3,677 research outputs found
Shocked by GRB 970228: the afterglow of a cosmological fireball
The location accuracy of the BeppoSAX Wide Field Cameras and acute
ground-based followup have led to the detection of a decaying afterglow in X
rays and optical light following the classical gamma-ray burst GRB 970228. The
afterglow in X rays and optical light fades as a power law at all wavelengths.
This behaviour was predicted for a relativistic blast wave that radiates its
energy when it decelerates by ploughing into the surrounding medium. Because
the afterglow has continued with unchanged behaviour for more than a month, its
total energy must be of order 10**51 erg, placing it firmly at a redshift of
order 1. Further tests of the model are discussed, some of which can be done
with available data, and implications for future observing strategies are
pointed out. We discuss how the afterglow can provide a probe for the nature of
the burst sources.Comment: 6 pages LaTeX, 1 postscript figure; minor edits, slightly more data
on light curve, MNRAS, IN PRESS (mid June/early July
Practical limitations in optical entanglement purification
Entanglement purification protocols play an important role in the
distribution of entangled systems, which is necessary for various quantum
information processing applications. We consider the effects of photo-detector
efficiency and bandwidth, channel loss and mode-mismatch on the operation of an
optical entanglement purification protocol. We derive necessary detector and
mode-matching requirements to facilitate practical operation of such a scheme,
without having to resort to destructive coincidence type demonstrations.Comment: 4 pages, 4 figure
Practical effects in the preparation of cluster states using weak non-linearities
We discuss experimental effects in the implementation of a recent scheme for
performing bus mediated entangling operations between qubits. Here a bus mode,
a strong coherent state, successively undergoes weak Kerr-type non-linear
interactions with qubits. A quadrature measurement on the bus then projects the
qubits into an entangled state. This approach has the benefit that entangling
gates are non-destructive, may be performed non-locally, and there is no need
for efficient single photon detection. In this paper we examine practical
issues affecting its experimental implementation. In particular, we analyze the
effects of post-selection errors, qubit loss, bus loss, mismatched coupling
rates and mode-mismatch. We derive error models for these effects and relate
them to realistic fault-tolerant thresholds, providing insight into realistic
experimental requirements.Comment: 8 pages, 5 figure
Distortions to Agricultural Incentives in Australia and New Zealand
In 1990, Australia and New Zealand were ranked around 25th and 35th in terms of GNP per capita, having been the highest-income countries in the world one hundred years earlier. The poor performance over that long period contrasts markedly with that of the past 15 years, when these two economies out-performed most other high-income countries. This difference in growth performance is due to major economic policy reforms during the past two to three decades. We provide new evidence on the extent of governmental distortions to agricultural incentives in particular in the two economies since the late 1940s, both directly and indirectly (and negatively) via manufacturing protection.Distorted incentives, agricultural and trade policy reform, Agricultural and Food Policy, F13, F14, Q17, Q18,
Impact of the wasting disease pathogen, Labryrinthula on the photobiology of eelgrass Zostera marina
Labyrinthula zosterae is clearly shown to be a primary pathogen of eelgrass Zostera marina L., not merely a secondary infection of senescent leaves or an indication of decomposition. The results of this investigation using a Diving-PAM fluorometer indicate that the regions of tissue photosynthetically compromised by Labyrinthula are substantially larger than previously thought. Labyrinthula moves through Zostera marina tissue at a rate of up to 0.8 mm hâ1 during daylight periods. The photosynthetic efficiency of apparently healthy green leaf tissue can be reduced by almost 50% in areas up to 3 mm from a necrotic region infected with Labyrinthula. Once a necrotic spot expands to bisect the eelgrass leaf, the condition of all acropetal tissue is diminished; leaf tissue up to 5 cm away has severely reduced photosynthetic activity
Turbulent mixing layers in supersonic protostellar outflows, with application to DG Tauri
Turbulent entrainment processes may play an important role in the outflows
from young stellar objects at all stages of their evolution. In particular,
lateral entrainment of ambient material by high-velocity, well-collimated
protostellar jets may be the cause of the multiple emission-line velocity
components observed in the microjet-scale outflows driven by classical T Tauri
stars. Intermediate-velocity outflow components may be emitted by a turbulent,
shock- excited mixing layer along the boundaries of the jet. We present a
formalism for describing such a mixing layer based on Reynolds decomposition of
quantities measuring fundamental properties of the gas. In this model, the
molecular wind from large disc radii provides a continual supply of material
for entrainment. We calculate the total stress profile in the mixing layer,
which allows us to estimate the dissipation of turbulent energy, and hence the
luminosity of the layer. We utilize MAPPINGS IV shock models to determine the
fraction of total emission that occurs in [Fe II] 1.644 {\mu}m line emission in
order to facilitate comparison to previous observations of the young stellar
object DG Tauri. Our model accurately estimates the luminosity and changes in
mass outflow rate of the intermediate-velocity component of the DG Tau
approaching outflow. Therefore, we propose that this component represents a
turbulent mixing layer surrounding the well-collimated jet in this object.
Finally, we compare and contrast our model to previous work in the field.Comment: 18 pages, 13 figures, accepted for publication in MNRA
Quantum gate characterization in an extended Hilbert space
We describe an approach for characterizing the process of quantum gates using
quantum process tomography, by first modeling them in an extended Hilbert
space, which includes non-qubit degrees of freedom. To prevent unphysical
processes from being predicted, present quantum process tomography procedures
incorporate mathematical constraints, which make no assumptions as to the
actual physical nature of the system being described. By contrast, the
procedure presented here ensures physicality by placing physical constraints on
the nature of quantum processes. This allows quantum process tomography to be
performed using a smaller experimental data set, and produces parameters with a
direct physical interpretation. The approach is demonstrated by example of
mode-matching in an all-optical controlled-NOT gate. The techniques described
are non-specific and could be applied to other optical circuits or quantum
computing architectures.Comment: 4 pages, 2 figures, REVTeX (published version
A new method of determining the initial size and Lorentz factor of gamma-ray burst fireballs using a thermal emission component
In recent years increasing evidence has emerged for a thermal component in
the gamma- and X-ray spectrum of the prompt emission phase in gamma-ray bursts.
The temperature and flux of the thermal component show a characteristic break
in the temporal behavior after a few seconds. We show here, that measurements
of the temperature and flux of the thermal component at early times (before the
break) allow the determination of the values of two of the least restricted
fireball model parameters: the size at the base of the flow and the outflow
bulk Lorentz factor. Relying on the thermal emission component only, this
measurement is insensitive to the inherent uncertainties of previous estimates
of the bulk motion Lorentz factor. We give specific examples of the use of this
method: for GRB970828 at redshift z=0.9578, we show that the physical size at
the base of the flow is r_0 = (2.9+-1.8)*10^8 Y_0^{-3/2} cm and the Lorentz
factor of the flow is Gamma = (305\+-28) Y_0^{1/4}, and for GRB990510 at
z=1.619, r_0=(1.7+-1.7)*10^8 Y_0^{-3/2} cm and Gamma=(384+-71) Y_0^{1/4}, where
Y = 1 Y_0 is the ratio between the total fireball energy and the energy emitted
in gamma- rays.Comment: Discussion added on gamma-ray emission efficiency. Accepted for
publication in Ap.J. Let
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