209 research outputs found
A NuSTAR observation of the reflection spectrum of the low mass X-ray binary 4U 1728-34
We report on a simultaneous NuSTAR and Swift observation of the neutron star
low-mass X-ray binary 4U 1728-34. We identified and removed four Type I X-ray
bursts during the observation in order to study the persistent emission. The
continuum spectrum is hard and well described by a black body with 1.5
keV and a cutoff power law with 1.5 and a cutoff temperature of 25
keV. Residuals between 6 and 8 keV provide strong evidence of a broad Fe
K line. By modeling the spectrum with a relativistically blurred
reflection model, we find an upper limit for the inner disk radius of . Consequently we find that km,
assuming M=1.4{\mbox{\rm\,M_{\mathord\odot}}} and . We also find an
upper limit on the magnetic field of G.Comment: 9 pages, 8 figure
Two-sided combinatorial volume bounds for non-obtuse hyperbolic polyhedra
We give a method for computing upper and lower bounds for the volume of a
non-obtuse hyperbolic polyhedron in terms of the combinatorics of the
1-skeleton. We introduce an algorithm that detects the geometric decomposition
of good 3-orbifolds with planar singular locus and underlying manifold the
3-sphere. The volume bounds follow from techniques related to the proof of
Thurston's Orbifold Theorem, Schl\"afli's formula, and previous results of the
author giving volume bounds for right-angled hyperbolic polyhedra.Comment: 36 pages, 19 figure
Probabilistic manipulation of entangled photons
We propose probabilistic controlled-NOT and controlled-phase gates for qubits
stored in the polarization of photons. The gates are composed of linear optics
and photon detectors, and consume polarization entangled photon pairs. The
fraction of the successful operation is only limited by the efficiency of the
Bell-state measurement. The gates work correctly under the use of imperfect
detectors and lossy transmission of photons. Combined with single-qubit gates,
they can be used for producing arbitrary polarization states and for designing
various quantum measurements.Comment: 4 pages, 3 figure
Talbot Oscillations and Periodic Focusing in a One-Dimensional Condensate
An exact theory for the density of a one-dimensional Bose-Einstein condensate
with hard core particle interactions is developed in second quantization and
applied to the scattering of the condensate by a spatially periodic impulse
potential. The boson problem is mapped onto a system of free fermions obeying
the Pauli exclusion principle to facilitate the calculation. The density
exhibits a spatial focusing of the probability density as well as a periodic
self-imaging in time, or Talbot effect. Furthermore, the transition from single
particle to many body effects can be measured by observing the decay of the
modulated condensate density pattern in time. The connection of these results
to classical and atom optical phase gratings is made explicit
Decoherence-Free Subspaces for Multiple-Qubit Errors: (II) Universal, Fault-Tolerant Quantum Computation
Decoherence-free subspaces (DFSs) shield quantum information from errors
induced by the interaction with an uncontrollable environment. Here we study a
model of correlated errors forming an Abelian subgroup (stabilizer) of the
Pauli group (the group of tensor products of Pauli matrices). Unlike previous
studies of DFSs, this type of errors does not involve any spatial symmetry
assumptions on the system-environment interaction. We solve the problem of
universal, fault-tolerant quantum computation on the associated class of DFSs.Comment: 22 pages, 4 figures. Sequel to quant-ph/990806
Neighborhoods of trees in circular orderings
In phylogenetics, a common strategy used to construct an evolutionary tree for a set of species X is to search in the space of all such trees for one that optimizes some given score function (such as the minimum evolution, parsimony or likelihood score). As this can be computationally intensive, it was recently proposed to restrict such searches to the set of all those trees that are compatible with some circular ordering of the set X. To inform the design of efficient algorithms to perform such searches, it is therefore of interest to find bounds for the number of trees compatible with a fixed ordering in the neighborhood of a tree that is determined by certain tree operations commonly used to search for trees: the nearest neighbor interchange (nni), the subtree prune and regraft (spr) and the tree bisection and reconnection (tbr) operations. We show that the size of such a neighborhood of a binary tree associated with the nni operation is independent of the tree’s topology, but that this is not the case for the spr and tbr operations. We also give tight upper and lower bounds for the size of the neighborhood of a binary tree for the spr and tbr operations and characterize those trees for which these bounds are attained
Quantum computing in optical microtraps based on the motional states of neutral atoms
We investigate quantum computation with neutral atoms in optical microtraps
where the qubit is implemented in the motional states of the atoms, i.e., in
the two lowest vibrational states of each trap. The quantum gate operation is
performed by adiabatically approaching two traps and allowing tunneling and
cold collisions to take place. We demonstrate the capability of this scheme to
realize a square-root of swap gate, and address the problem of double
occupation and excitation to other unwanted states. We expand the two-particle
wavefunction in an orthonormal basis and analyze quantum correlations
throughout the whole gate process. Fidelity of the gate operation is evaluated
as a function of the degree of adiabaticity in moving the traps. Simulations
are based on rubidium atoms in state-of-the-art optical microtraps with quantum
gate realizations in the few tens of milliseconds duration range.Comment: 11 pages, 7 figures, for animations of the gate operation, see
http://www.itp.uni-hannover.de/~eckert/na/index.htm
NuSTAR ground calibration: The Rainwater Memorial Calibration Facility (RaMCaF)
The Nuclear Spectroscopic Telescope Array (NuSTAR) is a NASA Small Explorer mission that will carry the first focusing hard X-ray (5-80 keV ) telescope to orbit. The ground calibration of the three flight optics was carried out at the Rainwater Memorial Calibration Facility (RaMCaF) built for this purpose. In this article we present the facility and its use for the ground calibration of the three optics
Trapping dust particles in the outer regions of protoplanetary disks
In order to explain grain growth to mm sized particles and their retention in
outer regions of protoplanetary disks, as it is observed at sub-mm and mm
wavelengths, we investigate if strong inhomogeneities in the gas density
profiles can slow down excessive radial drift and can help dust particles to
grow. We use coagulation/fragmentation and disk-structure models, to simulate
the evolution of dust in a bumpy surface density profile which we mimic with a
sinusoidal disturbance. For different values of the amplitude and length scale
of the bumps, we investigate the ability of this model to produce and retain
large particles on million years time scales. In addition, we introduced a
comparison between the pressure inhomogeneities considered in this work and the
pressure profiles that come from magnetorotational instability. Using the
Common Astronomy Software Applications ALMA simulator, we study if there are
observational signatures of these pressure inhomogeneities that can be seen
with ALMA. We present the favorable conditions to trap dust particles and the
corresponding calculations predicting the spectral slope in the mm-wavelength
range, to compare with current observations. Finally we present simulated
images using different antenna configurations of ALMA at different frequencies,
to show that the ring structures will be detectable at the distances of the
Taurus Auriga or Ophiucus star forming regions.Comment: Pages 15, Figures 14. Accepted for publication in Astronomy and
Astrophysic
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