5,996 research outputs found
Interplay between computable measures of entanglement and other quantum correlations
Composite quantum systems can be in generic states characterized not only by
entanglement, but also by more general quantum correlations. The interplay
between these two signatures of nonclassicality is still not completely
understood. In this work we investigate this issue focusing on computable and
observable measures of such correlations: entanglement is quantified by the
negativity N, while general quantum correlations are measured by the
(normalized) geometric quantum discord D_G. For two-qubit systems, we find that
the geometric discord reduces to the squared negativity on pure states, while
the relationship holds for arbitrary mixed states. The latter
result is rigorously extended to pure, Werner and isotropic states of two-qudit
systems for arbitrary d, and numerical evidence of its validity for arbitrary
states of a qubit and a qutrit is provided as well. Our results establish an
interesting hierarchy, that we conjecture to be universal, between two relevant
and experimentally friendly nonclassicality indicators. This ties in with the
intuition that general quantum correlations should at least contain and in
general exceed entanglement on mixed states of composite quantum systems.Comment: 10 pages, 4 figure
Extremal curves in nilpotent Lie groups
We classify extremal curves in free nilpotent Lie groups. The classification
is obtained via an explicit integration of the adjoint equation in Pontryagin
Maximum Principle. It turns out that abnormal extremals are precisely the
horizontal curves contained in algebraic varieties of a specific type. We also
extend the results to the nonfree case.Comment: 30 pages, final versio
The Hi-GAL compact source catalogue – I. The physical properties of the clumps in the inner Galaxy (−71∘.0 < ℓ < 67∘0)
Hi-GAL (Herschel InfraRed Galactic Plane Survey) is a large-scale survey of the Galactic plane, performed with Herschel in five infrared continuum bands between 70 and 500 μm. We present a band-merged catalogue of spatially matched sources and their properties derived from fits to the spectral energy distributions (SEDs) and heliocentric distances, based on the photometric catalogues presented in Molinari et al., covering the portion of Galactic plane −71∘.0 < ℓ < 67∘.0. The band-merged catalogue contains 100 922 sources with a regular SED, 24 584 of which show a 70-μm counterpart and are thus considered protostellar, while the remainder are considered starless. Thanks to this huge number of sources, we are able to carry out a preliminary analysis of early stages of star formation, identifying the conditions that characterize different evolutionary phases on a statistically significant basis. We calculate surface densities to investigate the gravitational stability of clumps and their potential to form massive stars. We also explore evolutionary status metrics such as the dust temperature, luminosity and bolometric temperature, finding that these are higher in protostellar sources compared to pre-stellar ones. The surface density of sources follows an increasing trend as they evolve from pre-stellar to protostellar, but then it is found to decrease again in the majority of the most evolved clumps. Finally, we study the physical parameters of sources with respect to Galactic longitude and the association with spiral arms, finding only minor or no differences between the average evolutionary status of sources in the fourth and first Galactic quadrants, or between ‘on-arm’ and ‘interarm’ positions
Chemical and kinematical properties of Galactic bulge stars surrounding the stellar system Terzan 5
As part of a study aimed at determining the kinematical and chemical
properties of Terzan 5, we present the first characterization of the bulge
stars surrounding this puzzling stellar system. We observed 615 targets located
well beyond the tidal radius of Terzan 5 and we found that their radial
velocity distribution is well described by a Gaussian function peaked at
=+21.0\pm4.6 km/s and with dispersion sigma_v=113.0\pm2.7 km/s. This is
the one of the few high-precision spectroscopic survey of radial velocities for
a large sample of bulge stars in such a low and positive latitude environment
(b=+1.7{\deg}). We found no evidence for the peak at \sim+200 km/s found
in Nidever et al. 2012. The strong contamination of many observed spectra by
TiO bands prevented us from deriving the iron abundance for the entire
spectroscopic sample, introducing a selection bias. The metallicity
distribution was finally derived for a sub-sample of 112 stars in a magnitude
range where the effect of the selection bias is negligible. The distribution is
quite broad and roughly peaked at solar metallicity ([Fe/H]\simeq+0.05 dex)
with a similar number of stars in the super-solar and in the sub-solar ranges.
The population number ratios in different metallicity ranges agree well with
those observed in other low-latitude bulge fields suggesting (i) the possible
presence of a plateau for |b|<4{\deg} for the ratio between stars in the
super-solar (0<[Fe/H]<0.5 dex) and sub-solar (-0.5<[Fe/H]<0 dex) metallicity
ranges; (ii) a severe drop of the metal-poor component ([Fe/H]<-0.5) as a
function of Galactic latitude.Comment: 27 pages, 9 figures, accepted for publication by Ap
Ceci n'est pas a globular cluster: the metallicity distribution of the stellar system Terzan 5
We present new determinations of the iron abundance for 220 stars belonging
to the stellar system Terzan 5 in the Galactic bulge. The spectra have been
acquired with FLAMES at the Very Large Telescope of the European Southern
Observatory and DEIMOS at the Keck II Telescope. This is by far the largest
spectroscopic sample of stars ever observed in this stellar system. From this
dataset, a subsample of targets with spectra unaffected by TiO bands was
extracted and statistically decontaminated from field stars. Once combined with
34 additional stars previously published by our group, a total sample of 135
member stars covering the entire radial extent of the system has been used to
determine the metallicity distribution function of Terzan 5. The iron
distribution clearly shows three peaks: a super-solar component at
[Fe/H] dex, accounting for 29% of the sample, a dominant sub-solar
population at [Fe/H] dex, corresponding to 62% of the total, and a
minor (6%) metal-poor component at [Fe/H] dex. Such a broad,
multi-modal metallicity distribution demonstrates that Terzan 5 is not a
genuine globular cluster but the remnant of a much more complex stellar system.Comment: 29 pages, 10 figures. Accepted for publication by Ap
Target annihilation by diffusing particles in inhomogeneous geometries
The survival probability of immobile targets, annihilated by a population of
random walkers on inhomogeneous discrete structures, such as disordered solids,
glasses, fractals, polymer networks and gels, is analytically investigated. It
is shown that, while it cannot in general be related to the number of distinct
visited points, as in the case of homogeneous lattices, in the case of bounded
coordination numbers its asymptotic behaviour at large times can still be
expressed in terms of the spectral dimension , and its exact
analytical expression is given. The results show that the asymptotic survival
probability is site independent on recurrent structures (),
while on transient structures () it can strongly depend on the
target position, and such a dependence is explicitly calculated.Comment: To appear in Physical Review E - Rapid Communication
Proper motions in Terzan 5: membership of the multi-iron sub-populations and first constrain to the orbit
By exploiting two sets of high-resolution images obtained with HST ACS/WFC
over a baseline of ~10 years we have measured relative proper motions of
~70,000 stars in the stellar system Terzan 5. The results confirm the
membership of the three sub-populations with different iron abudances
discovered in the system. The orbit of the system has been derived from a first
estimate of its absolute proper motion, obtained by using bulge stars as
reference. The results of the integration of this orbit within an axisymmetric
Galactic model exclude any external accretion origin for this cluster. Terzan 5
is known to have chemistry similar to the Galactic bulge; our findings support
a kinematic link between the cluster and the bulge, further strengthening the
possibility that Terzan 5 is the fossil remnant of one of the pristine clumps
that originated the bulge.Comment: 25 pages, 14 figures, accepted for publication by Ap
Polarization in the prompt emission of gamma-ray bursts and their afterglows
Synchrotron is considered the dominant emission mechanism in the production
of gamma-ray burst photons in the prompt as well as in the afterglow phase.
Polarization is a characteristic feature of synchrotron and its study can
reveal a wealth of information on the properties of the magnetic field and of
the energy distribution in gamma-ray burst jets. In this paper I will review
the theory and observations of gamma-ray bursts polarization. While the theory
is well established, observations have prove difficult to perform, due to the
weakness of the signal. The discriminating power of polarization observations,
however, cannot be overestimated.Comment: 16 pages, 9 figures, accepted for publication in the New Journal of
Physics focus issue on Gamma Ray Burst
Multi-time delay, multi-point Linear Stochastic Estimation of a cavity shear layer velocity from wall-pressure measurements
Multi-time-delay Linear Stochastic Estimation (MTD-LSE) technique is thoroughly described, focusing on its fundamental properties and potentialities. In the multi-time-delay ap- proach, the estimate of the temporal evolution of the velocity at a given location in the flow field is obtained from multiple past samples of the unconditional sources. The technique is applied to estimate the velocity in a cavity shear layer flow, based on wall-pressure measurements from multiple sensor
Measurement-induced disturbances and nonclassical correlations of Gaussian states
We study quantum correlations beyond entanglement in two-mode Gaussian states
of continuous variable systems, by means of the measurement-induced disturbance
(MID) and its ameliorated version (AMID). In analogy with the recent studies of
the Gaussian quantum discord, we define a Gaussian AMID by constraining the
optimization to all bi-local Gaussian positive operator valued measurements. We
solve the optimization explicitly for relevant families of states, including
squeezed thermal states. Remarkably, we find that there is a finite subset of
two-mode Gaussian states, comprising pure states, where non-Gaussian
measurements such as photon counting are globally optimal for the AMID and
realize a strictly smaller state disturbance compared to the best Gaussian
measurements. However, for the majority of two--mode Gaussian states the
unoptimized MID provides a loose overestimation of the actual content of
quantum correlations, as evidenced by its comparison with Gaussian discord.
This feature displays strong similarity with the case of two qubits. Upper and
lower bounds for the Gaussian AMID at fixed Gaussian discord are identified. We
further present a comparison between Gaussian AMID and Gaussian entanglement of
formation, and classify families of two-mode states in terms of their Gaussian
AMID, Gaussian discord, and Gaussian entanglement of formation. Our findings
provide a further confirmation of the genuinely quantum nature of general
Gaussian states, yet they reveal that non-Gaussian measurements can play a
crucial role for the optimized extraction and potential exploitation of
classical and nonclassical correlations in Gaussian states.Comment: 16 pages, 5 figures; new results added; to appear in Phys. Rev.
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