273 research outputs found
Spin Chains in an External Magnetic Field. Closure of the Haldane Gap and Effective Field Theories
We investigate both numerically and analytically the behaviour of a spin-1
antiferromagnetic (AFM) isotropic Heisenberg chain in an external magnetic
field. Extensive DMRG studies of chains up to N=80 sites extend previous
analyses and exhibit the well known phenomenon of the closure of the Haldane
gap at a lower critical field H_c1. We obtain an estimate of the gap below
H_c1. Above the lower critical field, when the correlation functions exhibit
algebraic decay, we obtain the critical exponent as a function of the net
magnetization as well as the magnetization curve up to the saturation (upper
critical) field H_c2. We argue that, despite the fact that the SO(3) symmetry
of the model is explicitly broken by the field, the Haldane phase of the model
is still well described by an SO(3) nonlinear sigma-model. A mean-field theory
is developed for the latter and its predictions are compared with those of the
numerical analysis and with the existing literature.Comment: 11 pages, 4 eps figure
Redundancy of classical and quantum correlations during decoherence
We analyze the time dependence of entanglement and total correlations between
a system and fractions of its environment in the course of decoherence. For the
quantum Brownian motion model we show that the entanglement and total
correlations have rather different dependence on the size of the environmental
fraction. Redundancy manifests differently in both types of correlations and
can be related with induced--classicality. To study this we introduce a new
measure of redundancy and compare it with the existing one.Comment: 6 pages, 4 figure
Qubits in phase space: Wigner function approach to quantum error correction and the mean king problem
We analyze and further develop a new method to represent the quantum state of
a system of qubits in a phase space grid of points (where
). The method, which was recently proposed by Wootters and co--workers
(Gibbons {\it et al.}, quant-ph/0401155), is based on the use of the elements
of the finite field to label the phase space axes. We present a
self--contained overview of the method, we give new insights on some of its
features and we apply it to investigate problems which are of interest for
quantum information theory: We analyze the phase space representation of
stabilizer states and quantum error correction codes and present a phase space
solution to the so--called ``mean king problem''.Comment: 18 pages, 16 figures; typos fixed, some minor corrections, figures of
the circuits were change
Finding critical points using improved scaling Ansaetze
Analyzing in detail the first corrections to the scaling hypothesis, we
develop accelerated methods for the determination of critical points from
finite size data. The output of these procedures are sequences of
pseudo-critical points which rapidly converge towards the true critical points.
In fact more rapidly than previously existing methods like the Phenomenological
Renormalization Group approach. Our methods are valid in any spatial
dimensionality and both for quantum or classical statistical systems. Having at
disposal fast converging sequences, allows to draw conclusions on the basis of
shorter system sizes, and can be extremely important in particularly hard cases
like two-dimensional quantum systems with frustrations or when the sign problem
occurs. We test the effectiveness of our methods both analytically on the basis
of the one-dimensional XY model, and numerically at phase transitions occurring
in non integrable spin models. In particular, we show how a new Homogeneity
Condition Method is able to locate the onset of the
Berezinskii-Kosterlitz-Thouless transition making only use of ground-state
quantities on relatively small systems.Comment: 16 pages, 4 figures. New version including more general Ansaetze
basically applicable to all case
Long-distance entanglement in spin systems
Most quantum system with short-ranged interactions show a fast decay of
entanglement with the distance. In this Letter, we focus on the peculiarity of
some systems to distribute entanglement between distant parties. Even in
realistic models, like the spin-1 Heisenberg chain, sizable entanglement is
present between arbitrarily distant particles. We show that long distance
entanglement appears for values of the microscopic parameters which do not
coincide with known quantum critical points, hence signaling a transition
detected only by genuine quantum correlations.Comment: RevTex, 5 pages, 7 .eps figures Two references added in published
versio
Charmed Baryons with
The width of a recently discovered excited charmed-strange baryon, a
candidate for a state with spin 3/2, is calculated. In the absence of
configuration mixing between the ground-state (spin-1/2) charmed-strange baryon
and the spin-1/2 state lying about 95 MeV above it,
one finds and , where the tilde denotes the partial
width with kinematic factors removed. Assuming a kinematic factor for P-wave
decay of , one predicts MeV, while the channel is closed. Some
suggestions are given for detecting the , the spin-3/2 charmed
nonstrange baryon, and the , the spin-3/2 charmed doubly-strange
baryon.Comment: 11 pages, latex, 2 uuencoded figures sent separatel
A quantum gate array can be programmed to evaluate the expectation value of any operator
A programmable gate array is a circuit whose action is controlled by input
data. In this letter we describe a special--purpose quantum circuit that can be
programmed to evaluate the expectation value of any operator acting on a
space of states of dimensions. The circuit has a program register whose
state encodes the operator whose expectation value is to be
evaluated. The method requires knowledge of the expansion of in a basis of
the space of operators. We discuss some applications of this circuit and its
relation to known instances of quantum state tomography.Comment: 4 pages, 3 figures include
Heavy Flavour Baryons in Hyper Central Model
Heavy flavor baryons containing single and double charm (beauty) quarks with
light flavor combinations are studied using the hyper central description of
the three-body problem. The confinement potential is assumed as hyper central
coulomb plus power potential with power index . The ground state
masses of the heavy flavor, and baryons are computed
for different power index, starting from 0.5 to 2.0. The predicted
masses are found to attain a saturated value in each case of quark combinations
beyond the power index .Comment: 10 pages, 4 figure
Quasiprobability distribution functions for periodic phase-spaces: I. Theoretical Aspects
An approach featuring -parametrized quasiprobability distribution
functions is developed for situations where a circular topology is observed.
For such an approach, a suitable set of angle-angular momentum coherent states
must be constructed in appropriate fashion.Comment: 13 pages, 3 figure
Band Distributions for Quantum Chaos on the Torus
Band distributions (BDs) are introduced describing quantization in a toral
phase space. A BD is the uniform average of an eigenstate phase-space
probability distribution over a band of toral boundary conditions. A general
explicit expression for the Wigner BD is obtained. It is shown that the Wigner
functions for {\em all} of the band eigenstates can be reproduced from the
Wigner BD. Also, BDs are shown to be closer to classical distributions than
eigenstate distributions. Generalized BDs, associated with sets of adjacent
bands, are used to extend in a natural way the Chern-index characterization of
the classical-quantum correspondence on the torus to arbitrary rational values
of the scaled Planck constant.Comment: 12 REVTEX page
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