36,245 research outputs found
Entanglement monotones
In the context of quantifying entanglement we study those functions of a
multipartite state which do not increase under the set of local
transformations. A mathematical characterization of these monotone magnitudes
is presented. They are then related to optimal strategies of conversion of
shared states. More detailed results are presented for pure states of bipartite
systems. It is show that more than one measure are required simultaneously in
order to quantify completely the non-local resources contained in a bipartite
pure state, while examining how this fact does not hold in the so-called
asymptotic limit. Finally, monotonicity under local transformations is proposed
as the only natural requirement for measures of entanglement.Comment: Revtex, 13 pages, no figures. Previous title: "On the
characterization of entanglement". Major changes in notation and structure.
Some new results, comments and references have been adde
Optimal distillation of a GHZ state
We present the optimal local protocol to distill a
Greenberger-Horne-Zeilinger (GHZ) state from a single copy of any pure state of
three qubits.Comment: RevTex, 4 pages, 2 figures. Published version, some references adde
The orbit of Beta Pic b as a transiting planet
In 1981, Beta Pictoris showed strong and rapid photometric variations
possibly due to a transiting giant planet. Later, a planetary mass companion to
the star, Beta Pic b, was identified using imagery. Observations at different
epochs (2003 and 2009-2015) detected the planet at a projected distance of 6 to
9 AU from the star and showed that the planet is on an edge-on orbit. The
observed motion is consistent with an inferior conjunction in 1981, and Beta
Pic b can be the transiting planet proposed to explain the photometric event
observed at that time. Assuming that the 1981 event is related to the transit
or the inferior conjunction of Beta Pic b on an edge-on orbit, we search for
the planetary orbit in agreement with all the measurements of the planet
position published so far. We find two different orbits that are compatible
with all these constraints: (i) an orbit with a period of 17.970.08 years
along with an eccentricity of around 0.12 and (ii) an orbit with a period of
36.380.13 years and a larger eccentricity of about 0.32. In the near
future, new imaging observations should allow us to discriminate between these
two different orbits. We also estimate the possible dates for the next
transits, which could take place as early as 2017 or 2018, even for a
long-period orbit.Comment: Accepted for publication in A&
Hydrogen column density evaluations toward Capella: consequences on the interstellar deuterium abundance
The deuterium abundance evaluation in the direction of Capella has for a long
time been used as a reference for the local interstellar medium (ISM) within
our Galaxy. We show here that broad and weak HI components could be present on
the Capella line of sight, leading to a large new additional systematic
uncertainty on the N(HI) evaluation.
The D/H ratio toward Capella is found to be equal to 1.67 (+/-0.3)x10^-5 with
almost identical chi^2 for all the fits (this range includes only the
systematic error; the 2 sigma statistical one is almost negligible in
comparison). It is concluded that D/H evaluations over HI column densities
below 10^19 cm^-2 (even perhaps below 10^20 cm^-2 if demonstrated by additional
observations) may present larger uncertainties than previously anticipated. It
is mentionned that the D/O ratio might be a better tracer for DI variations in
the ISM as recently measured by the Far Ultraviolet Spectroscopic Explorer
(FUSE).Comment: Accepted for publication in the Astrophysical Journal Letter
Entanglement renormalization and gauge symmetry
A lattice gauge theory is described by a redundantly large vector space that
is subject to local constraints, and can be regarded as the low energy limit of
an extended lattice model with a local symmetry. We propose a numerical
coarse-graining scheme to produce low energy, effective descriptions of lattice
models with a local symmetry, such that the local symmetry is exactly preserved
during coarse-graining. Our approach results in a variational ansatz for the
ground state(s) and low energy excitations of such models and, by extension, of
lattice gauge theories. This ansatz incorporates the local symmetry in its
structure, and exploits it to obtain a significant reduction of computational
costs. We test the approach in the context of the toric code with a magnetic
field, equivalent to Z2 lattice gauge theory, for lattices with up to 16 x 16
sites (16^2 x 2 = 512 spins) on a torus. We reproduce the well-known ground
state phase diagram of the model, consisting of a deconfined and spin polarized
phases separated by a continuous quantum phase transition, and obtain accurate
estimates of energy gaps, ground state fidelities, Wilson loops, and several
other quantities.Comment: reviewed version as published in PRB; this version includes a new
section about the accuracy of the results several corrections and added
citation
Approximate transformations and robust manipulation of bipartite pure state entanglement
We analyze approximate transformations of pure entangled quantum states by
local operations and classical communication, finding explicit conversion
strategies which optimize the fidelity of transformation. These results allow
us to determine the most faithful teleportation strategy via an initially
shared partially entangled pure state. They also show that procedures for
entanglement manipulation such as entanglement catalysis [Jonathan and Plenio,
Phys. Rev. Lett. 83, 3566 (1999)] are robust against perturbation of the states
involved, and motivate the notion of non-local fidelity, which quantifies the
difference in the entangled properties of two quantum states.Comment: 11 pages, 4 figure
Open source environment to define constraints in route planning for GIS-T
Route planning for transportation systems is strongly related to shortest path algorithms, an optimization problem extensively studied in the literature. To find the shortest path in a network one usually assigns weights to each branch to represent the difficulty of taking such branch. The weights construct a linear preference function ordering the variety of alternatives from the most to the least attractive.Postprint (published version
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