1,195 research outputs found
Chaotic Evolution in Quantum Mechanics
A quantum system is described, whose wave function has a complexity which
increases exponentially with time. Namely, for any fixed orthonormal basis, the
number of components required for an accurate representation of the wave
function increases exponentially.Comment: 8 pages (LaTeX 16 kB, followed by PostScript 2 kB for figure
Evolution of Liouville density of a chaotic system
An area-preserving map of the unit sphere, consisting of alternating twists
and turns, is mostly chaotic. A Liouville density on that sphere is specified
by means of its expansion into spherical harmonics. That expansion initially
necessitates only a finite number of basis functions. As the dynamical mapping
proceeds, it is found that the number of non-negligible coefficients increases
exponentially with the number of steps. This is to be contrasted with the
behavior of a Schr\"odinger wave function which requires, for the analogous
quantum system, a basis of fixed size.Comment: LaTeX 4 pages (27 kB) followed by four short PostScript files (2 kB +
2 kB + 1 kB + 4 kB
Asymmetry of bipartite quantum discord
It is known from the analysis of the density matrix for bipartite systems
that the quantum discord (as a measure of quantum correlations) depends on the
particular subsystem chosen for the projective measurements. We study asymmetry
of the discord in a simple physical model of two spin-1/2 particles with the
dipole-dipole interaction governed by the XY Hamiltonian in the inhomogeneous
magnetic field. The dependence of the above discord asymmetry on the Larmour
frequencies at both T=0 (the ground state) and has been investigated. It
is demonstrated, in particular, that the asymmetry is negligible for high
temperatures but it may become significant with the decrease in temperature.Comment: 5 pages 3 figure
Wigner's little group and Berry's phase for massless particles
The ``little group'' for massless particles (namely, the Lorentz
transformations that leave a null vector invariant) is isomorphic to
the Euclidean group E2: translations and rotations in a plane. We show how to
obtain explicitly the rotation angle of E2 as a function of and we
relate that angle to Berry's topological phase. Some particles admit both signs
of helicity, and it is then possible to define a reduced density matrix for
their polarization. However, that density matrix is physically meaningless,
because it has no transformation law under the Lorentz group, even under
ordinary rotations.Comment: 4 pages revte
All multipartite Bell correlation inequalities for two dichotomic observables per site
We construct a set of 2^(2^n) independent Bell correlation inequalities for
n-partite systems with two dichotomic observables each, which is complete in
the sense that the inequalities are satisfied if and only if the correlations
considered allow a local classical model. All these inequalities can be
summarized in a single, albeit non-linear inequality. We show that quantum
correlations satisfy this condition provided the state has positive partial
transpose with respect to any grouping of the n systems into two subsystems. We
also provide an efficient algorithm for finding the maximal quantum mechanical
violation of each inequality, and show that the maximum is always attained for
the generalized GHZ state.Comment: 11 pages, REVTe
Serological prevalence of Brucella spp. in feral pigs and sympatric cattle in the Pantanal of Mato Grosso do Sul, Brazil.
The aim of this study was to estimate the prevalence of anti-Brucella antibodies in feral pigs and sympatric cattle in the Pantanal sub-regions of Paiaguás and Nhecolândia. The study was conducted in Corumbá, State of Mato Grosso do Sul, Brazil. A total of 105 feral pigs and 256 cattle were sampled on 12 farms. Blood samples were collected from all the animals for serological diagnosis with buffered acidified antigen (BAA) for screening, confirmatory 2-Mercaptoethanol (2-ME) test, and comparative fluorescent polarization assay (FPA). The positive prevalence of feral pigs was 1% (1/105) in BAA and FPT, with no positive result confirmed of BAA in 2-ME. The prevalence of positive sampled cattle was 11.32% (29/256), 4.3% (10/256), and 7.42% (19/256) in the BAA, 2-ME, and FPT tests, respectively. The degree of agreement obtained among the serological tests in cattle was Kappa = 0.506 (p < 0.001), 95% CI (0.282-0.729). The results of serological tests showed that brucellosis is widespread in cattle herds of the studied region, but the same type of exposure to the agent did not occur in feral pigs according to the used diagnostic tests
Finite precision measurement nullifies the Kochen-Specker theorem
Only finite precision measurements are experimentally reasonable, and they
cannot distinguish a dense subset from its closure. We show that the rational
vectors, which are dense in S^2, can be colored so that the contradiction with
hidden variable theories provided by Kochen-Specker constructions does not
obtain. Thus, in contrast to violation of the Bell inequalities, no
quantum-over-classical advantage for information processing can be derived from
the Kochen-Specker theorem alone.Comment: 7 pages, plain TeX; minor corrections, interpretation clarified,
references update
Analytical study of non-linear transport across a semiconductor-metal junction
In this paper we study analytically a one-dimensional model for a
semiconductor-metal junction. We study the formation of Tamm states and how
they evolve when the semi-infinite semiconductor and metal are coupled
together. The non-linear current, as a function of the bias voltage, is studied
using the non-equilibrium Green's function method and the density matrix of the
interface is given. The electronic occupation of the sites defining the
interface has strong non-linearities as function of the bias voltage due to
strong resonances present in the Green's functions of the junction sites. The
surface Green's function is computed analytically by solving a quadratic matrix
equation, which does not require adding a small imaginary constant to the
energy. The wave function for the surface states is given
Representations of Coherent and Squeezed States in a -deformed Fock Space
We establish some of the properties of the states interpolating between
number and coherent states denoted by ; among them are the
reproducing of these states by the action of an operator-valued function on (the standard Fock space) and the fact that they can be regarded as
-deformed coherent bound states. In this paper we use them, as the basis of
our new Fock space which in this case are not orthogonal but normalized. Then
by some special superposition of them we obtain new representations for
coherent and squeezed states in the new basis. Finally the statistical
properties of these states are studied in detail.Comment: 13 pages, 4 Figure
Quantum Computation with Quantum Dots
We propose a new implementation of a universal set of one- and two-qubit
gates for quantum computation using the spin states of coupled single-electron
quantum dots. Desired operations are effected by the gating of the tunneling
barrier between neighboring dots. Several measures of the gate quality are
computed within a newly derived spin master equation incorporating decoherence
caused by a prototypical magnetic environment. Dot-array experiments which
would provide an initial demonstration of the desired non-equilibrium spin
dynamics are proposed.Comment: 12 pages, Latex, 2 ps figures. v2: 20 pages (very minor corrections,
substantial expansion), submitted to Phys. Rev.
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