264 research outputs found
Average fidelity between random quantum states
We analyze mean fidelity between random density matrices of size N, generated
with respect to various probability measures in the space of mixed quantum
states: Hilbert-Schmidt measure, Bures (statistical) measure, the measures
induced by partial trace and the natural measure on the space of pure states.
In certain cases explicit probability distributions for fidelity are derived.
Results obtained may be used to gauge the quality of quantum information
processing schemes.Comment: 15 revtex pages with 4 figures; Ver. 2: another distribution derived,
an extra figure included, Ver. 3: comments in introduction and conclusion
added ver 4. minor improvment
On the comparison of volumes of quantum states
This paper aims to study the \a-volume of \cK, an arbitrary subset of the
set of density matrices. The \a-volume is a generalization of the
Hilbert-Schmidt volume and the volume induced by partial trace. We obtain
two-side estimates for the \a-volume of \cK in terms of its Hilbert-Schmidt
volume. The analogous estimates between the Bures volume and the \a-volume
are also established. We employ our results to obtain bounds for the
\a-volume of the sets of separable quantum states and of states with positive
partial transpose (PPT). Hence, our asymptotic results provide answers for
questions listed on page 9 in \cite{K. Zyczkowski1998} for large in the
sense of \a-volume.
\vskip 3mm PACS numbers: 02.40.Ft, 03.65.Db, 03.65.Ud, 03.67.M
CP^n, or, entanglement illustrated
We show that many topological and geometrical properties of complex
projective space can be understood just by looking at a suitably constructed
picture. The idea is to view CP^n as a set of flat tori parametrized by the
positive octant of a round sphere. We pay particular attention to submanifolds
of constant entanglement in CP^3 and give a few new results concerning them.Comment: 28 pages, 9 figure
Bound states and scattering in quantum waveguides coupled laterally through a boundary window
We consider a pair of parallel straight quantum waveguides coupled laterally
through a window of a width in the common boundary. We show that such
a system has at least one bound state for any . We find the
corresponding eigenvalues and eigenfunctions numerically using the
mode--matching method, and discuss their behavior in several situations. We
also discuss the scattering problem in this setup, in particular, the turbulent
behavior of the probability flow associated with resonances. The level and
phase--shift spacing statistics shows that in distinction to closed
pseudo--integrable billiards, the present system is essentially non--chaotic.
Finally, we illustrate time evolution of wave packets in the present model.Comment: LaTeX text file with 12 ps figure
CERTAIN MODIFICATIONS OF AITKEN'S ACCELERATOR FOR SLOWLY CONVERGENT SEQUENCES
The paper presents three modifications of the Aitken accelerator for slowly or irregularly
convergent sequences. The first proposal, based on backward extension of a sequence,
is particularly useful if a small number of regular sequence terms is known. The next
two procedures - integral exponential and hyperbolic - can be applied to sequences with
irregular or disturbed convergence. Numerical examples show essential error reduction of
the limit of sequences under consideration
On the volume of the set of mixed entangled states II
The problem of of how many entangled or, respectively, separable states there
are in the set of all quantum states is investigated. We study to what extent
the choice of a measure in the space of density matrices describing
N--dimensional quantum systems affects the results obtained. We demonstrate
that the link between the purity of the mixed states and the probability of
entanglement is not sensitive to the measure chosen. Since the criterion of
partial transposition is not sufficient to distinguish all separable states for
N > 6, we develop an efficient algorithm to calculate numerically the
entanglement of formation of a given mixed quantum state, which allows us to
compute the volume of separable states for N=8 and to estimate the volume of
the bound entangled states in this case.Comment: 14 pages in Latex, Revtex + epsf; 7 figures in .ps included (one new
figure in the revised version, several minor changes
Geometry of entangled states
Geometric properties of the set of quantum entangled states are investigated.
We propose an explicit method to compute the dimension of local orbits for any
mixed state of the general K x M problem and characterize the set of
effectively different states (which cannot be related by local
transformations). Thus we generalize earlier results obtained for the simplest
2 x 2 system, which lead to a stratification of the 6D set of N=4 pure states.
We define the concept of absolutely separable states, for which all globally
equivalent states are separable.Comment: 16 latex pages, 4 figures in epsf, minor corrections, references
updated, to appear in Phys. Rev.
Truncations of Random Orthogonal Matrices
Statistical properties of non--symmetric real random matrices of size ,
obtained as truncations of random orthogonal matrices are
investigated. We derive an exact formula for the density of eigenvalues which
consists of two components: finite fraction of eigenvalues are real, while the
remaining part of the spectrum is located inside the unit disk symmetrically
with respect to the real axis. In the case of strong non--orthogonality,
const, the behavior typical to real Ginibre ensemble is found. In the
case with fixed , a universal distribution of resonance widths is
recovered.Comment: 4 pages, final revised version (one reference added, minor changes in
Introduction
On the structure of the body of states with positive partial transpose
We show that the convex set of separable mixed states of the 2 x 2 system is
a body of constant height. This fact is used to prove that the probability to
find a random state to be separable equals 2 times the probability to find a
random boundary state to be separable, provided the random states are generated
uniformly with respect to the Hilbert-Schmidt (Euclidean) distance. An
analogous property holds for the set of positive-partial-transpose states for
an arbitrary bipartite system.Comment: 10 pages, 1 figure; ver. 2 - minor changes, new proof of lemma
Geometric measure of entanglement and applications to bipartite and multipartite quantum states
The degree to which a pure quantum state is entangled can be characterized by
the distance or angle to the nearest unentangled state. This geometric measure
of entanglement, already present in a number of settings (see Shimony 1995 and
Barnum and Linden 2001), is explored for bipartite and multipartite pure and
mixed states. The measure is determined analytically for arbitrary two-qubit
mixed states and for generalized Werner and isotropic states, and is also
applied to certain multipartite mixed states. In particular, a detailed
analysis is given for arbitrary mixtures of three-qubit GHZ, W and inverted-W
states. Along the way, we point out connections of the geometric measure of
entanglement with entanglement witnesses and with the Hartree approximation
method.Comment: 13 pages, 11 figures, this is a combination of three previous
manuscripts (quant-ph/0212030, quant-ph/0303079, and quant-ph/0303158) made
more extensive and coherent. To appear in PR
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