21,111 research outputs found
Comment on "High Field Studies of Superconducting Fluctuations in High-Tc Cuprates. Evidence for a Small Gap distinct from the Large Pseudogap"
By using high magnetic field data to estimate the background conductivity,
Rullier-Albenque and coworkers have recently published [Phys.Rev.B 84, 014522
(2011)] experimental evidence that the in-plane paraconductivity in cuprates is
almost independent of doping. In this Comment we also show that, in contrast
with their claims, these useful data may be explained at a quantitative level
in terms of the Gaussian-Ginzburg-Landau approach for layered superconductors,
extended by Carballeira and coworkers to high reduced-temperatures by
introducing a total-energy cutoff [Phys.Rev.B 63, 144515 (2001)]. When
combined, these two conclusions further suggest that the paraconductivity in
cuprates is conventional, i.e., associated with fluctuating superconducting
pairs above the mean-field critical temperature.Comment: 9 pages, 1 figur
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
Limit to the radio emission from a putative central compact source in SN1993J
SN1993J in M81 is the best studied young radio-luminous supernova in the
Northern Hemisphere. We recently reported results from the analysis of a
complete set of VLBI observations of this supernova at 1.7, 2.3, 5.0, and 8.4
GHz, covering a time baseline of more than one decade. Those reported results
were focused on the kinematics of the expanding shock, the particulars of its
evolving non-thermal emission, the density profile of the circumstellar medium,
and the evolving free-free opacity by the supernova ejecta. In the present
paper, we complete our analysis by performing a search for any possible signal
from a compact source (i.e., a stellar-mass black hole or a young pulsar
nebula) at the center of the expanding shell. We have performed a stacking of
all our VLBI images at each frequency, after subtraction of our best-fit shell
model at each epoch, and measured the peak intensity in the stacked residual
image. Given the large amount of available global VLBI observations, the
stacking of all the residual images allows us to put upper limits to the
eventual emission of a putative compact central source at the level of
Jy at 5 GHz (or, more conservatively, Jy, if we
make a further correction for the ejecta opacity) and somewhat larger at other
wavelengths.Comment: 4 pages, 3 figures. Accepted for publication in A&
Optimal conversion of non--local unitary operations
We study when a non--local unitary operation acting on two --level systems
can probabilistically simulate another one when arbitrary local operations and
classical communication are allowed. We provide necessary and sufficient
conditions for the simulation to be possible. Probabilistic interconvertability
is used to define an equivalence relation between gates. We show that this
relation induces a finite number of classes, that we identify. In the case of
two qubits, two classes of non--local operations exist. We choose the CNOT and
SWAP as representatives of these classes. We show how the CNOT [SWAP] can be
deterministically converted into any operation of its class. We also calculate
the optimal probability of obtaining the CNOT [SWAP] from any operation of the
corresponding class and provide a protocol to achieve this task.Comment: 4 pages, no figure
Time-optimal Hamiltonian simulation and gate synthesis using homogeneous local unitaries
Motivated by experimental limitations commonly met in the design of solid
state quantum computers, we study the problems of non-local Hamiltonian
simulation and non-local gate synthesis when only homogeneous local unitaries
are performed in order to tailor the available interaction. Homogeneous (i.e.
identical for all subsystems) local manipulation implies a more refined
classification of interaction Hamiltonians than the inhomogeneous case, as well
as the loss of universality in Hamiltonian simulation. For the case of
symmetric two-qubit interactions, we provide time-optimal protocols for both
Hamiltonian simulation and gate synthesis.Comment: 7 page
Storing quantum dynamics in quantum states: stochastic programmable gate for U(1) operations
We show how quantum dynamics can be captured in the state of a quantum
system, in such a way that the system can be used to stochastically perform, at
a later time, the stored transformation perfectly on some other quantum system.
Thus programmable quantum gates for quantum information processing are feasible
if some probability of failure -that we show to decrease exponentially with the
size of the storing resources- is allowed.Comment: RevTex, 4 pages, 3 figures. Extension of quant-ph/0012067, including
several results concerning optimality of the scheme for storage of operation
Ground state entanglement in quantum spin chains
A microscopic calculation of ground state entanglement for the XY and
Heisenberg models shows the emergence of universal scaling behavior at quantum
phase transitions. Entanglement is thus controlled by conformal symmetry. Away
from the critical point, entanglement gets saturated by a mass scale. Results
borrowed from conformal field theory imply irreversibility of entanglement loss
along renormalization group trajectories. Entanglement does not saturate in
higher dimensions which appears to limit the success of the density matrix
renormalization group technique. A possible connection between majorization and
renormalization group irreversibility emerges from our numerical analysis.Comment: 26 pages, 16 figures, added references, minor changes. Final versio
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