14,891 research outputs found
Thermalization, Error-Correction, and Memory Lifetime for Ising Anyon Systems
We consider two-dimensional lattice models that support Ising anyonic
excitations and are coupled to a thermal bath. We propose a phenomenological
model for the resulting short-time dynamics that includes pair-creation,
hopping, braiding, and fusion of anyons. By explicitly constructing topological
quantum error-correcting codes for this class of system, we use our
thermalization model to estimate the lifetime of the quantum information stored
in the encoded spaces. To decode and correct errors in these codes, we adapt
several existing topological decoders to the non-Abelian setting. We perform
large-scale numerical simulations of these two-dimensional Ising anyon systems
and find that the thresholds of these models range between 13% to 25%. To our
knowledge, these are the first numerical threshold estimates for quantum codes
without explicit additive structure.Comment: 34 pages, 9 figures; v2 matches the journal version and corrects a
misstatement about the detailed balance condition of our Metropolis
simulations. All conclusions from v1 are unaffected by this correctio
Understanding person acquisition using an interactive activation and competition network
Face perception is one of the most developed visual skills that humans display, and recent work has attempted to examine the mechanisms involved in face perception through noting how neural networks achieve the same performance. The purpose of the present paper is to extend this approach to look not just at human face recognition, but also at human face acquisition. Experiment 1 presents empirical data to describe the acquisition over time of appropriate representations for newly encountered faces. These results are compared with those of Simulation 1, in which a modified IAC network capable of modelling the acquisition process is generated. Experiment 2 and Simulation 2 explore the mechanisms of learning further, and it is demonstrated that the acquisition of a set of associated new facts is easier than the acquisition of individual facts in isolation of one another. This is explained in terms of the advantage gained from additional inputs and mutual reinforcement of developing links within an interactive neural network system. <br/
Rotational quenching rate coefficients for H_2 in collisions with H_2 from 2 to 10,000 K
Rate coefficients for rotational transitions in H_2 induced by H_2 impact are
presented. Extensive quantum mechanical coupled-channel calculations based on a
recently published (H_2)_2 potential energy surface were performed. The
potential energy surface used here is presumed to be more reliable than
surfaces used in previous work. Rotational transition cross sections with
initial levels J <= 8 were computed for collision energies ranging between
0.0001 and 2.5 eV, and the corresponding rate coefficients were calculated for
the temperature range 2 < T <10,000 K. In general, agreement with earlier
calculations, which were limited to 100-6000 K, is good though discrepancies
are found at the lowest and highest temperatures. Low-density-limit cooling
functions due to para- and ortho-H_2 collisions are obtained from the
collisional rate coefficients. Implications of the new results for non-thermal
H_2 rotational distributions in molecular regions are also investigated
The molecular environment of massive star forming cores associated with Class II methanol maser emission
Methanol maser emission has proven to be an excellent signpost of regions
undergoing massive star formation (MSF). To investigate their role as an
evolutionary tracer, we have recently completed a large observing program with
the ATCA to derive the dynamical and physical properties of molecular/ionised
gas towards a sample of MSF regions traced by 6.7 GHz methanol maser emission.
We find that the molecular gas in many of these regions breaks up into multiple
sub-clumps which we separate into groups based on their association
with/without methanol maser and cm continuum emission. The temperature and
dynamic state of the molecular gas is markedly different between the groups.
Based on these differences, we attempt to assess the evolutionary state of the
cores in the groups and thus investigate the role of class II methanol masers
as a tracer of MSF.Comment: 5 pages, 1 figure, IAU Symposium 242 Conference Proceeding
Infrared images of reflection nebulae and Orion's bar: Fluorescent molecular hydrogen and the 3.3 micron feature
Images were obtained of the (fluorescent) molecular hydrogen 1-0 S(1) line, and of the 3.3 micron emission feature, in Orion's Bar and three reflection nebulae. The emission from these species appears to come from the same spatial locations in all sources observed. This suggests that the 3.3 micron feature is excited by the same energetic UV-photons which cause the molecular hydrogen to fluoresce
Intercellular signaling as a cause of cell death in cyclically impacted cartilage explants
AbstractRecently, in vitro cartilage studies have shown that impact loading can produce structural damage and osteoarthritis-like changes, including tissue swelling, collagen denaturation, and cell death.Objective This study was to determine whether a signal for cell death moves through the cartilage matrix, resulting in the spread of cell death over time from impacted to unimpacted regions.Design Cyclic impacts were applied to the 2mm core of 4mm cartilage discs. Post-impact culturing extended for 3, 6 or 21 days and occurred in one of two ways. In one, discs were cultured intact. In the second, cores were removed immediately after cessation of impact and cores and rings cultured separately. Cells in apoptosis and later stage necrosis were monitored using the TUNEL assay.Results The extent of cell death in impacted samples increased with increased duration of post-impact culturing. At the early time, the majority of cell death was located in the regions of direct impact whereas after extended culture, the extent of cell death was similar in the surrounding unimpacted regions and in the impacted core region. However, the physical separation of the impacted core from the surrounding, non-impacted ring regions immediately after impact, and prior to independent culture, kept the level of cell death in the surrounding ring close to control levels, even after 21 days of incubation.Discussion These findings indicate that soluble intercellular signalling is involved in the spreading of cell death through the cartilage matrix, and that its effects can be prevented by physical isolation of the surrounding ring from the impacted core
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