories can be recovered in the
present superspace formulation for super-Lie-algebra realization of symplectic
3-algebras.Comment: 37 pages, minor changes, published in PR
Subleading corrections to parity-violating pion photoproduction
We compute the photon asymmetry BÎł for near threshold parity-violating (PV) pion photoproduction through subleading order. We show that subleading contributions involve a new combination of PV couplings not included in previous analyses of hadronic PV. We argue that existing constraints on the leading order contribution to BÎłâobtained from the PV Îł-decay of 18Fâsuggest that the impact of the subleading contributions may be more significant than expected from naturalness arguments
Quantum information transfer and models for black hole mechanics
General features of information transfer between quantum subsystems, via
unitary evolution, are investigated, with applications to the problem of
information transfer from a black hole to its surroundings. A particularly
direct form of quantum information transfer is "subspace transfer," which can
be characterized by saturation of a subadditivity inequality. We also describe
more general unitary quantum information transfer, and categorize different
models for black hole evolution. Evolution that only creates paired excitations
inside/outside the black hole is shown not to extract information, but
information-transferring models exist both in the "saturating" and
"non-saturating" category. The former more closely capture thermodynamic
behavior; the latter generically have enhanced energy flux, beyond that of
Hawking.Comment: 31 pages, harvmac. v2: nomenclature change, minor added explanation.
v3: small corrections/rewordings; improved figure; version to match
publication in PR
A multi-view approach to cDNA micro-array analysis
The official published version can be obtained from the link below.Microarray has emerged as a powerful technology that enables biologists to study thousands of genes simultaneously, therefore, to obtain a better understanding of the gene interaction and regulation mechanisms. This paper is concerned with improving the processes involved in the analysis of microarray image data. The main focus is to clarify an image's feature space in an unsupervised manner. In this paper, the Image Transformation Engine (ITE), combined with different filters, is investigated. The proposed methods are applied to a set of real-world cDNA images. The MatCNN toolbox is used during the segmentation process. Quantitative comparisons between different filters are carried out. It is shown that the CLD filter is the best one to be applied with the ITE.This work was supported in part by the Engineering and Physical Sciences Research
Council (EPSRC) of the UK under Grant GR/S27658/01, the National Science Foundation of China under Innovative Grant 70621001, Chinese Academy of Sciences
under Innovative Group Overseas Partnership Grant, the BHP Billiton Cooperation of Australia Grant, the International Science and Technology Cooperation Project of China
under Grant 2009DFA32050 and the Alexander von Humboldt Foundation of Germany
Loschmidt echo and fidelity decay near an exceptional point
Non-Hermitian classical and open quantum systems near an exceptional point
(EP) are known to undergo strong deviations in their dynamical behavior under
small perturbations or slow cycling of parameters as compared to Hermitian
systems. Such a strong sensitivity is at the heart of many interesting
phenomena and applications, such as the asymmetric breakdown of the adiabatic
theorem, enhanced sensing, non-Hermitian dynamical quantum phase transitions
and photonic catastrophe. Like for Hermitian systems, the sensitivity to
perturbations on the dynamical evolution can be captured by Loschmidt echo and
fidelity after imperfect time reversal or quench dynamics. Here we disclose a
rather counterintuitive phenomenon in certain non-Hermitian systems near an EP,
namely the deceleration (rather than acceleration) of the fidelity decay and
improved Loschmidt echo as compared to their Hermitian counterparts, despite
large (non-perturbative) deformation of the energy spectrum introduced by the
perturbations. This behavior is illustrated by considering the fidelity decay
and Loschmidt echo for the single-particle hopping dynamics on a tight-binding
lattice under an imaginary gauge field.Comment: 11 pages, 6 figures, to appear in Annalen der Physi
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