8,099 research outputs found
Observing gravitational lensing effects by Sgr A* with GRAVITY
The massive black hole at the Galactic center Sgr A* is surrounded by a
cluster of stars orbiting around it. Light from these stars is bent by the
gravitational field of the black hole, giving rise to several phenomena:
astrometric displacement of the primary image, the creation of a secondary
image that may shift the centroid of Sgr A*, magnification effects on both
images. The near-to-come second generation VLTI instrument GRAVITY will perform
observations in the Near Infrared of the Galactic Center at unprecedented
resolution, opening the possibility of observing such effects. Here we
investigate the observability limits for GRAVITY of gravitational lensing
effects on the S-stars in the parameter space [DLS,gamma,K], where DLS is the
distance between the lens and the source, gamma is the alignment angle of the
source, and K is the source apparent magnitude in the K-band. The easiest
effect to be observed in the next years is the astrometric displacement of
primary images. In particular the shift of the star S17 from its Keplerian
orbit will be detected as soon as GRAVITY becomes operative. For exceptional
configurations it will be possible to detect effects related to the spin of the
black hole or Post-Newtonian orders in the deflection.Comment: 29 pages, 9 figures, in press on Ap
Gravitational Lensing of stars in the central arcsecond of our Galaxy
In the neighborhood of Sgr A*, several stars (S2, S12, S14, S1, S8, S13)
enjoy an accurate determination of their orbital parameters. General Relativity
predicts that the central black hole acts as a gravitational lens on these
stars, generating a secondary image and two infinite series of relativistic
images. For each of these six stars, we calculate the light curves for the
secondary and the first two relativistic images, in the Schwarzschild black
hole hypothesis, throughout their orbital periods. The curves are peaked around
the periapse epoch, but two subpeaks may arise in nearly edge-on orbits, when
the source is behind or in front of Sgr A*. We show that for most of these
stars the secondary image should be observable during its brightness peak. In
particular, S14 is the best candidate, since its secondary image reaches K=23.3
with an angular separation of 0.125 mas from the apparent horizon of the
central black hole. The detection of such images by future instruments could
represent the first observation of gravitational lensing beyond the weak field
approximation.Comment: 28 pages, 9 figures, in press on Ap
Time Delay in Black Hole Gravitational Lensing as a Distance Estimator
We calculate the time delay between different relativistic images formed by
black hole gravitational lensing in the strong field limit. For spherically
symmetric black holes, it turns out that the time delay between the first two
images is proportional to the minimum impact angle. Their ratio gives a very
interesting and precise measure of the distance of the black hole. Moreover,
using also the separation between the images and their luminosity ratio, it is
possible to extract the mass of the black hole. The time delay for the black
hole at the center of our Galaxy is just few minutes, but for supermassive
black holes with M=10^8 - 10^9 solar masses in the neighbourhood of the Local
Group the time delay amounts to few days, thus being measurable with a good
accuracy.Comment: 8 pages, 3 figure
Quantum tomography and nonlocality
We present a tomographic approach to the study of quantum nonlocality in
multipartite systems. Bell inequalities for tomograms belonging to a generic
tomographic scheme are derived by exploiting tools from convex geometry. Then,
possible violations of these inequalities are discussed in specific tomographic
realizations providing some explicit examples.Comment: 23 pages, 2 figures, contribution to the special issue of Physica
Scripta celebrating 150 years of Margarita and Vladimir I. Man'k
Radiation Pressure Induced Einstein-Podolsky-Rosen Paradox
We demonstrate the appearance of Einstein-Podolsky-Rosen (EPR) paradox when a
radiation field impinges on a movable mirror. The, the possibility of a local
realism test within a pendular Fabry-Perot cavity is shown to be feasible.Comment: 4 pages ReVTeX, 1 eps figur
RADIS: Remote Attestation of Distributed IoT Services
Remote attestation is a security technique through which a remote trusted
party (i.e., Verifier) checks the trustworthiness of a potentially untrusted
device (i.e., Prover). In the Internet of Things (IoT) systems, the existing
remote attestation protocols propose various approaches to detect the modified
software and physical tampering attacks. However, in an interoperable IoT
system, in which IoT devices interact autonomously among themselves, an
additional problem arises: a compromised IoT service can influence the genuine
operation of other invoked service, without changing the software of the
latter. In this paper, we propose a protocol for Remote Attestation of
Distributed IoT Services (RADIS), which verifies the trustworthiness of
distributed IoT services. Instead of attesting the complete memory content of
the entire interoperable IoT devices, RADIS attests only the services involved
in performing a certain functionality. RADIS relies on a control-flow
attestation technique to detect IoT services that perform an unexpected
operation due to their interactions with a malicious remote service. Our
experiments show the effectiveness of our protocol in validating the integrity
status of a distributed IoT service.Comment: 21 pages, 10 figures, 2 table
Methods for Estimating Capacities and Rates of Gaussian Quantum Channels
Optimization methods aimed at estimating the capacities of a general Gaussian
channel are developed. Specifically evaluation of classical capacity as maximum
of the Holevo information is pursued over all possible Gaussian encodings for
the lossy bosonic channel, but extension to other capacities and other Gaussian
channels seems feasible. Solutions for both memoryless and memory channels are
presented. It is first dealt with single use (single-mode) channel where the
capacity dependence from channel's parameters is analyzed providing a full
classification of the possible cases. Then it is dealt with multiple uses
(multi-mode) channel where the capacity dependence from the (multi-mode)
environment state is analyzed when both total environment energy and
environment purity are fixed. This allows a fair comparison among different
environments, thus understanding the role of memory (inter-mode correlations)
and phenomenon like superadditivity of the capacity. The developed methods are
also used for deriving transmission rates with heterodyne and homodyne
measurements at the channel output. Classical capacity and transmission rates
are presented within a unique framework where the rates can be treated as
logarithmic approximations of the capacity.Comment: 39 pages, 30 figures. New results and graphs were added. Errors and
misprints were corrected. To appear in IEEE Trans. Inf. T
Reconstructing the density operator by using generalized field quadratures
The Wigner function for one and two-mode quantum systems is explicitely
expressed in terms of the marginal distribution for the generic linearly
transformed quadratures. Then, also the density operator of those systems is
written in terms of the marginal distribution of these quadratures. Some
examples to apply this formalism, and a reduction to the usual optical homodyne
tomography are considered.Comment: 17 pages, Latex,accepted by Quantum and Semiclassical Optic
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