3,102 research outputs found
Freezing distributed entanglement in spin chains
We show how to freeze distributed entanglement that has been created from the
natural dynamics of spin chain systems. The technique that we propose simply
requires single-qubit operations and isolates the entanglement in specific
qubits at the ends of branches. Such frozen entanglement provides a useful
resource, for example for teleportation or distributed quantum processing. The
scheme can be applied to a wide range of systems -- including actual spin
systems and alternative qubit embodiments in strings of quantum dots, molecules
or atoms.Comment: 5 pages, to appear in Phys. Rev. A (Rapid Communication
Entanglement from density measurements: analytical density-functional for the entanglement of strongly correlated fermions
We derive an analytical density functional for the single-site entanglement
of the one-dimensional homogeneous Hubbard model, by means of an approximation
to the linear entropy. We show that this very simple density functional
reproduces quantitatively the exact results. We then use this functional as
input for a local density approximation to the single-site entanglement of
inhomogeneous systems. We illustrate the power of this approach in a
harmonically confined system, which could simulate recent experiments with
ultracold atoms in optical lattices as well as in a superlattice and in an
impurity system. The impressive quantitative agreement with numerical
calculations -- which includes reproducing subtle signatures of the particle
density stages -- shows that our density-functional can provide entanglement
calculations for actual experiments via density measurements. Next we use our
functional to calculate the entanglement in disordered systems. We find that,
at contrast with the expectation that disorder destroys the entanglement, there
exist regimes for which the entanglement remains almost unaffected by the
presence of disordered impurities.Comment: 6 pages, 3 figure
Effect of confinement potential geometry on entanglement in quantum dot-based nanostructures
We calculate the spatial entanglement between two electrons trapped in a
nanostructure for a broad class of confinement potentials, including single and
double quantum dots, and core-shell quantum dot structures.
By using a parametrized confinement potential, we are able to switch from one
structure to the others with continuity and to analyze how the entanglement is
influenced by the changes in the confinement geometry. We calculate the
many-body wave function by `exact' diagonalization of the time independent
Schr\"odinger equation. We discuss the relationship between the entanglement
and specific cuts of the wave function, and show that the wave function at a
single highly symmetric point could be a good indicator for the entanglement
content of the system. We analyze the counterintuitive relationship between
spatial entanglement and Coulomb interaction, which connects maxima (minima) of
the first to minima (maxima) of the latter. We introduce a potential quantum
phase transition which relates quantum states characterized by different
spatial topology. Finally we show that by varying shape, range and strength of
the confinement potential, it is possible to induce strong and rapid variations
of the entanglement between the two electrons. This property may be used to
tailor nanostructures according to the level of entanglement required by a
specific application.Comment: 10 pages, 8 figures and 1 tabl
Geometry induced entanglement transitions in nanostructures
We model quantum dot nanostructures using a one-dimensional system of two
interacting electrons. We show that strong and rapid variations may be induced
in the spatial entanglement by varying the nanostructure geometry. We
investigate the position-space information entropy as an indicator of the
entanglement in this system. We also consider the expectation value of the
Coulomb interaction and the ratio of this expectation to the expectation of the
confining potential and their link to the entanglement. We look at the first
derivative of the entanglement and the position-space information entropy to
infer information about a possible quantum phase transition.Comment: 3 pages, 2 figures, to appear in Journal of Applied Physic
Comparing persistence diagrams through complex vectors
The natural pseudo-distance of spaces endowed with filtering functions is
precious for shape classification and retrieval; its optimal estimate coming
from persistence diagrams is the bottleneck distance, which unfortunately
suffers from combinatorial explosion. A possible algebraic representation of
persistence diagrams is offered by complex polynomials; since far polynomials
represent far persistence diagrams, a fast comparison of the coefficient
vectors can reduce the size of the database to be classified by the bottleneck
distance. This article explores experimentally three transformations from
diagrams to polynomials and three distances between the complex vectors of
coefficients.Comment: 11 pages, 4 figures, 2 table
Dry grasslands of Hippocrepido glaucae-Stipion austroitalicae in the Pollino Massif (Calabria, Italy)
Rocky pastures dominated by Stipa austroitalica in the south-east of Italy were classified within an endemic alliance, Hippocrepido glaucae-Stipion austroitalicae, originally assigned to a Balkan order
(Scorzoneretalia villosae). Actually, the distribution area of S. austroitalica extends further westwards and large patches are found on the south-east side of the Pollino Massif. This study aims to describe and characterise the plant communities dominated by S. austroitalica in this area and analyse their floristic and chorological relationships with other associations of Hippocrepido-Stipion. Moreover, their syntaxonomy is discussed in the context of the Italian and south European dry grasslands biogeography. The grasslands were studied on the basis of 19 phytosociological relevés. A larger data set, including 185 relevés with S. austroitalica, was used to visualise the relationships among the associations through nonmetric multi-dimensional scaling ordination. The results allowed the description of a new association, Bupleuro gussonei-Stipetum austroitalicae, classifi ed within Hippocrepido-Stipion. As a consequence, the alliance synrange was extended up to the Pollino Massif. The Hip pocrepido-Stipion, together with Cytiso spinescentis-Bromion erecti, was arranged in Euphorbietalia myrsinitidis, an endemic order of the Italian peninsula. The proposed scheme upgrades the syntaxonomy and nomenclature of the dry grasslands vegetation of central and southern Italy
The Timing of Nine Globular Cluster Pulsars
We have used the Robert C. Byrd Green Bank Telescope to time nine previously
known pulsars without published timing solutions in the globular clusters M62,
NGC 6544, and NGC 6624. We have full timing solutions that measure the spin,
astrometric, and (where applicable) binary parameters for six of these pulsars.
The remaining three pulsars (reported here for the first time) were not
detected enough to establish solutions. We also report our timing solutions for
five pulsars with previously published solutions, and find good agreement with
past authors, except for PSR J1701-3006B in M62. Gas in this system is probably
responsible for the discrepancy in orbital parameters, and we have been able to
measure a change in the orbital period over the course of our observations.
Among the pulsars with new solutions we find several binary pulsars with very
low mass companions (members of the so-called "black widow" class) and we are
able to place constraints on the mass-to-light ratio in two clusters. We
confirm that one of the pulsars in NGC 6624 is indeed a member of the rare
class of non-recycled pulsars found in globular clusters. We also have measured
the orbital precession and Shapiro delay for a relativistic binary in NGC 6544.
If we assume that the orbital precession can be described entirely by general
relativity, which is likely, we are able to measure the total system mass
(2.57190(73) M_sun) and companion mass (1.2064(20) M_sun), from which we derive
the orbital inclination [sin(i) = 0.9956(14)] and the pulsar mass (1.3655(21)
M_sun), the most precise such measurement ever obtained for a millisecond
pulsar. The companion is the most massive known around a fully recycled pulsar.Comment: Published in ApJ; 33 pages, 5 figures, 7 table
GMRT Discovery of A Millisecond Pulsar in a Very Eccentric Binary System
We report the discovery of the binary millisecond pulsar J0514-4002A, which
is the first known pulsar in the globular cluster NGC 1851 and the first pulsar
discovered using the Giant Metrewave Radio Telescope (GMRT). The pulsar has a
rotational period of 4.99 ms, an orbital period of 18.8 days, and the most
eccentric pulsar orbit yet measured (e = 0.89). The companion has a minimum
mass of 0.9 M_sun and its nature is presently unclear. After accreting matter
from a low-mass companion star which spun it up to a (few) millisecond spin
period, the pulsar eventually exchanged the low-mass star for its more massive
present companion. This is exactly the same process that could form a system
containing a millisecond pulsar and a black hole; the discovery of NGC 1851A
demonstrates that such systems might exist in the Universe, provided that
stellar mass black holes exist in globular clusters.Comment: 12 pages (referee format), 3 figures, accepted for publication in
Astrophysical Journal Letter
Intrinsic electric field effects on few-particle interactions in coupled GaN quantum dots
We study the multi-exciton optical spectrum of vertically coupled GaN/AlN
quantum dots with a realistic three-dimensional direct-diagonalization approach
for the description of few-particle Coulomb-correlated states. We present a
detailed analysis of the fundamental properties of few-particle/exciton
interactions peculiar of nitride materials. The giant intrinsic electric fields
and the high electron/hole effective masses give rise to different effects
compared to GaAs-based quantum dots: intrinsic exciton-exciton coupling,
non-molecular character of coupled dot exciton wavefunction, strong dependence
of the oscillator strength on the dot height, large ground state energy shift
for dots separated by different barriers. Some of these effects make GaN/AlN
quantum dots interesting candidates in quantum information processing.Comment: 23 pages, 8 figures, 1 tabl
Massive Cosmologies
We explore the cosmological solutions of a recently proposed extension of
General Relativity with a Lorentz-invariant mass term. We show that the same
constraint that removes the Boulware-Deser ghost in this theory also prohibits
the existence of homogeneous and isotropic cosmological solutions.
Nevertheless, within domains of the size of inverse graviton mass we find
approximately homogeneous and isotropic solutions that can well describe the
past and present of the Universe. At energy densities above a certain crossover
value, these solutions approximate the standard FRW evolution with great
accuracy. As the Universe evolves and density drops below the crossover value
the inhomogeneities become more and more pronounced. In the low density regime
each domain of the size of the inverse graviton mass has essentially non-FRW
cosmology. This scenario imposes an upper bound on the graviton mass, which we
roughly estimate to be an order of magnitude below the present-day value of the
Hubble parameter. The bound becomes especially restrictive if one utilizes an
exact self-accelerated solution that this theory offers. Although the above are
robust predictions of massive gravity with an explicit mass term, we point out
that if the mass parameter emerges from some additional scalar field
condensation, the constraint no longer forbids the homogeneous and isotropic
cosmologies. In the latter case, there will exist an extra light scalar field
at cosmological scales, which is screened by the Vainshtein mechanism at
shorter distances.Comment: 21 page
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