2,018 research outputs found
Testing excitation models of rapidly oscillating Ap stars with interferometry
Rapidly oscillating Ap stars are unique objects in the potential they offer
to study the interplay between a number of important physical phenomena, in
particular, pulsations, magnetic fields, diffusion, and convection.
Nevertheless, the simple understanding of how the observed pulsations are
excited in these stars is still in progress. In this work we perform a test to
what is possibly the most widely accepted excitation theory for this class of
stellar pulsators. The test is based on the study of a subset of members of
this class for which stringent data on the fundamental parameters are available
thanks to interferometry. For three out of the four stars considered in this
study, we find that linear, non-adiabatic models with envelope convection
suppressed around the magnetic poles can reproduce well the frequency region
where oscillations are observed. For the fourth star in our sample no agreement
is found, indicating that a new excitation mechanism must be considered. For
the three stars whose observed frequencies can be explained by the excitation
models under discussion, we derive the minimum angular extent of the region
where convection must be suppressed. Finally, we find that the frequency
regions where modes are expected to be excited in these models is very
sensitive to the stellar radius. This opens the interesting possibility of
determining this quantity and related ones, such as the effective temperature
or luminosity, from comparison between model predictions and observations, in
other targets for which these parameters are not well determined.Comment: Accepted for publication in the MNRA
Entangled inputs cannot make imperfect quantum channels perfect
Entangled inputs can enhance the capacity of quantum channels, this being one
of the consequences of the celebrated result showing the non-additivity of
several quantities relevant for quantum information science. In this work, we
answer the converse question (whether entangled inputs can ever render noisy
quantum channels have maximum capacity) to the negative: No sophisticated
entangled input of any quantum channel can ever enhance the capacity to the
maximum possible value; a result that holds true for all channels both for the
classical as well as the quantum capacity. This result can hence be seen as a
bound as to how "non-additive quantum information can be". As a main result, we
find first practical and remarkably simple computable single-shot bounds to
capacities, related to entanglement measures. As examples, we discuss the qubit
amplitude damping and identify the first meaningful bound for its classical
capacity.Comment: 5 pages, 2 figures, an error in the argument on the quantum capacity
corrected, version to be published in the Physical Review Letter
Solos do campo experimental da Embrapa Milho e Sorgo: suas características e classificação no novo Sistema Brasileiro.
bitstream/CNPS/11831/1/bpd05_2002_milho_sorgo.pd
Schmidt balls around the identity
Robustness measures as introduced by Vidal and Tarrach [PRA, 59, 141-155]
quantify the extent to which entangled states remain entangled under mixing.
Analogously, we introduce here the Schmidt robustness and the random Schmidt
robustness. The latter notion is closely related to the construction of Schmidt
balls around the identity. We analyse the situation for pure states and provide
non-trivial upper and lower bounds. Upper bounds to the random Schmidt-2
robustness allow us to construct a particularly simple distillability
criterion. We present two conjectures, the first one is related to the radius
of inner balls around the identity in the convex set of Schmidt number
n-states. We also conjecture a class of optimal Schmidt witnesses for pure
states.Comment: 7 pages, 1 figur
Are all maximally entangled states pure?
We study if all maximally entangled states are pure through several
entanglement monotones. In the bipartite case, we find that the same conditions
which lead to the uniqueness of the entropy of entanglement as a measure of
entanglement, exclude the existence of maximally mixed entangled states. In the
multipartite scenario, our conclusions allow us to generalize the idea of
monogamy of entanglement: we establish the \textit{polygamy of entanglement},
expressing that if a general state is maximally entangled with respect to some
kind of multipartite entanglement, then it is necessarily factorized of any
other system.Comment: 5 pages, 1 figure. Proof of theorem 3 corrected e new results
concerning the asymptotic regime include
Short-reach MCF-based systems employing KK Receivers and feedforward neural networks for ICXT mitigation
This paper proposes and evaluates the use of machine learning (ML) techniques for mitigating the effect of the random inter-core crosstalk (ICXT) on 256 Gb/s short-reach systems employing weakly coupled multicore fiber (MCF) and Kramers–Kronig (KK) receivers. The performance improvement provided by the k-means clustering, k nearest neighbor (KNN) and feedforward neural network (FNN) techniques are assessed and compared with the system performance obtained without employing ML. The FNN proves to significantly improve the system performance by mitigating the impact of the ICXT on the received signal. This is achieved by employing only 10 neurons in the hidden layer and four input features for the training phase. It has been shown that k-means or KNN techniques do not provide performance improvement compared to the system without using ML. These conclusions are valid for direct detection MCF-based short-reach systems with the product between the skew (relative time delay between cores) and the symbol rate much lower than one (skew×symbol rate≪1). By employing the proposed FNN, the bit error rate (BER) always stood below 10−1.8 on all the time fractions under analysis (compared with 100 out of 626 occurrences above the BER threshold when ML was not used). For the BER threshold of 10−1.8 and compared with the standard system operating without employing ML techniques, the system operating with the proposed FNN shows a received optical power improvement of almost 3 dB.info:eu-repo/semantics/publishedVersio
The quantum one-time pad in the presence of an eavesdropper
A classical one-time pad allows two parties to send private messages over a
public classical channel -- an eavesdropper who intercepts the communication
learns nothing about the message. A quantum one-time pad is a shared quantum
state which allows two parties to send private messages or private quantum
states over a public quantum channel. If the eavesdropper intercepts the
quantum communication she learns nothing about the message. In the classical
case, a one-time pad can be created using shared and partially private
correlations. Here we consider the quantum case in the presence of an
eavesdropper, and find the single letter formula for the rate at which the two
parties can send messages using a quantum one-time pad
Caracterização da ictiofauna durante as estações de inverno e verão na ressaca da Lagoa dos Índios, Macapá - AP.
Resumo simples
Efficient and feasible state tomography of quantum many-body systems
We present a novel method to perform quantum state tomography for
many-particle systems which are particularly suitable for estimating states in
lattice systems such as of ultra-cold atoms in optical lattices. We show that
the need for measuring a tomographically complete set of observables can be
overcome by letting the state evolve under some suitably chosen random circuits
followed by the measurement of a single observable. We generalize known results
about the approximation of unitary 2-designs, i.e., certain classes of random
unitary matrices, by random quantum circuits and connect our findings to the
theory of quantum compressed sensing. We show that for ultra-cold atoms in
optical lattices established techniques like optical super-lattices, laser
speckles, and time-of-flight measurements are sufficient to perform fully
certified, assumption-free tomography. Combining our approach with tensor
network methods - in particular the theory of matrix-product states - we
identify situations where the effort of reconstruction is even constant in the
number of lattice sites, allowing in principle to perform tomography on
large-scale systems readily available in present experiments.Comment: 10 pages, 3 figures, minor corrections, discussion added, emphasizing
that no single-site addressing is needed at any stage of the scheme when
implemented in optical lattice system
Experimental implementation of a NMR entanglement witness
Entanglement witnesses (EW) allow the detection of entanglement in a quantum
system, from the measurement of some few observables. They do not require the
complete determination of the quantum state, which is regarded as a main
advantage. On this paper it is experimentally analyzed an entanglement witness
recently proposed in the context of Nuclear Magnetic Resonance (NMR)
experiments to test it in some Bell-diagonal states. We also propose some
optimal entanglement witness for Bell-diagonal states. The efficiency of the
two types of EW's are compared to a measure of entanglement with tomographic
cost, the generalized robustness of entanglement. It is used a GRAPE algorithm
to produce an entangled state which is out of the detection region of the EW
for Bell-diagonal states. Upon relaxation, the results show that there is a
region in which both EW fails, whereas the generalized robustness still shows
entanglement, but with the entanglement witness proposed here with a better
performance
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