726 research outputs found
Qubit Teleportation and Transfer across Antiferromagnetic Spin Chains
We explore the capability of spin-1/2 chains to act as quantum channels for
both teleportation and transfer of qubits. Exploiting the emergence of
long-distance entanglement in low-dimensional systems [Phys. Rev. Lett. 96,
247206 (2006)], here we show how to obtain high communication fidelities
between distant parties. An investigation of protocols of teleportation and
state transfer is presented, in the realistic situation where temperature is
included. Basing our setup on antiferromagnetic rotationally invariant systems,
both protocols are represented by pure depolarizing channels. We propose a
scheme where channel fidelity close to one can be achieved on very long chains
at moderately small temperature.Comment: 5 pages, 4 .eps figure
Data report: paleomagnetic and environmental magnetic properties of sediments from site 1202 (kuroshio current)
We present paleomagnetic and mineral magnetic results from ocean
sediments from the southern Okinawa Trough (west Pacific). We obtained
samples from two holes from Ocean Drilling Program Site 1202
and determined the natural remanent magnetization, magnetic susceptibility,
anhysteretic remanent magnetization (ARM), hysteresis properties,
and thermomagnetic behavior. Hole 1202A was studied between
100 and 120 meters below seafloor (mbsf) and Hole 1202B between 0
and 140 mbsf, both at 1-cm resolution. Hysteresis properties and thermomagnetic
behavior were measured on selected samples. The measurements
show a stable magnetization carried by pseudo-singledomain-
sized low-titanium magnetite. Magnetic inclinations are predominantly
positive and record the Brunhes (C1n) normal polarity
chron. Susceptibility and ARM, as well as the environmentally significant
rock magnetic ratios (ARM/k and ARM30 mT/ARM0 mT), reflect
changes in sediment input from Taiwan and the East China Sea continental
shelf changes in the path of the Kuroshio Current and changes
in climatic conditions
State-dependent TMS reveals representation of affective body movements in the anterior intraparietal cortex
In humans, recognition of others’ actions involves a cortical network that comprises, among other cortical regions, the posterior superior temporal sulcus (pSTS), where biological motion is coded and the anterior intraparietal suclus (aIPS), where movement information is elaborated in terms of meaningful goal directed actions. This action observation system (AOS) is thought to encode neutral voluntary actions, and possibly some aspects of affective motor repertoire, but the role of the AOS’ areas in processing affective kinematic information has never been examined. Here we investigated whether the action observation system plays a role in representing dynamic emotional bodily expressions. In the first experiment, we assessed behavioural adaptation effects of observed affective movements. Participants watched series of happy or fearful whole-body point-light displays (PLDs) as adapters and were then asked to perform an explicit categorization of the emotion expressed in test PLDs. Participants were slower when categorizing any of the two emotions as long as it was congruent with the emotion in the adapter sequence. We interpreted this effect as adaptation to the emotional content of PLDs. In the second experiment, we combined this paradigm with TMS applied over either the right aIPS, pSTS and the right half of the occipital pole (corresponding to Brodmann’s area 17 and serving as control) to examine the neural locus of the adaptation effect. TMS over the aIPS (but not over the other sites) reversed the behavioural cost of adaptation, specifically for fearful contents. This demonstrates that aIPS contains an explicit representation of affective body movements
CSI 2264: Simultaneous optical and X-ray variability in pre-Main Sequence stars. I: Time resolved X-ray spectral analysis during optical dips and accretion bursts in stars with disks
Pre-main sequence stars are variable sources. In stars with disks, this
variability is related to the morphology of the inner circumstellar region
(<0.1 AU) and that of the photosphere and corona, all impossible to be
spatially resolved with present day techniques. This has been the main
motivation for the Coordinated Synoptic Investigation of NGC 2264. In this
paper, we focus on the stars with disks. We analyze the X-ray spectral
properties extracted during optical bursts and dips in order to unveil the
nature of these phenomena. We analyze simultaneous CoRoT and Chandra/ACIS-I
observations to search for coherent optical and X-ray flux variability in stars
with disks. Then, stars are analyzed in two different samples. In stars with
variable extinction, we look for a simultaneous increase of optical extinction
and X-ray absorption during the optical dips; in stars with accretion bursts,
we search for soft X-ray emission and increasing X-ray absorption during the
bursts. Results. We find evidence for coherent optical and X-ray flux
variability among the stars with variable extinction. In 9/24 stars with
optical dips, we observe a simultaneous increase of X-ray absorption and
optical extinction. In seven dips, it is possible to calculate the NH/AV ratio
in order to infer the composition of the obscuring material. In 5/20 stars with
optical accretion bursts, we observe increasing soft X-ray emission during the
bursts that we associate to the emission of accreting gas. It is not surprising
that these properties are not observed in all the stars with dips and bursts,
since favorable geometric configurations are required. The observed variable
absorption during the dips is mainly due to dust-free material in accretion
streams. In stars with accretion bursts, we observe on average a larger soft
X-ray spectral component not observed in non accreting stars.Comment: Accepted for publication by Astronomy & Astrophysic
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
Rapidly-converging methods for the location of quantum critical points from finite-size data
We analyze in detail, beyond the usual scaling hypothesis, the finite-size
convergence of static quantities toward the thermodynamic limit. In this way we
are able to obtain sequences of pseudo-critical points which display a faster
convergence rate as compared to currently used methods. The approaches are
valid in any spatial dimension and for any value of the dynamic exponent. We
demonstrate the effectiveness of our methods both analytically on the basis of
the one dimensional XY model, and numerically considering c = 1 transitions
occurring in non integrable spin models. In particular, we show that these
general methods are able to locate precisely the onset of the
Berezinskii-Kosterlitz-Thouless transition making only use of ground-state
properties on relatively small systems.Comment: 9 pages, 2 EPS figures, RevTeX style. Updated to published versio
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
Fidelity approach to quantum phase transitions
We review briefly the quantum fidelity approach to quantum phase transitions
in a pedagogical manner. We try to relate all established but scattered results
on the leading term of the fidelity into a systematic theoretical framework,
which might provide an alternative paradigm for understanding quantum critical
phenomena. The definition of the fidelity and the scaling behavior of its
leading term, as well as their explicit applications to the one-dimensional
transverse-field Ising model and the Lipkin-Meshkov-Glick model, are introduced
at the graduate-student level. In addition, we survey also other types of
fidelity approach, such as the fidelity per site, reduced fidelity,
thermal-state fidelity, operator fidelity, etc; as well as relevant works on
the fidelity approach to quantum phase transitions occurring in various
many-body systems.Comment: 41 pages, 31 figures. We apologize if we omit acknowledging your
relevant works. Do tell. An updated version with clearer figures can be found
at: http://www.phy.cuhk.edu.hk/~sjgu/fidelitynote.pd
Stable particles in anisotropic spin-1 chains
Motivated by field-theoretic predictions we investigate the stable
excitations that exist in two characteristic gapped phases of a spin-1 model
with Ising-like and single-ion anisotropies. The sine-Gordon theory indicates a
region close to the phase boundary where a stable breather exists besides the
stable particles, that form the Haldane triplet at the Heisenberg isotropic
point. The numerical data, obtained by means of the Density Matrix
Renormalization Group, confirm this picture in the so-called large-D phase for
which we give also a quantitative analysis of the bound states using standard
perturbation theory. However, the situation turns out to be considerably more
intricate in the Haldane phase where, to the best of our data, we do not
observe stable breathers contrarily to what could be expected from the
sine-Gordon model, but rather only the three modes predicted by a novel
anisotropic extension of the Non-Linear Sigma Model studied here by means of a
saddle-point approximation.Comment: 8 pages, 7 eps figures, svjour clas
ISO-ChaI 52: a weakly-accreting young stellar object with a dipper light curve
We report on the discovery of periodic dips in the multiband lightcurve of
ISO-ChaI 52, a young stellar object in the Chamaeleon I dark cloud. This is one
among the peculiar objects that display very low or negligible accretion both
in their UV continuum and spectral lines, although they present a remarkable
infrared excess emission characteristic of optically-thick circumstellar disks.
We have analyzed a VLT/X-Shooter spectrum with the tool ROTFIT to determine the
stellar parameters. The latter, along with photometry from our campaign with
the REM telescope and from the literature, have allowed us to model the
spectral energy distribution and to estimate the size and temperature of the
inner and outer disk. From the rotational period of the star/disk system of
3.45 days we estimate a disk inclination of 36. The depth of the dips
in different bands has been used to gain information about the occulting
material. A single extinction law is not able to fit the observed behavior,
while a two-component model of a disk warp composed of a dense region with a
gray extinction and an upper layer with an ISM-type extinction provides a
better fit of the data.Comment: 11 pages, 11 figures; accepted for publication in Astronomy &
Astrophysic
- …