11,256 research outputs found
Separating the classical and quantum information via quantum cloning
An application of quantum cloning to optimally interface a quantum system
with a classical observer is presented, in particular we describe a procedure
to perform a minimal disturbance measurement on a single qubit by adopting a
1->2 cloning machine followed by a generalized measurement on a single clone
and the anti-clone or on the two clones. Such scheme has been applied to
enhance the transmission fidelity over a lossy quantum channel.Comment: 4 pages, 2figure
Capacity analysis of suburban rail networks
As is well known, capacity evaluation and the identification of bottlenecks on rail networks are complex issues depending upon several technical elements. This is even more perceptible in metropolitan areas where different services (freight, long distance, metro/regional, etc.) are operated using the same limited infrastructures; as a consequence, these facilities may represent bottlenecks of the rail system since they are often highly utilized and congested. This paper tries to explore the issue of capacity evaluation of complex rail networks, proposing synthetic indicators
and analyses for feasibility studies or strategic planning. The presented methodology suggests taking into account the main differences in infrastructure characteristics (e.g. single or double lines, signalling systems, terminus or passing stations, etc.) and rail services (e.g. diverse rolling stock, various frequencies, average distances and number of stops, etc.) in order to propose a general approach applicable for capacity analysis of a network as a whole, hence evaluating the utilization rate and the congestion on both lines and stations. To better explore and
validate the methodology, an application to a line of the Naples’ suburban network is presented. The results confirm the applicability and effectiveness of the proposed approach; the outcomes indicate the capacity utilization rate of the considered facilities, pointing out likely bottlenecks and possible actions to improve the system efficiency
Experimental Purification of Single Qubits
We report the experimental realization of the purification protocol for
single qubits sent through a depolarization channel. The qubits are associated
with polarization encoded photon particles and the protocol is achieved by
means of passive linear optical elements. The present approach may represent a
convenient alternative to the distillation and error correction protocols of
quantum information.Comment: 10 pages, 2 figure
BeppoSAX LECS background subtraction techniques
We present 3 methods for the subtraction of non-cosmic and unresolved cosmic
backgrounds observed by the Low-Energy Concentrator Spectrometer (LECS)
on-board BeppoSAX. Removal of these backgrounds allows a more accurate modeling
of the spectral data from point and small-scale extended sources. At high
(>|25| degree) galactic latitudes, subtraction using a standard background
spectrum works well. At low galactic latitudes, or in complex regions of the
X-ray sky, two alternative methods are presented. The first uses counts
obtained from two semi-annuli near the outside of the LECS field of view to
estimate the background at the source location. The second method uses ROSAT
Position Sensitive Proportional Counter (PSPC) all-sky survey data to estimate
the LECS background spectrum for a given pointing position. A comparison of the
results from these methods provides an estimate of the systematic
uncertainties. For high galactic latitude fields, all 3 methods give 3 sigma
confidence uncertainties of <0.9 10^-3 count/s (0.1-10 keV), or <1.5 10^-3
count/s (0.1-2 keV). These correspond to 0.1-2.0 keV fluxes of 0.7-1.8 and
0.5-1.1 10^-13 erg/cm2/s for a power-law spectrum with a photon index of 2 and
photoelectric absorption of 3 10^20 and 3 10^21 atom/cm2, respectively. At low
galactic latitudes, or in complex regions of the X-ray sky, the uncertainties
are a factor ~2.5 higher.Comment: 13 pages. Accepted for publication in A&A
Low EUV Luminosities Impinging on Protoplanetary Disks
The amount of high-energy stellar radiation reaching the surface of
protoplanetary disks is essential to determine their chemistry and physical
evolution. Here, we use millimetric and centimetric radio data to constrain the
EUV luminosity impinging on 14 disks around young (~2-10Myr) sun-like stars.
For each object we identify the long-wavelength emission in excess to the dust
thermal emission, attribute that to free-free disk emission, and thereby
compute an upper limit to the EUV reaching the disk. We find upper limits lower
than 10 photons/s for all sources without jets and lower than photons/s for the three older sources in our sample. These latter
values are low for EUV-driven photoevaporation alone to clear out
protoplanetary material in the timescale inferred by observations. In addition,
our EUV upper limits are too low to reproduce the [NeII] 12.81 micron
luminosities from three disks with slow [NeII]-detected winds. This indicates
that the [NeII] line in these sources primarily traces a mostly neutral wind
where Ne is ionized by 1 keV X-ray photons, implying higher photoevaporative
mass loss rates than those predicted by EUV-driven models alone. In summary,
our results suggest that high-energy stellar photons other than EUV may
dominate the dispersal of protoplanetary disks around sun-like stars.Comment: Accepted for publication to The Astrophysical Journa
On Fermionic T-duality of Sigma modes on AdS backgrounds
We study the fermionic T-duality symmetry of integrable Green-Schwarz sigma
models on AdS backgrounds. We show that the sigma model on
background is self-dual under fermionic T-duality. We also construct new
integrable sigma models on . These backgrounds could be
realized as supercosets of SU supergroups for arbitrary , but could also be
realized as supercosets of OSp supergroups for . We find that the
supercosets based on SU supergroups are self-dual under fermionic T-duality,
while the supercosets based on OSp supergroups are not. However, the reasons of
OSp supercosets being not self-dual under fermionic T-duality are different.
For case, corresponding to background, the
failure is due to the singular fermionic quadratic terms, just like
case. For case, the failure is due to the
shortage of right number of -symmetry to gauge away the fermionic
degrees of freedom, even though the fermionic quadratic term is not singular
any more. More general, for the supercosets of the OSp supergroups with
superalgebra , including and
backgrounds, the sigma models are not self-dual under fermionic T-duality as
well, obstructed by the -symmetry.Comment: 17 pages; Clarfications on kappa symmetries, references
added;Published versio
Resolved Young Binary Systems And Their Disks
We have conducted a survey of young single and multiple systems in the Taurus–Auriga star-forming region with the Atacama Large Millimeter Array (ALMA), substantially improving both the spatial resolution and sensitivity with which individual protoplanetary disks in these systems have been observed. These ALMA observations can resolve binary separations as small as 25–30 au and have an average 3σ detection level of 0.35 mJy, equivalent to a disk mass of 4 × 10−5 M ⊙ for an M3 star. Our sample was constructed from stars that have an infrared excess and/or signs of accretion and have been classified as Class II. For the binary and higher-order multiple systems observed, we detect λ = 1.3 mm continuum emission from one or more stars in all of our target systems. Combined with previous surveys of Taurus, our 21 new detections increase the fraction of millimeter-detected disks to over 75% in all categories of stars (singles, primaries, and companions) earlier than spectral type M6 in the Class II sample. Given the wealth of other information available for these stars, this has allowed us to study the impact of multiplicity with a much larger sample. While millimeter flux and disk mass are related to stellar mass as seen in previous studies, we find that both primary and secondary stars in binary systems with separations of 30–4200 au have lower values of millimeter flux as a function of stellar mass than single stars. We also find that for these systems, the circumstellar disk around the primary star does not dominate the total disk mass in the system and contains on average 62% of the total mass
Resolved Young Binary Systems And Their Disks
We have conducted a survey of young single and multiple systems in the Taurus–Auriga star-forming region with the Atacama Large Millimeter Array (ALMA), substantially improving both the spatial resolution and sensitivity with which individual protoplanetary disks in these systems have been observed. These ALMA observations can resolve binary separations as small as 25–30 au and have an average 3σ detection level of 0.35 mJy, equivalent to a disk mass of 4 × 10−5 M ⊙ for an M3 star. Our sample was constructed from stars that have an infrared excess and/or signs of accretion and have been classified as Class II. For the binary and higher-order multiple systems observed, we detect λ = 1.3 mm continuum emission from one or more stars in all of our target systems. Combined with previous surveys of Taurus, our 21 new detections increase the fraction of millimeter-detected disks to over 75% in all categories of stars (singles, primaries, and companions) earlier than spectral type M6 in the Class II sample. Given the wealth of other information available for these stars, this has allowed us to study the impact of multiplicity with a much larger sample. While millimeter flux and disk mass are related to stellar mass as seen in previous studies, we find that both primary and secondary stars in binary systems with separations of 30–4200 au have lower values of millimeter flux as a function of stellar mass than single stars. We also find that for these systems, the circumstellar disk around the primary star does not dominate the total disk mass in the system and contains on average 62% of the total mass
The Second INTEGRAL AGN Catalogue
The INTEGRAL mission provides a large data set for studying the hard X-ray
properties of AGN and allows testing of the unified scheme for AGN. We present
analysis of INTEGRAL IBIS/ISGRI, JEM-X, and OMC data for 199 AGN supposedly
detected by INTEGRAL above 20 keV. The data analysed here allow a significant
spectral extraction on 148 objects and an optical variability study of 57 AGN.
The slopes of the hard X-ray spectra of Seyfert 1 and Seyfert~2 galaxies are
found to be consistent within the uncertainties, whereas higher cut-off
energies and lower luminosities are measured for the more absorbed / type 2
AGN. The intermediate Seyfert 1.5 objects exhibit hard X-ray spectra consistent
with those of Seyfert 1. When applying a Compton reflection model, the
underlying continua appear the same in Seyfert 1 and 2 with photon index 2, and
the reflection strength is about R = 1, when assuming different inclination
angles. A significant correlation is found between the hard X-ray and optical
luminosity and the mass of the central black hole in the sense that the more
luminous objects appear to be more massive. There is also a general trend
toward the absorbed sources and type 2 AGN having lower Eddington ratios. The
black holemass appears to form a fundamental plane together with the optical
and X-ray luminosity of the form Lv being proportional to Lx^0.6 M^0.2, similar
to that found between radio luminosity Lr, Lx, and M. The unified model for
Seyfert galaxies seems to hold, showing in hard X-rays that the central engine
is the same in Seyfert 1 and 2, but seen under different inclination angles and
absorption. (Abridged)Comment: 26 pages, 16 figures, accepted for publication in A&A. Corrections by
language editor included in version
Influence of the Lower Hybrid Drift Instability on the onset of Magnetic Reconnection
Two-dimensional and three-dimensional kinetic simulation results reveal the
importance of the Lower-Hybrid Drift Instability LHDI to the onset of magnetic
reconnection. Both explicit and implicit kinetic simulations show that the LHDI
heats electrons anisotropically and increases the peak current density. Linear
theory predicts these modifications can increase the growth rate of the tearing
instability by almost two orders of magnitude and shift the fastest growing
modes to significantly shorter wavelengths. These predictions are confirmed by
nonlinear kinetic simulations in which the growth and coalescence of small
scale magnetic islands leads to a rapid onset of large scale reconnection
- …