14,248 research outputs found
Beamed Core Antimatter Propulsion: Engine Design and Optimization
A conceptual design for beamed core antimatter propulsion is reported, where
electrically charged annihilation products directly generate thrust after being
deflected and collimated by a magnetic nozzle. Simulations were carried out
using the Geant4 (Geometry and tracking) software toolkit released by the CERN
accelerator laboratory for Monte Carlo simulation of the interaction of
particles with matter and fields. Geant permits a more sophisticated and
comprehensive design and optimization of antimatter engines than the software
environment for simulations reported by prior researchers. The main finding is
that effective exhaust speeds Ve ~ 0.69c (where c is the speed of light) are
feasible for charged pions in beamed core propulsion, a major improvement over
the Ve ~ 0.33c estimate based on prior simulations. The improvement resulted
from optimization of the geometry and the field configuration of the magnetic
nozzle. Moreover, this improved performance is realized using a magnetic field
on the order of 10 T at the location of its highest magnitude. Such a field
could be produced with today's technology, whereas prior nozzle designs
anticipated and required major advances in this area. The paper also briefly
reviews prospects for production of the fuel needed for a beamed core engine.Comment: To be published in J. of British Interplanetary Society; this
post-referee version has updated references and Fig. 4 is changed for better
reproduction in b/
Non-equilibrium theory of charge qubit decoherence in the quantum point contact measurement
A non-equilibrium theory describing the charge qubit dynamics measured by a
quantum point contact is developed based on Schwinger-Keldysh's approach. Using
the real-time diagram technique, we derive the master equation to all orders in
perturbation expansions. The non-Markovian processes in the qubit dynamics is
naturally taken into account. The qubit decoherence, in particular, the
influence of the tunneling-electron fluctuation in the quantum point contact
with a longer time correlation, is studied in the framework. We consider the
Lorentzian-type spectral density to characterize the channel mixture of the
electron tunneling processes induced by the measurement and determine the
correlation time scale of the tunneling-electron fluctuation. The result shows
that as the quantum point contact is casted with a narrower profile of the
spectral density, tunneling electrons can propagate with a longer time
correlation and lead to the non-Markovian processes of the qubit dynamics. The
qubit electron in the charge qubit will be driven coherently. The quantum point
contact measurement with the minimum deviation of the electron tunneling
processes prevents the qubit state from the decoherence.Comment: 14 pages, 7 figure
HIBERT: Document Level Pre-training of Hierarchical Bidirectional Transformers for Document Summarization
Neural extractive summarization models usually employ a hierarchical encoder
for document encoding and they are trained using sentence-level labels, which
are created heuristically using rule-based methods. Training the hierarchical
encoder with these \emph{inaccurate} labels is challenging. Inspired by the
recent work on pre-training transformer sentence encoders
\cite{devlin:2018:arxiv}, we propose {\sc Hibert} (as shorthand for {\bf
HI}erachical {\bf B}idirectional {\bf E}ncoder {\bf R}epresentations from {\bf
T}ransformers) for document encoding and a method to pre-train it using
unlabeled data. We apply the pre-trained {\sc Hibert} to our summarization
model and it outperforms its randomly initialized counterpart by 1.25 ROUGE on
the CNN/Dailymail dataset and by 2.0 ROUGE on a version of New York Times
dataset. We also achieve the state-of-the-art performance on these two
datasets.Comment: to appear in ACL 201
Supersolid with nontrivial topological spin textures in spin-orbit-coupled Bose gases
Supersolid is a long-sought exotic phase of matter, which is characterized by
the coexistence of a diagonal long-range order of solid and an off-diagonal
long-range order of superfluid. Possible candidates to realize such a phase
have been previously considered, including hard-core bosons with long-range
interaction and soft-core bosons. Here we demonstrate that an ultracold atomic
condensate of hard-core bosons with contact interaction can establish a
supersolid phase when simultaneously subjected to spin-orbit coupling and a
spin-dependent periodic potential. This supersolid phase is accompanied by
topologically nontrivial spin textures, and is signaled by the separation of
momentum distribution peaks, which can be detected via time-of-flight
measurements. We also discuss possibilities to produce and observe the
supersolid phase for realistic experimental situations
Manipulating quantum information on the controllable systems or subspaces
In this paper, we explore how to constructively manipulate qubits by rotating
Bloch spheres. It is revealed that three-rotation and one-rotation Hamiltonian
controls can be constructed to steer qubits when two tunable Hamiltonian
controls are available. It is demonstrated in this research that local-wave
function controls such as Bang-Bang, triangle-function and quadratic function
controls can be utilized to manipulate quantum states on the Bloch sphere. A
new kind of time-energy performance index is proposed to trade-off time and
energy resource cost, in which control magnitudes are optimized in terms of
this kind of performance. It is further exemplified that this idea can be
generalized to manipulate encoded qubits on the controllable subspace
Spin-orbit Coupled Bose-Einstein Condensates in Spin-dependent Optical Lattices
We investigate the ground-state properties of spin-orbit coupled
Bose-Einstein condensates in spin-dependent optical lattices. The competition
between the spin-orbit coupling strength and the depth of the optical lattice
leads to a rich phase diagram. Without spin-orbit coupling, the spin-dependent
optical lattices separate the condensates into alternating spin domains with
opposite magnetization directions. With relatively weak spin-orbit coupling,
the spin domain wall is dramatically changed from N\'{e}el wall to Bloch wall.
For sufficiently strong spin-orbit coupling, vortex chains and antivortex
chains are excited in the spin-up and spin-down domains respectively,
corresponding to the formation of a lattice composed of meron-pairs and
antimeron-pairs in the pseudospin representation. We also discuss how to
observe these phenomena in real experiments
A new scheme for probabilistic teleportation and its potential applications
We propose a novel scheme for probabilistic teleportation when the
information of the partially entangled state is only available for the sender.
This is in contrast with the fact that the receiver must know the non-maximally
entangled state in previous typical schemes for the teleportation.
Additionally, we illustrate two potential applications of the novel scheme to
probabilistically teleport an unknown two-level quantum state from a sender to
a receiver with the help of an assistant, who plays distinguishable roles under
different communication conditions, and the applications indicate that our
proposal could improve the security and enlarge the applied range of
probabilistic teleportation.Comment: 11 pages, 2 figure
Transport Model Studies of the Baryon-Rich Quark-Gluon Plasma formed in Heavy Ion Collisions
Heavy ion collisions in the low energy run at Relativistic Heavy Ion Collider
(RHIC) and future Facility for Antiproton and Ion Research (FAIR) in Germany
are expected to produce a quark-gluon plasma that has a finite baryon chemical
potential, allowing thus the possibility to study the location of the critical
endpoint in the QCD phase diagram. In this talk, using a multiphase transport
model, that includes interactions in both initial partonic and final hadronic
matters and the transition between these two phases of matter, we discuss the
effects of partonic interactions on observables such as the elliptic flow that
have played essential roles in studying the properties of the net baryon free
quark-gluon plasma produced in heavy ion collisions at higher energies at RHIC.
Also, we study the effect of density fluctuations due to a first-order
transition between the quark-gluon plasma and hadronic matter on fluctuations
of hadron mean transverse momentum and produced deuteron number as well as on
two-pion correlations. We further discuss the possibility of studying the
mechanism of charm energy loss in the baryon-rich quark-gluon plasma and the
properties of phi mesons in hot-dense matter produced in these collisions.Comment: 15 pages, 7 pages. Contribution to the 5th International Workshop on
Critical Point and Onset of Deconfinement, Brookhaven National Laboratory,
Long Island, New York, USA, June 8-12, 200
The magnetization degree of the outflow powering the highly-polarized reverse shock emission of GRB 120308A
GRB 120308A, a long duration ray burst detected by {\it Swift}, was
distinguished by a highly-polarized early optical afterglow emission that
strongly suggests an ordered magnetic field component in the emitting region.
In this work we model the optical and X-ray emission in the reverse and forward
shock scenario and show that the strength of the magnetic field in reverse
shock region is times stronger than that in the forward shock region.
Consequently the outflow powering the highly-polarized reverse shock optical
emission was mildly-magnetized at a degree a few percent.
Considering the plausible magnetic energy dissipation in both the acceleration
and prompt emission phases of the Gamma-ray Burst (GRB) outflow, the afterglow
data of GRB 120308A provides us the compelling evidence that at least for some
GRBs a non-ignorable fraction of the energy was released in the form of
Poynting-flux, confirming the finding firstly made in the reverse-forward shock
emission modeling of the optical afterglow of GRB 990123 (Fan et al. 2002;
Zhang et al. 2003)
Fermi/LAT observations of Lobe-dominant Radio Galaxy 3C 207 and Possible Radiation Region of the Gamma-Rays
3C 207 is a lobe-dominant radio galaxy with one sided jet and the bright
knots in kpc-Mpc scale were resolved in the radio, optical and X-ray bands. It
was confirmed as a gamma-ray emitter with Fermi/LAT, but it is uncertain
whether the gamma-ray emission region is the core or knots due to the low
spatial resolution of Fermi/LAT. We present an analysis of its Fermi/LAT data
in the past 9 years. Different from the radio and optical emission from the
core, it is found that the gamma-ray emission is steady without detection of
flux variation over 2 sigma confidence level. This likely implies that the
gamma-ray emission is from its knots. We collect the radio, optical, and X-ray
data of knot-A, the closest knot from the core at 1 arcsec, and compile its
spectral energy distribution (SED). Although the single-zone
synchrotron+SSC+IC/CMB model by assuming knot-A at rest can reproduce the SED
in the radio-optical-X-ray band, the predicted gamma-ray flux is lower than the
LAT observations and the derived magnetic field strength deviates the
equipartition condition with 3 orders of magnitude. Assuming that knot-A is
relativistically moving, its SED from radio to gamma-ray bands would be well
represented with the single-zone synchrotron+SSC+IC/CMB model under the
equipartition condition. These results likely suggest that the gamma-ray
emission may be from knot-A via the IC/CMB process and the knot should have
relativistical motion. The jet power derived from our model parameters is also
roughly consistent with the kinetic power estimated with the radio data.Comment: 7 pages, 2 figures, accepted for publication in RA
- …