30,644 research outputs found
Antiresonance and interaction-induced localization in spin and qubit chains with defects
We study a spin chain with an anisotropic XXZ coupling in an external field.
Such a chain models several proposed types of a quantum computer. The chain
contains a defect with a different on-site energy. The interaction between
excitations is shown to lead to two-excitation states localized next to the
defect. In a resonant situation scattering of excitations on each other might
cause decay of an excitation localized on the defect. We find that destructive
quantum interference suppresses this decay. Numerical results confirm the
analytical predictions.Comment: Updated versio
Many-particle confinement by constructed disorder and quantum computing
Many-particle confinement (localization) is studied for a 1D system of
spinless fermions with nearest-neighbor hopping and interaction, or
equivalently, for an anisotropic Heisenberg spin-1/2 chain. This system is
frequently used to model quantum computers with perpetually coupled qubits. We
construct a bounded sequence of site energies that leads to strong
single-particle confinement of all states on individual sites. We show that
this sequence also leads to a confinement of all many-particle states in an
infinite system for a time that scales as a high power of the reciprocal
hopping integral. The confinement is achieved for strong interaction between
the particles while keeping the overall bandwidth of site energies
comparatively small. The results show viability of quantum computing with
time-independent qubit coupling.Comment: An invited paper for the topical issue of J. Opt. B on quantum
contro
Melhoramento genético do feijão-caupi na Embrapa Semi-Árido.
Procedimentos para hibridações, avanços de gerações e competições; análises estatísticas dos ensaios avançados; análises para a qualidade tecnológica dos grãos; avaliações em macroparcelas em nível de propriedades rurais; seleção de linhagens avançadas nos cruzamentos com Epace 10 e BR 14 Gurguéia; análises para a qualidade tecnológica dos grãos; avaliações em macroparcelas em nível de propriedades rurais; seleção de linhagens de crescimento determinado e porte ereto; seleção de linhagens tipo ?Canapu? tolerantes às principais viroses; integração de melhoramento clássico e molecular; desenvolvimento de linhagens com propriedades de alimentos funcionaisbitstream/CPATSA/36702/1/SDC204.pd
Nonlinear viscosity and velocity distribution function in a simple longitudinal flow
A compressible flow characterized by a velocity field is
analyzed by means of the Boltzmann equation and the Bhatnagar-Gross-Krook
kinetic model. The sign of the control parameter (the longitudinal deformation
rate ) distinguishes between an expansion () and a condensation ()
phenomenon. The temperature is a decreasing function of time in the former
case, while it is an increasing function in the latter. The non-Newtonian
behavior of the gas is described by a dimensionless nonlinear viscosity
, that depends on the dimensionless longitudinal rate . The
Chapman-Enskog expansion of in powers of is seen to be only
asymptotic (except in the case of Maxwell molecules). The velocity distribution
function is also studied. At any value of , it exhibits an algebraic
high-velocity tail that is responsible for the divergence of velocity moments.
For sufficiently negative , moments of degree four and higher may diverge,
while for positive the divergence occurs in moments of degree equal to or
larger than eight.Comment: 18 pages (Revtex), including 5 figures (eps). Analysis of the heat
flux plus other minor changes added. Revised version accepted for publication
in PR
Estudo comparativo entre os metadados da Agência de Informação Embrapa e do Acervo Documental do AINFO.
A embrapa Informática Agropecuária é a principal responsável pela concretização de duas importantes iniciativas no âmbito da organização da informação: o AINFO e a Agência de Informação Embrapa.bitstream/CNPTIA/10650/1/comtec66.pdfAcesso em: 28 maio 2008
Using quantum state protection via dissipation in a quantum-dot molecule to solve the Deutsch problem
The wide set of control parameters and reduced size scale make semiconductor
quantum dots attractive candidates to implement solid-state quantum
computation. Considering an asymmetric double quantum dot coupled by tunneling,
we combine the action of a laser field and the spontaneous emission of the
excitonic state to protect an arbitrary superposition state of the indirect
exciton and ground state. As a by-product we show how to use the protected
state to solve the Deutsch problem.Comment: 8 pages, 1 figure, 2 table
Transport Processes in Metal-Insulator Granular Layers
Tunnel transport processes are considered in a square lattice of metallic
nanogranules embedded into insulating host to model tunnel conduction in real
metal/insulator granular layers. Based on a simple model with three possible
charging states (, or 0) of a granule and three kinetic processes
(creation or recombination of a pair, and charge transfer) between
neighbor granules, the mean-field kinetic theory is developed. It describes the
interplay between charging energy and temperature and between the applied
electric field and the Coulomb fields by the non-compensated charge density.
The resulting charge and current distributions are found to be essentially
different in the free area (FA), between the metallic contacts, or in the
contact areas (CA), beneath those contacts. Thus, the steady state dc transport
is only compatible with zero charge density and ohmic resistivity in FA, but
charge accumulation and non-ohmic behavior are \emph{necessary} for conduction
over CA. The approximate analytic solutions are obtained for characteristic
regimes (low or high charge density) of such conduction. The comparison is done
with the measurement data on tunnel transport in related experimental systems.Comment: 10 pages, 11 figures, 1 reference corrected, acknowlegments adde
Continuous dynamic assimilation of the inner region data in hydrodynamics modelling: optimization approach
In meteorological and oceanological studies the classical approach for finding the numerical solution of the regional model consists in formulating and solving a Cauchy-Dirichlet problem. The boundary conditions are obtained by linear interpolation of coarse-grid data provided by a global model. Errors in boundary conditions due to interpolation may cause large deviations from the correct regional solution. The methods developed to reduce these errors deal with continuous dynamic assimilation of known global data available inside the regional domain. One of the approaches of this assimilation procedure performs a nudging of large-scale components of regional model solution to large-scale global data components by introducing relaxation forcing terms into the regional model equations. As a result, the obtained solution is not a valid numerical solution to the original regional model. Another approach is the use a four-dimensional variational data assimilation procedure which is free from the above-mentioned shortcoming. In this work we formulate the joint problem of finding the regional model solution and data assimilation as a PDE-constrained optimization problem. Three simple model examples (ODE Burgers equation, Rossby-Oboukhov equation, Korteweg-de Vries equation) are considered in this paper. Numerical experiments indicate that the optimization approach can significantly improve the precision of the regional solution
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