3,927 research outputs found
Training course for radiation safety technicians
Course of instruction includes sections on basic information, natural radioactivity, properties of alpha, beta, gamma, X rays, and neutrons, concepts of radiation units and dose determinations, shielding, biological effects, background radiation, radiation protection standards, and internal dose calculation
Structured Near-Optimal Channel-Adapted Quantum Error Correction
We present a class of numerical algorithms which adapt a quantum error
correction scheme to a channel model. Given an encoding and a channel model, it
was previously shown that the quantum operation that maximizes the average
entanglement fidelity may be calculated by a semidefinite program (SDP), which
is a convex optimization. While optimal, this recovery operation is
computationally difficult for long codes. Furthermore, the optimal recovery
operation has no structure beyond the completely positive trace preserving
(CPTP) constraint. We derive methods to generate structured channel-adapted
error recovery operations. Specifically, each recovery operation begins with a
projective error syndrome measurement. The algorithms to compute the structured
recovery operations are more scalable than the SDP and yield recovery
operations with an intuitive physical form. Using Lagrange duality, we derive
performance bounds to certify near-optimality.Comment: 18 pages, 13 figures Update: typos corrected in Appendi
Resolved Kinematics of Runaway and Field OB Stars in the Small Magellanic Cloud
We use GAIA DR2 proper motions of the RIOTS4 field OB stars in the Small
Magellanic Cloud (SMC) to study the kinematics of runaway stars. The data
reveal that the SMC Wing has a systemic peculiar motion relative to the SMC Bar
of (v_RA, v_Dec) = (62 +/-7, -18+/-5) km/s and relative radial velocity +4.5
+/- 5.0 km/s. This unambiguously demonstrates that these two regions are
kinematically distinct: the Wing is moving away from the Bar, and towards the
Large Magellanic Cloud with a 3-D velocity of 64 +/- 10 km/s. This is
consistent with models for a recent, direct collision between the Clouds. We
present transverse velocity distributions for our field OB stars, confirming
that unbound runaways comprise on the order of half our sample, possibly more.
Using eclipsing binaries and double-lined spectroscopic binaries as tracers of
dynamically ejected runaways, and high-mass X-ray binaries (HMXBs) as tracers
of runaways accelerated by supernova kicks, we find significant contributions
from both populations. The data suggest that HMXBs have lower velocity
dispersion relative to dynamically ejected binaries, consistent with the former
corresponding to less energetic supernova kicks that failed to unbind the
components. Evidence suggests that our fast runaways are dominated by
dynamical, rather than supernova, ejections.Comment: Accepted to ApJ Letters. 10 pages, 4 figure
Optimum Quantum Error Recovery using Semidefinite Programming
Quantum error correction (QEC) is an essential element of physical quantum
information processing systems. Most QEC efforts focus on extending classical
error correction schemes to the quantum regime. The input to a noisy system is
embedded in a coded subspace, and error recovery is performed via an operation
designed to perfectly correct for a set of errors, presumably a large subset of
the physical noise process. In this paper, we examine the choice of recovery
operation. Rather than seeking perfect correction on a subset of errors, we
seek a recovery operation to maximize the entanglement fidelity for a given
input state and noise model. In this way, the recovery operation is optimum for
the given encoding and noise process. This optimization is shown to be
calculable via a semidefinite program (SDP), a well-established form of convex
optimization with efficient algorithms for its solution. The error recovery
operation may also be interpreted as a combining operation following a quantum
spreading channel, thus providing a quantum analogy to the classical diversity
combining operation.Comment: 7 pages, 3 figure
Physical Conditions in Quasar Outflows: VLT Observations of QSO 2359-1241
We analyze the physical conditions of the outflow seen in QSO 2359-1241 (NVSS
J235953-124148), based on high resolution spectroscopic VLT observations. This
object was previously studied using Keck/HIRES data. The main improvement over
the HIRES results is our ability to accurately determine the number density of
the outflow. For the major absorption component, level population from five
different Fe II excited level yields n_H=10^4.4 cm^-3 with less than 20%
scatter. We find that the Fe ii absorption arises from a region with roughly
constant conditions and temperature greater than 9000 K, before the ionization
front where temperature and electron density drop. Further, we model the
observed spectra and investigate the effects of varying gas metalicities and
the spectral energy distribution of the incident ionizing radiation field. The
accurately measured column densities allow us to determine the ionization
parameter log(U) = -2.4 and total column density of the outflow (log(N_H) =
20.6 cm^-2). Combined with the number density finding, these are stepping
stones towards determining the mass flux and kinetic luminosity of the outflow,
and therefore its importance to AGN feedback processes.Comment: 21 pages, 3 figures (accepted for publication in the ApJ
Runaway OB Stars in the Small Magellanic Cloud: Dynamical Versus Supernova Ejections
Runaway OB stars are ejected from their parent clusters via two mechanisms,
both involving multiple stars: the dynamical ejection scenario (DES) and the
binary supernova scenario (BSS). We constrain the relative contributions from
these two ejection mechanisms in the Small Magellanic Cloud (SMC) using data
for 304 field OB stars from the spatially complete, Runaways and Isolated
O-Type Star Spectroscopic Survey of the SMC (RIOTS4). We obtain stellar masses
and projected rotational velocities for the sample using RIOTS4
spectra, and use transverse velocities from DR2
proper motions. Kinematic analyses of the masses, , non-compact
binaries, high-mass X-ray binaries, and Oe/Be stars largely support predictions
for the statistical properties of the DES and BSS populations. We find that
dynamical ejections dominate over supernova ejections by a factor of
in the SMC, and our results suggest a high frequency of DES runaways and binary
ejections. Objects seen as BSS runaways also include two-step ejections of
binaries that are reaccelerated by SN kicks. We find that two-step runaways
likely dominate the BSS runaway population. Our results further imply that any
contribution from field OB star formation is small. Finally, our
data strongly support the post-mass-transfer model for the origin of classical
Oe/Be stars, providing a simple explanation for the bimodality in the distribution and high, near-critical, Oe/Be rotation velocities. The close
correspondence of Oe/Be stars with BSS predictions implies that the
emission-line disks are long-lived.Comment: 20 pages, 7 figures, 3 tables. Accepted by Ap
Discovery of a [WO] central star in the planetary nebula Th 2-A
% context About 2500 planetary nebulae are known in our Galaxy but only 224
have central stars with reported spectral types in the Strasbourg-ESO Catalogue
of Galactic Planetary Nebulae (Acker et al. 1992; Acker et al. 1996) % aims We
have started an observational program aiming to increase the number of PN
central stars with spectral classification. % methods By means of spectroscopy
and high resolution imaging, we identify the position and true nature of the
central star. We carried out low resolution spectroscopic observations at
CASLEO telescope, complemented with medium resolution spectroscopy performed at
Gemini South and Magellan telescopes. % results As a first outcome of this
survey, we present for the first time the spectra of the central star of the PN
Th 2-A. These spectra show emission lines of ionized C and O, typical in
Wolf-Rayet stars. % conclusions We identify the position of that central star,
which is not the brightest one of the visual central pair. We classify it as of
type [WO 3]pec, which is consistent with the high excitation and dynamical age
of the nebula.Comment: 3 pages and 2 figures. Paper recommended for publication in A&
Dynamics of lattice spins as a model of arrhythmia
We consider evolution of initial disturbances in spatially extended systems
with autonomous rhythmic activity, such as the heart. We consider the case when
the activity is stable with respect to very smooth (changing little across the
medium) disturbances and construct lattice models for description of
not-so-smooth disturbances, in particular, topological defects; these models
are modifications of the diffusive XY model. We find that when the activity on
each lattice site is very rigid in maintaining its form, the topological
defects - vortices or spirals - nucleate a transition to a disordered,
turbulent state.Comment: 17 pages, revtex, 3 figure
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