29,196 research outputs found
Submillimeter polarization and variability of quasar PKS 1830-211
Polarization from active galactic nuclei is interpreted as a signpost of the
role of magnetic fields in the launch and collimation of their relativistic
radio jets. Here, we report the detection of a clear polarization signal from
ALMA observations of the gravitationally lensed quasar PKS 1830-211 at
submillimeter wavelengths (Band 9, 650 GHz). Applying a
differential-polarimetry technique to the two compact lensed images of the
quasar, we estimate a fractional polarization of ~5% for one lensed image,
while the other appears nearly unpolarized, which implies that the polarization
activity varies on a timescale of a few weeks. With additional ALMA Band 7 and
8 (between 300-500 GHz) concomitant data, we constrain a Faraday rotation of a
few rad m. We also observe flux-density variability of ~10%
within one hour in Band 9. This work illustrates that a differential analysis
can extract high-accuracy information (flux-density ratio and polarimetry) free
of calibration issues from resolved sources in the submillimeter domain.Comment: 7 pages, 6 figures, accepted for publications in A&
Using gravitational lensed images to investigate the intrinsic AGN variability
We discuss about how the relative flux densities among the images of
gravitationally-lensed active galactic nuclei, AGN, can be used to study the
intrinsic AGN variability with high accuracy. Multi-frequency monitoring
observations of resolved gravitational lenses can allow us to detect signals of
very weak variability and also provide information about the jet opacity and
structure. As an example, we investigate the variability of the flux-density
ratio between the two lensed images of the blazar B0218+357, using
dual-frequency cm-wave observations. Similar to our previously reported
submm-wave observations of the lensed blazar PKS1830-211, we observe a clear
chromatic variability, starting short before an increase in the flux-density of
the blazar. The evolution of the flux-density ratios between the blazar images
shows a more clear and rich structure than that of the mere lightcurves of each
individual image. The accuracy in the ratio measurements is allowing us to see
variability episodes in the blazar that are weaker than the natural scatter in
the absolute flux-density measurements. A simple opacity model in the jet is
used to consistently explain the difference between the flux-density-ratio
evolution at the two frequencies.Comment: 5 pages, 2 figures. Accepted for publication in A&A. Final versio
Quantum phase transitions in fully connected spin models: an entanglement perspective
We consider a set of fully connected spins models that display first- or
second-order transitions and for which we compute the ground-state entanglement
in the thermodynamical limit. We analyze several entanglement measures
(concurrence, R\'enyi entropy, and negativity), and show that, in general,
discontinuous transitions lead to a jump of these quantities at the transition
point. Interestingly, we also find examples where this is not the case.Comment: 9 pages, 7 figures, published versio
Strong monotonicity in mixed-state entanglement manipulation
A strong entanglement monotone, which never increases under local operations
and classical communications (LOCC), restricts quantum entanglement
manipulation more strongly than the usual monotone since the usual one does not
increase on average under LOCC. We propose new strong monotones in mixed-state
entanglement manipulation under LOCC. These are related to the decomposability
and 1-positivity of an operator constructed from a quantum state, and reveal
geometrical characteristics of entangled states. These are lower bounded by the
negativity or generalized robustness of entanglement.Comment: 6 pages and 1 figure. A brief discussion about the connection to
asymptotic distillability was adde
The joys of permutation symmetry: direct measurements of entanglement
So-called direct measurements of entanglement are collective measurements on
multiple copies of a (bipartite or multipartite) quantum system that directly
provide one a value for some entanglement measure, such as the concurrence for
bipartite states. Multiple copies are needed since the entanglement of a mixed
state is not a linear function of the density matrix. Unfortunately, so far all
experimental implementations of direct measurements made unverified assumptions
about the form of the states, and, therefore, do not qualify as entanglement
verification tests. I discuss how a direct measurement can be turned into a
quantitative entanglement verification test by exploiting a recent theorem by
Renner (R. Renner, Nature Physics 3, 645 (2007)).Comment: 4 pages, 3 figure
Creation and Manipulation of Anyons in the Kitaev Model
We analyze the effect of local spin operators in the Kitaev model on the
honeycomb lattice. We show, in perturbation around the isolated-dimer limit,
that they create Abelian anyons together with fermionic excitations which are
likely to play a role in experiments. We derive the explicit form of the
operators creating and moving Abelian anyons without creating fermions and show
that it involves multi-spin operations. Finally, the important experimental
constraints stemming from our results are discussed.Comment: 4 pages, 3 figures, published versio
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