2,335 research outputs found
Macroscopically local correlations can violate information causality
Although quantum mechanics is a very successful theory, its foundations are
still a subject of intense debate. One of the main problems is the fact that
quantum mechanics is based on abstract mathematical axioms, rather than on
physical principles. Quantum information theory has recently provided new ideas
from which one could obtain physical axioms constraining the resulting
statistics one can obtain in experiments. Information causality and macroscopic
locality are two principles recently proposed to solve this problem. However
none of them were proven to define the set of correlations one can observe. In
this paper, we present an extension of information causality and study its
consequences. It is shown that the two above-mentioned principles are
inequivalent: if the correlations allowed by nature were the ones satisfying
macroscopic locality, information causality would be violated. This gives more
confidence in information causality as a physical principle defining the
possible correlation allowed by nature.Comment: are welcome. 6 pages, 4 figs. This is the originally submitted
version. The published version contains some bounds on quantum realizations
of d2dd isotropic boxes (table 1), found by T. Vertesi, who kindly shared
them with u
Dimension witnesses and quantum state discrimination
Dimension witnesses allow one to test the dimension of an unknown physical
system in a device-independent manner, that is, without placing assumptions
about the functioning of the devices used in the experiment. Here we present
simple and general dimension witnesses for quantum systems of arbitrary Hilbert
space dimension. Our approach is deeply connected to the problem of quantum
state discrimination, hence establishing a strong link between these two
research topics. Finally, our dimension witnesses can distinguish between
classical and quantum systems of the same dimension, making them potentially
useful for quantum information processing.Comment: 5 page
Variability in the Lambda Orionis cluster substellar domain
We present the first results on variability of very low mass stars and brown
dwarfs belonging to the 5 Myr Lambda Orionis cluster (Collinder 69). We have
monitored almost continuously in the J filter a small area of the cluster which
includes 12 possible members of the cluster during one night. Some members have
turned to be short-term variable. One of them, LOri167, has a mass close to the
planetary mass limit and its variability might be due to instabilities produced
by the deuterium burning, although other mechanism cannot be ruled out.Comment: Accepted for Astronomische Nachrichten. Workshop "Ultra-low-mass star
formation and evolution", La Palma June 200
VOSA: Virtual Observatory SED Analyzer. An application to the Collinder 69 open cluster
The physical properties of almost any kind of astronomical object can be
derived by fitting synthetic spectra or photometry extracted from theoretical
models to observational data.
We want to develop an automatic procedure to perform this kind of fittings to
a relatively large sample of members of a stellar association and apply this
methodology to the case of Collinder 69.
We combine the multiwavelength data of our sources and follow a work-flow to
derive the physical parameters of the sources. The key step of the work-flow is
performed by a new VO-tool, VOSA. All the steps in this process are done in a
VO environment.
We present this new tool, and provide physical parameters such as T, gravity, luminosity, etc. for 170 candidate members to Collinder
69, and an upper-limit for the age of this stellar association.
This kind of studies of star forming regions, clusters, etc. produces a huge
amount of data, very tedious to analyse using the traditional methodology.
Thus, they are excellent examples where to apply the VO capabilities.Comment: Accepted for publication in A&
Activation of Non-Local Quantum Resources
We find two two-qubit states such that any number of copies of one state or
the other cannot violate the CHSH Bell inequality. However, their tensor
product can produce a CHSH violation of at least 2.023. We also identify a
CHSH-local state such that two copies of it are CHSH-violating. The tools
employed here can be easily adapted to find instances of non-locality
activation in arbitrary Bell scenarios
Perturbations in Hybrid Loop Quantum Cosmology: Continuum Limit in Fourier Space
We analyze the passage to a continuum limit of the mode spectrum of
primordial perturbations around flat cosmological spacetimes in hybrid Loop
Quantum Cosmology, showing that this limit can be reached even if one starts by
considering a finite fiducial cell as spatial slice. We focus our attention on
regimes in which the background cosmology follows the effective dynamics of
Loop Quantum Cosmology, although we comment on extensions of our arguments
beyond this regime, as well as to some formalisms other than the hybrid
approach. Whereas the perturbed system can be described in an invariant way
under changes of the fiducial volume using the standard variables of the
improved prescription for Loop Quantum Cosmology, we show that the desired
continuum limit can be established by means of scaling transformations of the
physical volume when this volume grows unboundedly. These transformations lead
to a model with a continuum of modes and independent of any scale of reference
for the physical volume. For the sake of comparison, we also consider an
alternative road to the continuum in Fourier space that has been employed in
geometrodynamics and is based on the use of scaling transformations of the
fiducial volume, together with variables that are independent of them.Comment: 13 page
A numerical power spectrum for electroencephalographic processing
The record of the oscillations of the electric potential of the human brain provides useful information about the mind activity at rest and during the achievement of sensory and cognitive processing tasks. The use of appropriate quantitative tools assigning numerical values to the observed variable is necessary to define good descriptors of the electroencephalogram, allowing comparisons between different recordings. In this line, we propose a numerical method for the spectral and temporal reconstruction of a brain signal. The convergence of the procedure is analyzed, providing results of the concerned approximation error. In a second part of the text, we use the methodology described to the quantification of the bioelectric variations produced in the brain waves for the execution of a test of attention related to military simulation
A near-infrared survey for new low-mass members in Alpha Per
We present a near-infrared (K'-band) survey of 0.7 square degree area in the
Alpha Persei open cluster (age = 90 Myr, distance = 182 pc) carried out with
the Omega-Prime camera on the Calar Alto 3.5-m telescope. Combining optical
data (Rc and Ic) obtained with the KPNO/MOSA detector and presented in Stauffer
et al. (1999) with the K' observations, a sample of new candidate members has
been extracted from the optical-infrared colour-magnitude diagram. The location
of these candidates in the colour-colour diagram suggests that two-thirds of
them are actually reddened background giants. About 20 new candidate members
with masses between 0.3 and 0.04 Msun are added to the 400 known Alpha Per
cluster members. If they are indeed Alpha Per members, four of the new
candidates would be brown dwarfs. We discuss the advantages and drawbacks of
the near-infrared survey as compared to the optical selection method. We also
describe the outcome of optical spectroscopy obtained with the Twin
spectrograph on the Calar Alto 3.5-m telescope for about 30 candidates,
including selected members from the optical sample presented in Barrado y
Navascues et al. (2002) and from our joint optical/infrared catalogue. These
results argue in favour of the optical selection method for this particular
cluster.Comment: 13 pages, 10 figures, 3 tables and 1 electronic table, accepted for
publication in A&
Pure state estimation and the characterization of entanglement
A connection between the state estimation problem and the separability
problem is noticed and exploited to find efficient numerical algorithms to
solve the first one. Based on these ideas, we also derive a systematic method
to obtain upper bounds on the maximum local fidelity when the states are
distributed among several distant parties.Comment: Closer to published versio
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