4,181 research outputs found
Slower-than-Light Spin-1/2 Particles Endowed with Negative Mass Squared
Extending in a straightforward way the standard Dirac theory, we study a
quantum mechanical wave-equation describing free spinning particles --which we
propose to call "Pseudotachyons" (PT's)-- which behave like tachyons in the
momentum space, but like subluminal particles (v<c) in the ordinary space. This
is allowed since, as it happens in every quantum theory for spin-1/2 particles,
the momentum operator (that is conserved) and the velocity operator (that is
not) are independent operators, which refer to independent quantities. As a
consequence, at variance with ordinary Dirac particles, for PT's the average
velocity is not equal to the classical velocity, but actually to the velocity
"dual" of the classical velocity. The speed of PT's is therefore smaller than
the speed of light. Since a lot of experimental data seems to involve a
negative mass squared for neutrinos, we suggest that these particles might be
PT's, travelling, because of their very small mass, at subluminal speeds very
close to c. The present theory is shown to be separately invariant under the C,
P, T transformations; the covariance under Lorentz transformations is also
proved. Furthermore, we derive the kinematical constraints linking 4-impulse,
4-velocity and 4-polarization of free PT'sComment: LaTeX; 20 page
Astrophysical constraints on unparticle-inspired models of gravity
We use stellar dynamics arguments to constrain the relevant parameters of
ungravity inspired models. We show that resulting bounds do constrain the
parameters of the theory of unparticles, as far as its energy scale satisfies
the condition and is close to unity.Comment: 5 pages, 4 figure
The ground state of a class of noncritical 1D quantum spin systems can be approximated efficiently
We study families H_n of 1D quantum spin systems, where n is the number of
spins, which have a spectral gap \Delta E between the ground-state and
first-excited state energy that scales, asymptotically, as a constant in n. We
show that if the ground state |\Omega_m> of the hamiltonian H_m on m spins,
where m is an O(1) constant, is locally the same as the ground state
|\Omega_n>, for arbitrarily large n, then an arbitrarily good approximation to
the ground state of H_n can be stored efficiently for all n. We formulate a
conjecture that, if true, would imply our result applies to all noncritical 1D
spin systems. We also include an appendix on quasi-adiabatic evolutions.Comment: 9 pages, 1 eps figure, minor change
Optimal discrimination of quantum operations
We address the problem of discriminating with minimal error probability two
given quantum operations. We show that the use of entangled input states
generally improves the discrimination. For Pauli channels we provide a complete
comparison of the optimal strategies where either entangled or unentangled
input states are used.Comment: 4 pages, no figure
Second-order Democratic Aggregation
Aggregated second-order features extracted from deep convolutional networks
have been shown to be effective for texture generation, fine-grained
recognition, material classification, and scene understanding. In this paper,
we study a class of orderless aggregation functions designed to minimize
interference or equalize contributions in the context of second-order features
and we show that they can be computed just as efficiently as their first-order
counterparts and they have favorable properties over aggregation by summation.
Another line of work has shown that matrix power normalization after
aggregation can significantly improve the generalization of second-order
representations. We show that matrix power normalization implicitly equalizes
contributions during aggregation thus establishing a connection between matrix
normalization techniques and prior work on minimizing interference. Based on
the analysis we present {\gamma}-democratic aggregators that interpolate
between sum ({\gamma}=1) and democratic pooling ({\gamma}=0) outperforming both
on several classification tasks. Moreover, unlike power normalization, the
{\gamma}-democratic aggregations can be computed in a low dimensional space by
sketching that allows the use of very high-dimensional second-order features.
This results in a state-of-the-art performance on several datasets
Gravitational Lensing Bound On The Average Redshift Of Gamma Ray Bursts In Models With Evolving Lenses
Identification of gravitationally lensed Gamma Ray Bursts (GRBs) in the BATSE
4B catalog can be used to constrain the average redshift of the GRBs.
In this paper we investigate the effect of evolving lenses on the of
GRBs in different cosmological models of universe. The cosmological parameters
$\Omega$ and $\Lambda$ have an effect on the of GRBs. The other factor
which can change the of GRBs is higher in evolving model of galaxies as compared to
non-evolving models of galaxies.Comment: 23 pages,one plain LaTeX file with three postscript figures This is
modified version with recent BATSE efficiency parameter and with the latest F
paramete
How did the discussion go: Discourse act classification in social media conversations
We propose a novel attention based hierarchical LSTM model to classify
discourse act sequences in social media conversations, aimed at mining data
from online discussion using textual meanings beyond sentence level. The very
uniqueness of the task is the complete categorization of possible pragmatic
roles in informal textual discussions, contrary to extraction of
question-answers, stance detection or sarcasm identification which are very
much role specific tasks. Early attempt was made on a Reddit discussion
dataset. We train our model on the same data, and present test results on two
different datasets, one from Reddit and one from Facebook. Our proposed model
outperformed the previous one in terms of domain independence; without using
platform-dependent structural features, our hierarchical LSTM with word
relevance attention mechanism achieved F1-scores of 71\% and 66\% respectively
to predict discourse roles of comments in Reddit and Facebook discussions.
Efficiency of recurrent and convolutional architectures in order to learn
discursive representation on the same task has been presented and analyzed,
with different word and comment embedding schemes. Our attention mechanism
enables us to inquire into relevance ordering of text segments according to
their roles in discourse. We present a human annotator experiment to unveil
important observations about modeling and data annotation. Equipped with our
text-based discourse identification model, we inquire into how heterogeneous
non-textual features like location, time, leaning of information etc. play
their roles in charaterizing online discussions on Facebook
Gravitational lensing constraint on the cosmic equation of state
Recent redshift-distance measurements of Type Ia supernovae (SNe Ia) at
cosmological distances suggest that two-third of the energy density of the
universe is dominated by dark energy component with an effective negative
pressure. This dark energy component is described by the equation of state
. We use gravitational lensing statistics to
constrain the equation of state of this dark energy. We use ,
image separation distribution function of lensed quasars, as a tool to probe
. We find that for the observed range of ,
should lie between in order to have five lensed quasars
in a sample of 867 optical quasars. This limit is highly sensitive to lens and
Schechter parameters and evolution of galaxies.Comment: Modified results and inclusion of calculations with new set of
parameter
A time-dependent Tsirelson's bound from limits on the rate of information gain in quantum systems
We consider the problem of distinguishing between a set of arbitrary quantum
states in a setting in which the time available to perform the measurement is
limited. We provide simple upper bounds on how well we can perform state
discrimination in a given time as a function of either the average energy or
the range of energies available during the measurement. We exhibit a specific
strategy that nearly attains this bound. Finally, we consider several
applications of our result. First, we obtain a time-dependent Tsirelson's bound
that limits the extent of the Bell inequality violation that can be in
principle be demonstrated in a given time t. Second, we obtain a
Margolus-Levitin type bound when considering the special case of distinguishing
orthogonal pure states.Comment: 15 pages, revtex, 1 figur
Using post-measurement information in state discrimination
We consider a special form of state discrimination in which after the
measurement we are given additional information that may help us identify the
state. This task plays a central role in the analysis of quantum cryptographic
protocols in the noisy-storage model, where the identity of the state
corresponds to a certain bit string, and the additional information is
typically a choice of encoding that is initially unknown to the cheating party.
We first provide simple optimality conditions for measurements for any such
problem, and show upper and lower bounds on the success probability. For a
certain class of problems, we furthermore provide tight bounds on how useful
post-measurement information can be. In particular, we show that for this class
finding the optimal measurement for the task of state discrimination with
post-measurement information does in fact reduce to solving a different problem
of state discrimination without such information. However, we show that for the
corresponding classical state discrimination problems with post-measurement
information such a reduction is impossible, by relating the success probability
to the violation of Bell inequalities. This suggests the usefulness of
post-measurement information as another feature that distinguishes the
classical from a quantum world.Comment: 10 pages, 4 figures, revtex, v2: published version, minor change
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