2,901 research outputs found
Single-copy entanglement in a gapped quantum spin chain
The single-copy entanglement of a given many-body system is defined [J.
Eisert and M. Cramer, Phys. Rev. A. 72, 042112 (2005)] as the maximal
entanglement deterministically distillable from a bipartition of a single
specimen of that system. For critical (gapless) spin chains, it was recently
shown that this is exactly half the von Neumann entropy [R. Orus, J. I.
Latorre, J. Eisert, and M. Cramer, Phys. Rev. A 73, 060303(R) (2006)], itself
defined as the entanglement distillable in the asymptotic limit: i.e. given an
infinite number of copies of the system. It is an open question as to what the
equivalent behaviour for gapped systems is. In this paper, I show that for the
paradigmatic spin-S Affleck-Kennedy-Lieb-Tasaki chain (the archetypal gapped
chain), the single-copy entanglement is equal to the von Neumann entropy: i.e.
all the entanglement present may be distilled from a single specimen.Comment: Typos corrected; accepted for publication in Phys. Rev. Lett.;
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VLBI observations of SN 2008iz: I. Expansion velocity and limits on anisotropic expansion
We present observations of the recently discovered supernova 2008iz in M82
with the VLBI High Sensitivity Array at 22 GHz, the Very Large Array at
frequencies of 1.4, 4.8, 8.4, 22 and 43 GHz, and the Chandra X-ray observatory.
The supernova was clearly detected on two VLBI images, separated by 11 months.
The source shows a ring-like morphology and expands with a velocity of ~23000
km/s. The most likely explosion date is in mid February 2008. The measured
expansion speed is a factor of ~2 higher than expected under the assumption
that synchrotron self-absorption dominates the light curve at the peak,
indicating that this absorption mechanism may not be important for the radio
emission. We find no evidence for an asymmetric explosion. The VLA spectrum
shows a broken power law, indicating that the source was still optically thick
at 1.4 GHz in April 2009. Finally, we report upper limits on the X-ray emission
from SN 2008iz and a second radio transient recently discovered by MERLIN
observations.Comment: accepted Astronomy & Astrophysics, 9 pages, 8 figures, also available
at http://www.mpifr-bonn.mpg.de/staff/abrunthaler/pub.shtm
8.4GHz VLBI observations of SN2004et in NGC6946
We report on 8.4GHz Very Long Baseline Interferometry (VLBI) observations of
the type II-P supernova SN2004et in the spiral galaxy NGC 6946, made on 20
February 2005 (151 days after explosion). The Very Large Array (VLA) flux
density was 1.230.07 mJy, corresponding to an isotropic luminosity at
8.4GHz of (4.450.3) erg s Hz and a brightness
temperature of (1.30.3) K. We also provide an improved
source position, accurate to about 0.5 mas in each coordinate. The VLBI image
shows a clear asymmetry. From model fitting of the size of the radio emission,
we estimate a minimum expansion velocity of 15,7002,000 km s. This
velocity is more than twice the expected mean expansion velocity estimated from
a synchrotron self-absorbed emission model, thus suggesting that synchrotron
self-absorption is not relevant for this supernova. With the benefit of an
optical spectrum obtained 12 days after explosion, we favor an emission model
which consists of two hot spots on an underlying expanding shell of width
comparable to that of SN 1993J.Comment: Accepted for publication in A&A (22/05/07
The nuclear starburst in Arp 299-A: From the 5.0 GHz VLBI radio light-curves to its core-collapse supernova rate
The nuclear region of the Luminous Infra-red Galaxy Arp 299-A hosts a recent
( Myr), intense burst of massive star formation which is expected to
lead to numerous core-collapse supernovae (CCSNe). Previous VLBI observations,
carried out with the EVN at 5.0 GHz and with the VLBA at 2.3 and 8.4 GHz,
resulted in the detection of a large number of compact, bright, non-thermal
sources in a region \lsim150 pc in size. We aim at establishing the nature of
all non-thermal, compact components in Arp 299-A, as well as estimating its
core-collapse supernova rate. We use multi-epoch European VLBI Network (EVN)
observations taken at 5.0 GHz to image with milliarcsecond resolution the
compact radio sources in the nuclear region of Arp 299-A. We also use one
single-epoch 5.0 GHz Multi-Element Radio Linked Interferometer Network (MERLIN)
observation to image the extended emission in which the compact radio sources
--traced by our EVN observations-- are embedded. Twenty-six compact sources are
detected, 8 of them are new objects not previously detected. The properties of
all detected objects are consistent with them being a mixed population of CCSNe
and SNRs. We find clear evidence for at least two new CCSNe, implying a lower
limit to the CCSN rate of \nu_{\rm SN}\gsim0.80 SN/yr indicating that the
bulk of the current star formation in Arp 299-A is taking place in the
innermost pc. Our MERLIN observations trace a region of diffuse,
extended emission which is co-spatial to the region where all compact sources
are found. From this diffuse, non-thermal radio emission we obtain an
independent estimate for the core-collapse supernova rate, which is in the
range - 0.65 SN/yr, roughly in agreement with previous
estimates and our direct estimate of the CCSN rate from the compact radio
emission.Comment: 13 pages, 5 figures, accepted for publication on Astronomy &
Astrophysic
Further results on the cross norm criterion for separability
In the present paper the cross norm criterion for separability of density
matrices is studied. In the first part of the paper we determine the value of
the greatest cross norm for Werner states, for isotropic states and for Bell
diagonal states. In the second part we show that the greatest cross norm
criterion induces a novel computable separability criterion for bipartite
systems. This new criterion is a necessary but in general not a sufficient
criterion for separability. It is shown, however, that for all pure states, for
Bell diagonal states, for Werner states in dimension d=2 and for isotropic
states in arbitrary dimensions the new criterion is necessary and sufficient.
Moreover, it is shown that for Werner states in higher dimensions (d greater
than 2), the new criterion is only necessary.Comment: REVTeX, 19 page
23 GHz VLBI Observations of SN 2008ax
We report on phase-referenced 23 GHz Very-Long-Baseline-Interferometry (VLBI)
observations of the type IIb supernova SN 2008ax, made with the Very Long
Baseline Array (VLBA) on 2 April 2008 (33 days after explosion). These
observations resulted in a marginal detection of the supernova. The total flux
density recovered from our VLBI image is 0.80.3 mJy (one standard
deviation). As it appears, the structure may be interpreted as either a
core-jet or a double source. However, the supernova structure could be somewhat
confused with a possible close by noise peak. In such a case, the recovered
flux density would decrease to 0.480.12 mJy, compatible with the flux
densities measured with the VLA at epochs close in time to our VLBI
observations. The lowest average expansion velocities derived from our
observations are km s (case of a double
source) and km s (taking the weaker source
component as a spurious, close by, noise peak, which is the more likely
interpretation). These velocities are 7.3 and 2 times higher, respectively,
than the maximum ejecta velocity inferred from optical-line observations.Comment: 4 pages, 3 figures. Accepted in A&A on 24/03/200
The impact of Charlson comorbidity index on the functional capacity of COVID-19 survivors: a prospective cohort study with one-year follow-up
Objective: To determine the association between the Charlson comorbidity index (CCI) score after discharge with 6-min walk test (6MWT) 1 year after discharge in a cohort of COVID-19 survivors. Methods: In this prospective study, data were collected from a consecutive sample of patients hospitalized for COVID-19. The CCI score was calculated from the comorbidity data. The main outcome was the distance walked in the 6MWT at 1 year after discharge. Associations between CCI and meters covered in the 6MWT were assessed through crude and adjusted linear regressions. The model was adjusted for possible confounding factors (sex, days of hospitalization, and basal physical capacity through sit-to-stand test one month after discharge). Results: A total of 41 patients were included (mean age 58.8 +/- 12.7 years, 20/21 men/women). A significant association was observed between CCI and 6MWT (meters): (i) crude model: beta = -18.7, 95% CI = -34.7 to -2.6, p < 0.05; (ii) model adjusted for propensity score including sex, days of hospitalization, and sit-to-stand: beta = -23.0, 95% CI = -39.1 to -6.8, p < 0.05. Conclusions: A higher CCI score after discharge indicates worse performance on the 6MWT at 1-year follow-up in COVID-19 survivors. The CCI score could also be used as a screening tool to make important clinical decisions
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