14,725 research outputs found
One-way quantum computation with four-dimensional photonic qudits
We consider the possibility of performing linear optical quantum computation
making use of extra photonic degrees of freedom. In particular we focus on the
case where we use photons as quadbits. The basic 2-quadbit cluster state is a
hyper-entangled state across polarization and two spatial mode degrees of
freedom. We examine the non-deterministic methods whereby such states can be
created from single photons and/or Bell pairs, and then give some mechanisms
for performing higher-dimensional fusion gates.Comment: 10 figures (typos are corrected
Structures Produced by the Collision of Extragalactic Jets with Dense Clouds
We have investigated how several parameters can affect the results of a
collision between an extragalactic jet and a dense, intergalactic cloud,
through a series of hydrodynamic simulations. Such collisions are often
suggested to explain the distorted structures of some radio jets. However,
theoretical studies of this mechanism are in conflict over whether it can
actually reproduce the observations.
The parameters are the Mach number, and the relative densities of the jet and
the cloud to the ambient medium. Using a simple prescription we have produced
synthetic radio images for comparison with observations. These show that a
variety of structures may be produced from simple jet-cloud collisions. We
illustrate this with a few examples, and examine the details in one case. In
most cases we do not see a clear, sustained deflection. Lighter jets are
completely disrupted. The most powerful jets produce a hotspot at the impact
which outshines any jet emission and erode the cloud too quickly to develop a
deflected arm. It appears that moderate Mach numbers and density contrasts are
needed to produce bends in the radio structure. This explains the apparent
conflict between theoretical studies, as conclusions were based on different
values of these parameters. Shocks are produced in the ambient medium that
might plausibly reproduce the observed alignment of the extended emission line
regions with the radio axis.Comment: 21 pages, 11 figures. Submitted to MNRAS. Also available in html
version at http://www.doc.mmu.ac.uk/STAFF/S.Higgins/jcmnpaper/jc_mn.htm
Adaptive weight estimator for quantum error correction
Quantum error correction of a surface code or repetition code requires the
pairwise matching of error events in a space-time graph of qubit measurements,
such that the total weight of the matching is minimized. The input weights
follow from a physical model of the error processes that affect the qubits.
This approach becomes problematic if the system has sources of error that
change over time. Here we show how the weights can be determined from the
measured data in the absence of an error model. The resulting adaptive decoder
performs well in a time-dependent environment, provided that the characteristic
time scale of the variations is greater than , with the duration of one error-correction cycle and
the typical error probability per qubit in one cycle.Comment: 5 pages, 4 figure
Aerosol-assisted metallo-organic chemical vapour deposition of Bi2Se3 films using single-molecule precursors: the crystal structure of bismuth(m) dibutyldiselenocarbamate
The complexes [Bi{Se2CN(C2H5)2}3], [Bi{Se2CN(C4H9)2}3], [Bi{Se2CN(CH3)(C4H9)}3] and
[Bi{Se2CN(CH3)(C6H13)}3] have been synthesized and characterized structurally using IR, 1H and 13C NMR. In
addition, the crystal structure of [Bi{Se2CN(C4H9)2}3] was determined by single-crystal X-ray diffraction,
showing the bismuth centre coordinated to three dialkyldiselenocarbamate ligands through the selenium donor
atoms. The Bi(III) compounds were used as precursors for the deposition of Bi2Se3 films on glass substrates
through aerosol-assisted metallo-organic chemical vapour deposition (AA-MOCVD)
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: II. 1D hydrodynamical models of wind driven shocks
Following the early Swift X-ray observations of the latest outburst of the
recurrent nova RS Ophiuchi in February 2006 (Paper I), we present new 1D
hydrodynamical models of the system which take into account all three phases of
the remnant evolution. The models suggest a novel way of modelling the system
by treating the outburst as a sudden increase then decrease in wind mass-loss
rate and velocity. The differences between this wind model and previous
Primakoff-type simulations are described. A more complex structure, even in 1D,
is revealed through the presence of both forward and reverse shocks, with a
separating contact discontinuity. The effects of radiative cooling are
investigated and key outburst parameters such as mass-loss rate, ejecta
velocity and mass are varied. The shock velocities as a function of time are
compared to the ones derived in Paper I. We show how the manner in which the
matter is ejected controls the evolution of the shock and that for a
well-cooled remnant, the shock deceleration rate depends on the amount of
energy that is radiated away.Comment: 9 pages, 5 figure
Gamma-Ray Bursts observed by XMM-Newton
Analysis of observations with XMM-Newton have made a significant contribution
to the study of Gamma-ray Burst (GRB) X-ray afterglows. The effective area,
bandpass and resolution of the EPIC instrument permit the study of a wide
variety of spectral features. In particular, strong, time-dependent, soft X-ray
emission lines have been discovered in some bursts. The emission mechanism and
energy source for these lines pose major problems for the current generation of
GRB models. Other GRBs have intrinsic absorption, possibly related to the
environment around the progenitor, or possible iron emission lines similar to
those seen in GRBs observed with BeppoSAX. Further XMM-Newton observations of
GRBs discovered by the Swift satellite should help unlock the origin of the GRB
phenomenon over the next few years.Comment: To appear in proceedings of the "XMM-Newton EPIC Consortium meeting,
Palermo, 2003 October 14-16", published in Memorie della Societa Astronomica
Italian
The Distance to Nova V959 Mon from VLA Imaging
Determining reliable distances to classical novae is a challenging but
crucial step in deriving their ejected masses and explosion energetics. Here we
combine radio expansion measurements from the Karl G. Jansky Very Large Array
with velocities derived from optical spectra to estimate an expansion parallax
for nova V959 Mon, the first nova discovered through its gamma-ray emission. We
spatially resolve the nova at frequencies of 4.5-36.5 GHz in nine different
imaging epochs. The first five epochs cover the expansion of the ejecta from
2012 October to 2013 January, while the final four epochs span 2014 February to
2014 May. These observations correspond to days 126 through 199 and days 615
through 703 after the first detection of the nova. The images clearly show a
non-spherical ejecta geometry. Utilizing ejecta velocities derived from 3D
modelling of optical spectroscopy, the radio expansion implies a distance
between 0.9 +/- 0.2 and 2.2 +/- 0.4 kpc, with a most probable distance of 1.4
+/- 0.4 kpc. This distance implies a gamma-ray luminosity much less than the
prototype gamma-ray-detected nova, V407 Cyg, possibly due to the lack of a red
giant companion in the V959 Mon system. V959 Mon also has a much lower
gamma-ray luminosity than other classical novae detected in gamma-rays to date,
indicating a range of at least a factor of 10 in the gamma-ray luminosities for
these explosions.Comment: 11 pages, 8 figures, 3 tables, submitted to ApJ 2015-01-21, under
revie
Twisted Fermi surface of a thin-film Weyl semimetal
The Fermi surface of a conventional two-dimensional electron gas is
equivalent to a circle, up to smooth deformations that preserve the orientation
of the equi-energy contour. Here we show that a Weyl semimetal confined to a
thin film with an in-plane magnetization and broken spatial inversion symmetry
can have a topologically distinct Fermi surface that is twisted into a
\mbox{figure-8} opposite orientations are coupled at a crossing which is
protected up to an exponentially small gap. The twisted spectral response to a
perpendicular magnetic field is distinct from that of a deformed Fermi
circle, because the two lobes of a \mbox{figure-8} cyclotron orbit give
opposite contributions to the Aharonov-Bohm phase. The magnetic edge channels
come in two counterpropagating types, a wide channel of width and a narrow channel of width (with
the magnetic length and the momentum separation
of the Weyl points). Only one of the two is transmitted into a metallic
contact, providing unique magnetotransport signatures.Comment: V4: 10 pages, 14 figures. Added figure and discussion about
"uncrossing deformations" of oriented contours, plus minor corrections.
Published in NJ
Spectral evolution and the onset of the X-ray GRB afterglow
Based on light curves from the Swift Burst Analyser, we investigate whether a
`dip' feature commonly seen in the early-time hardness ratios of Swift-XRT data
could arise from the juxtaposition of the decaying prompt emission and rising
afterglow. We are able to model the dip as such a feature, assuming the
afterglow rises as predicted by Sari & Piran (1999). Using this model we
measure the initial bulk Lorentz factor of the fireball. For a sample of 23
GRBs we find a median value of Gamma_0=225, assuming a constant-density
circumburst medium; or Gamma_0=93 if we assume a wind-like medium.Comment: 4 pages, 3 figures. To appear in the proceedings of GRB 2010,
Annapolis November 2010. (AIP Conference proceedings
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