14,261 research outputs found
A Variational Algorithm for Bayesian Variable Selection
There has been an intense development on the estimation of a sparse
regression coefficient vector in statistics, machine learning and related
fields. In this paper, we focus on the Bayesian approach to this problem, where
sparsity is incorporated by the so-called spike-and-slab prior on the
coefficients. Instead of replying on MCMC for posterior inference, we propose a
fast and scalable algorithm based on variational approximation to the posterior
distribution. The updating scheme employed by our algorithm is different from
the one proposed by Carbonetto and Stephens (2012). Those changes seem crucial
for us to show that our algorithm can achieve asymptotic consistency even when
the feature dimension diverges exponentially fast with the sample size.
Empirical results have demonstrated the effectiveness and efficiency of the
proposed algorithm
Discriminative Similarity for Clustering and Semi-Supervised Learning
Similarity-based clustering and semi-supervised learning methods separate the
data into clusters or classes according to the pairwise similarity between the
data, and the pairwise similarity is crucial for their performance. In this
paper, we propose a novel discriminative similarity learning framework which
learns discriminative similarity for either data clustering or semi-supervised
learning. The proposed framework learns classifier from each hypothetical
labeling, and searches for the optimal labeling by minimizing the
generalization error of the learned classifiers associated with the
hypothetical labeling. Kernel classifier is employed in our framework. By
generalization analysis via Rademacher complexity, the generalization error
bound for the kernel classifier learned from hypothetical labeling is expressed
as the sum of pairwise similarity between the data from different classes,
parameterized by the weights of the kernel classifier. Such pairwise similarity
serves as the discriminative similarity for the purpose of clustering and
semi-supervised learning, and discriminative similarity with similar form can
also be induced by the integrated squared error bound for kernel density
classification. Based on the discriminative similarity induced by the kernel
classifier, we propose new clustering and semi-supervised learning methods
Speeding Up Neural Machine Translation Decoding by Cube Pruning
Although neural machine translation has achieved promising results, it
suffers from slow translation speed. The direct consequence is that a trade-off
has to be made between translation quality and speed, thus its performance can
not come into full play. We apply cube pruning, a popular technique to speed up
dynamic programming, into neural machine translation to speed up the
translation. To construct the equivalence class, similar target hidden states
are combined, leading to less RNN expansion operations on the target side and
less \$\mathrm{softmax}\$ operations over the large target vocabulary. The
experiments show that, at the same or even better translation quality, our
method can translate faster compared with naive beam search by \$3.3\times\$ on
GPUs and \$3.5\times\$ on CPUs.Comment: 11pages, 11 figures, EMNLP-2018 conferenc
Critical Current Anomaly at the Topological Quantum Phase Transition in a Majorana Josephson Junction
Majorana bound states in topological Josephson junctions induce a 4? period
current-phase relation. Direct detection of the 4? periodicity is complicated
by the quasiparticle poisoning. We reveal that Majorana bound states are also
signaled by the anomalous enhancement on the critical current of the junction.
We show the landscape of the critical current for a nanowire Josephson junction
under a varying Zeeman field, and reveal a sharp step feature at the
topological quantum phase transition point, which comes from the anomalous
enhancement of the critical current at the topological regime. In multi-band
wires, the anomalous enhancement disappears for an even number of bands, where
the Majorana bound states fuse into Andreev bound states. This anomalous
critical current enhancement directly signals the existence of the Majorana
bound states, and also provides a valid signature for the topological quantum
phase transition.Comment: 20 pages, 4 figure
Landau-Zener-St\"uckelberg Interferometry for Majorana Qubit
Stimulated by a very recent experiment observing successfully two
superconducting states with even- and odd-number of electrons in a nanowire
topological superconductor as expected from the existence of two end Majorana
quasiparticles (MQs) [Albrecht \textit{et al.}, Nature \textbf{531}, 206
(2016)], we propose a way to manipulate Majorana qubit exploiting quantum
tunneling effects. The prototype setup consists of two one-dimensional (1D)
topological superconductors coupled by a tunneling junction which can be
controlled by gate voltage. We show that, upon current injection, the time
evolution of superconducting phase difference at the junction induces an
oscillation in energy levels of the Majorana parity states, whereas the
level-crossing is avoided by a small coupling energy of MQs in the individual
1D superconductors. This results in a Landau-Zener-St\"{u}ckelberg (LZS)
interference between the Majorana parity states. Adjusting the current pulse
and gate voltage, one can build a LZS interferometry which provides an
arbitrary manipulation of the Majorana qubit. The LZS rotation of Majorana
qubit can be monitored by the microwave radiated from the junction
VMAV-C: A Deep Attention-based Reinforcement Learning Algorithm for Model-based Control
Recent breakthroughs in Go play and strategic games have witnessed the great
potential of reinforcement learning in intelligently scheduling in uncertain
environment, but some bottlenecks are also encountered when we generalize this
paradigm to universal complex tasks. Among them, the low efficiency of data
utilization in model-free reinforcement algorithms is of great concern. In
contrast, the model-based reinforcement learning algorithms can reveal
underlying dynamics in learning environments and seldom suffer the data
utilization problem. To address the problem, a model-based reinforcement
learning algorithm with attention mechanism embedded is proposed as an
extension of World Models in this paper. We learn the environment model through
Mixture Density Network Recurrent Network(MDN-RNN) for agents to interact, with
combinations of variational auto-encoder(VAE) and attention incorporated in
state value estimates during the process of learning policy. In this way, agent
can learn optimal policies through less interactions with actual environment,
and final experiments demonstrate the effectiveness of our model in control
problem
Manipulating the Majorana Qubit with the Landau-Zener-St\"{u}ckelberg Interference
Constructing a universal operation scheme for Majorana qubits remains a
central issue for the topological quantum computation. We study the
Landau-Zener-St\"{u}ckelberg interference in a Majorana qubit and show that
this interference can be used to achieve controllable operations. The Majorana
qubit consists of an rf SQUID with a topological nanowire Josephson junction
which hosts Majorana bound states. In the SQUID, a magnetic flux pulse can
drive the quantum evolution of the Majorana qubit. The qubit experiences two
Landau-Zener transitions when the amplitude of the pulse is tuned around the
superconducting flux quanta . The Landau-Zener-St\"{u}ckelberg
interference between the two transitions rotates the Majorana qubit, with the
angle controlled by the time scale of the pulse. This rotation operation
implements a high-speed single-qubit gate on the Majorana qubit, which is a
necessary ingredient for the topological quantum computation
An efficient ab-initio quasiharmonic approach for the thermodynamics of solids
A first-principles approach called the {\it{self-consistent quasiharmonic
approximation}} (SC-QHA) method is formulated to calculate the thermal
expansion, thermomechanics, and thermodynamic functions of solids at finite
temperatures with both high efficiency and accuracy. The SC-QHA method requires
fewer phonon calculations than the conventional QHA method, and also
facilitates the convenient analysis of the microscopic origins of macroscopic
thermal phenomena. The superior performance of the SC-QHA method is
systematically examined by comparing it with the conventional QHA method and
experimental measurements on silicon, diamond, and alumina. It is then used to
study the effects of pressure on the anharmonic lattice properties of diamond
and alumina. The thermal expansion and thermomechanics of CaTiO,
which is a recently discovered important ferroelectric ceramic with a complex
crystal structure that is computationally challenging for the conventional QHA
method, are also calculated using the formulated SC-QHA method. The SC-QHA
method can significantly reduce the computational expense for various
quasiharmonic thermal properties especially when there are a large number of
structures to consider or when the solid is structurally complex. It is
anticipated that the algorithm will be useful for a variety of fields,
including oxidation, corrosion, high-pressure physics, ferroelectrics, and
high-throughput structure screening when temperature effects are required to
accurately describe realistic properties
Proposal for a flux qubit in a dc SQUID with the period Josephson effect
Constructing qubits which are suitable for quantum computation remains a
notable challenge. Here, we propose a superconducting flux qubit in a dc SQUID
structure, formed by a conventional insulator Josephson junction and a
topological nanowire Josephson junction with Majorana bound states. The zero
energy Majorana bound states transport period Josephson currents in the
nanowire junction. The interplay between this period Josephson effect
and the convectional period Josephson effect in the insulator junction
induces a double-well potential energy landscape in the SQUID. As a result, the
two lowest energy levels of the SQUID are isolated from other levels. These two
levels show contradicting circulating supercurrents, thus can be used as a flux
qubit. We reveal that this flux qubit has the merits of stability to external
noises, tolerance to the deviation of system parameters, and scalability to
large numbers. Furthermore, we demonstrate how to couple this flux qubit with
the Majorana qubit by tuning the junction parameters, and how to use this
coupling to manipulate the Majorana qubit
GeV excess in the Milky Way: The Role of Diffuse Galactic gamma ray Emission template
Several groups have analyzed the publicly-available Fermi-LAT data and
reported a spatially extended ray excess of around GeV from the
region surrounding the Galactic Center that might originate from annihilation
of dark matter particles with a rest mass GeV. In this work
we examine the role of the diffuse Galactic gamma ray emission (DGE) templates
played in suppressing the GeV excess. For such a purpose, we adopt in total 128
background templates that have been generated by Ackermann et al.
\cite{FermiLAT:2012aa} in the study of the {Fermi-LAT} observations of the
diffuse gamma ray emission considering the effects of cosmic rays and the
interstellar medium. The possible GeV excess, assumed to follow the spatial
distribution of the prompt gamma-rays produced in the annihilation of dark
matter particles taking a generalized NFW profile with an inner slope
, has been analyzed in some regions of interest. The introduction
of such an additional component centered at the Galactic center is found to
have improved the goodness of fit to the data significantly in all background
template models regardless of whether the excess spectrum is fixed or not. Our
results thus suggest that the presence of a statistically significant GeV
excess in the inner Galaxy is robust thought its spectrum depends on the DGE
model adopted in the analysis. The possible physical origin of the GeV excess
component is discussed and in the dark matter model the annihilation cross
section of such particles is evaluated.Comment: 14 pages, 9 figures. Accepted for publication in PRD, moderate
revision but main conclusions unchange
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