4,471 research outputs found
Engineering exotic phases for topologically-protected quantum computation by emulating quantum dimer models
We use a nonperturbative extended contractor renormalization (ENCORE) method
for engineering quantum devices for the implementation of topologically
protected quantum bits described by an effective quantum dimer model on the
triangular lattice. By tuning the couplings of the device, topological
protection might be achieved if the ratio between effective two-dimer
interactions and flip amplitudes lies in the liquid phase of the phase diagram
of the quantum dimer model. For a proposal based on a quantum Josephson
junction array [L. B. Ioffe {\it et al.}, Nature (London) {\bf 415}, 503
(2002)] our results show that optimal operational temperatures below 1 mK can
only be obtained if extra interactions and dimer flips, which are not present
in the standard quantum dimer model and involve three or four dimers, are
included. It is unclear if these extra terms in the quantum dimer Hamiltonian
destroy the liquid phase needed for quantum computation. Minimizing the effects
of multi-dimer terms would require energy scales in the nano-Kelvin regime. An
alternative implementation based on cold atomic or molecular gases loaded into
optical lattices is also discussed, and it is shown that the small energy
scales involved--implying long operational times--make such a device
impractical. Given the many orders of magnitude between bare couplings in
devices, and the topological gap, the realization of topological phases in
quantum devices requires careful engineering and large bare interaction scales.Comment: 12 pages, 10 figure
Neutrino Telescopes' Sensitivity to Dark Matter
The nature of the dark matter of the Universe is yet unknown and most likely
is connected with new physics. The search for its composition is under way
through direct and indirect detection. Fundamental physical aspects such as
energy threshold, geometry and location are taken into account to investigate
proposed neutrino telescopes of km^3 volume sensitivities to dark matter. These
sensitivities are just sufficient to test a few WIMP scenarios. Telescopes of
km^3 volume, such as IceCube, can definitely discover or exclude superheavy (M
> 10^10 GeV) Strong Interacting Massive Particles (Simpzillas). Smaller
neutrino telescopes such as ANTARES, AMANDA-II and NESTOR can probe a large
region of the Simpzilla parameter space.Comment: 28 pages, 9 figure
An adaptive cross approximation (ACA) for the extended boundary element method (XBEM) in anisotropic materials
The extended boundary element method (XBEM) is a modification from the standard BEM, where enrichment functions are embedded into the BEM formulation. The results obtained with this method were seen to be accurate and stable, being especially useful for fracture problems. However, the method suffers from the presence of a linear system containing unsymmetric and fully populated matrices which needs to be solved in order to get the solution of the boundary problem. This can be computationally expensive for problems dealing with multiple cracks for instance. Adaptive cross approximation (ACA) is used to reduce the number of operations necessary to solve the linear system of equations for a fracture problem with an anisotropic material
Reação de plantas de pimenta longa (Piper hispidinervium) a isolados de Fusarium solani f.sp. piperis.
Publicado também em: Fitopatologia Brasileira, v. 22, n. 1, p. 112, mar. 1997
Fluctuating Dimension in a Discrete Model for Quantum Gravity Based on the Spectral Principle
The spectral principle of Connes and Chamseddine is used as a starting point
to define a discrete model for Euclidean quantum gravity. Instead of summing
over ordinary geometries, we consider the sum over generalized geometries where
topology, metric and dimension can fluctuate. The model describes the geometry
of spaces with a countable number of points, and is related to the Gaussian
unitary ensemble of Hermitian matrices. We show that this simple model has two
phases. The expectation value . Moreover, the
space-time dimension is a dynamical observable in our model, and plays
the role of an order parameter. The computation of is discussed and
an upper bound is found, .Comment: 10 pages, no figures. Third version: This new version emphasizes the
spectral principle rather than the spectral action. Title has been changed
accordingly. We also reformulated the computation of the dimension, and added
a new reference. To appear in Physical Review Letter
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