10,328 research outputs found
Optimal quantum repeaters for qubits and qudits
A class of optimal quantum repeaters for qubits is suggested. The schemes are
minimal, i.e. involve a single additional probe qubit, and optimal, i.e.
provide the maximum information adding the minimum amount of noise. Information
gain and state disturbance are quantified by fidelities which, for our schemes,
saturate the ultimate bound imposed by quantum mechanics for randomly
distributed signals. Special classes of signals are also investigated, in order
to improve the information-disturbance trade-off. Extension to higher
dimensional signals (qudits) is straightforward.Comment: Revised version. To appear in PR
X-ray Dichroism and the Pseudogap Phase of Cuprates
A recent polarized x-ray absorption experiment on the high temperature
cuprate superconductor Bi2Sr2CaCu2O8 indicates the presence of broken parity
symmetry below the temperature, T*, where a pseudogap appears in photoemission.
We critically analyze the x-ray data, and conclude that a parity-breaking
signal of the kind suggested is unlikely based on the crystal structures
reported in the literature. Possible other origins of the observed dichroism
signal are discussed. We propose x-ray scattering experiments that can be done
in order to determine whether such alternative interpretations are valid or
not.Comment: final version to be published in Phys Rev B: some calculational
details added, clarification of XNLD contamination and biaxiality, more
discussion on possible space groups and previous optics result
Photon subtracted states and enhancement of nonlocality in the presence of noise
We address nonlocality of continuous variable systems in the presence of
dissipation and noise. Three nonlocality tests have been considered, based on
the measurement of displaced-parity, field-quadrature and pseudospin-operator,
respectively. Nonlocality of twin beam has been investigated, as well as that
of its non-Gaussian counterparts obtained by inconclusive subtraction of
photons. Our results indicate that: i) nonlocality of twin beam is degraded but
not destroyed by noise; ii) photon subtraction enhances nonlocality in the
presence of noise, especially in the low-energy regime.Comment: 12 pages, 7 figure
The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk
We analyze high quality abundances data of solar neighborhood stars and show
that there are two distinct regimes of [alpha/Fe] versus age which we identify
as the epochs of the thick and thin disk formation. A tight correlation between
metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk
stars, implying that this population formed from a well mixed ISM, over a time
scale of 4-5 Gyr. Thick disk stars vertical velocity dispersion correlate with
age, with the youngest objects having as small scale heights as those of thin
disk stars. A natural consequence of these two results is that a vertical
metallicity gradient is expected in this population. We suggest that the thick
disk set the initial conditions for the formation of the inner thin disk. This
provides also an explanation of the apparent coincidence between the step in
metallicity at 7-10 kpc in the thin disk and the confinment of the thick disk
at about R<10 kpc. We suggest that the outer thin disk developped outside the
influence of the thick disk, but also that the high alpha-enrichment of the
outer regions may originate from a primordial pollution by the gas expelled
from the thick disk. Local metal-poor thin disk stars, whose properties are
best explained by an origin in the outer disk, are shown to be as old as the
youngest thick disk (9-10 Gyr), implying that the outer thin disk started to
form while the thick disk formation was still on-going in the inner Galaxy. We
point out that, given the tight age-abundance relations in the thick disk, an
inside-out process would give rise to a radial gradient in abundances in this
population which is not observed. Finally, we argue that the data discussed
here leave little room for radial migration, either to have contaminated the
solar vicinity, or, to have redistributed stars in significant proportion
across the solar annulus.Comment: Accepted in A&A, Revised version with new figures and extended
discussio
Propagation of UHECRs in cosmological backgrounds: some results from SimProp
Ultra-High-Energy Cosmic Ray (UHECR) nuclei propagating in cosmological
radiation backgrounds produce secondary particles detectable at Earth. SimProp
is a one dimensional code for extragalactic propagation of UHECR nuclei,
inspired by the kinetic approach of Aloisio et al. As in this approach, only a
subset of nuclei and nuclear channels are used as representative. We discuss
the validation of the code and present applications to UHECR experimental
results. In particular we present the expected fluxes of neutrinos produced in
some astrophysical scenario.Comment: Poster presented by A. Di Matteo at the 33rd International Cosmic Ray
Conference, Rio De Janeiro (Brasil) July 2-9 201
Cosmogenic neutrinos and ultra-high energy cosmic ray models
We use an updated version of {\it SimProp}, a Monte Carlo simulation scheme
for the propagation of ultra-high energy cosmic rays, to compute cosmogenic
neutrino fluxes expected on Earth in various scenarios. These fluxes are
compared with the newly detected IceCube events at PeV energies and with recent
experimental limits at EeV energies of the Pierre Auger Observatory. This
comparison allows us to draw some interesting conclusions about the source
models for ultra-high energy cosmic rays. We will show how the available
experimental observations are almost at the level of constraining such models,
mainly in terms of the injected chemical composition and cosmological evolution
of sources. The results presented here will also be important in the evaluation
of the discovery capabilities of the future planned ultra-high energy cosmic
ray and neutrino observatories.Comment: 15 pages, 8 figures, some reference added, version accepted for
publication in JCA
When the Milky Way turned off the lights: APOGEE provides evidence of star formation quenching in our Galaxy
Quenching, the cessation of star formation, is one of the most significant
events in the life cycle of galaxies. We show here the first evidence that the
Milky Way experienced a generalised quenching of its star formation at the end
of its thick disk formation 9 Gyr ago. Elemental abundances of stars
studied as part of the APOGEE survey reveal indeed that in less than 2
Gyr the star formation rate in our Galaxy dropped by an order-of-magnitude.
Because of the tight correlation between age and alpha abundance, this event
reflects in the dearth of stars along the inner disk sequence in the
[Fe/H]-[/Fe] plane. Before this phase, which lasted about 1.5 Gyr, the
Milky Way was actively forming stars. Afterwards, the star formation resumed at
a much lower level to form the thin disk. These events are very well matched by
the latest observation of MW-type progenitors at high redshifts. In late type
galaxies, quenching is believed to be related to a long and secular exhaustion
of gas. In our Galaxy, it occurred on a much shorter time scale, while the
chemical continuity before and after the quenching indicates that it was not
due to the exhaustion of the gas. While quenching is generally associated with
spheroids, our results show that it also occurs in galaxies like the Milky Way,
possibly when they are undergoing a morphological transition from thick to thin
disks. Given the demographics of late type galaxies in the local universe, in
which classical bulges are rare, we suggest further that this may hold true
generally in galaxies with mass lower than or approximately , where
quenching could be directly a consequence of thick disk formation. We emphasize
that the quenching phase in the Milky Way could be contemporaneous with, and
related to, the formation of the bar. We sketch a scenario on how a strong bar
may inhibit star formation.Comment: 17 pages, 8 figures. Published versio
Ouabain-induced cytoplasmic vesicles and their role in cell volume maintenance
Cellular swelling is controlled by an active mechanism of cell volume regulation driven by a Na+/K+-dependent ATPase and by
aquaporins which translocate water along the osmotic gradient. Na+/K+-pump may be blocked by ouabain, a digitalic derivative,
by inhibition of ATP, or by drastic ion alterations of extracellular fluid. However, it has been observed that some tissues are still able
to control their volume despite the presence of ouabain, suggesting the existence of other mechanisms of cell volume control. In
1977, by correlating electron microscopy observation with ion and water composition of liver slices incubated in differentmetabolic
conditions in the presence or absence of ouabain, we observed that hepatocytes were able to control their volume extruding water
and recovering ion composition in the presence of ouabain. In particular, hepatocytes were able to sequester ions and water in
intracellular vesicles and then secrete themat the bile canaliculus pole.We named this âvesicularmechanismof cell volume control.â
Afterward, thismechanism has been confirmed by us and other laboratories in several mammalian tissues.This review summarizes
evidences regarding this mechanism, problems that are still pending, and questions that need to be answered. Finally, we shortly
review the importance of cell volume control in some human pathological conditions
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