12,569 research outputs found
On the Nature of Precursors in the Radio Pulsar Profiles
In the average profiles of several radio pulsars, the main pulse is
accompanied by the preceding component. This so called precursor is known for
its distinctive polarization, spectral, and fluctuation properties. Recent
single-pulse observations hint that the sporadic activity at the extreme
leading edge of the pulse may be prevalent in pulsars. We for the first time
propose a physical mechanism of this phenomenon. It is based on the induced
scattering of the main pulse radiation into the background. We show that the
scattered component is directed approximately along the ambient magnetic field
and, because of rotational aberration in the scattering region, appears in the
pulse profile as a precursor to the main pulse. Our model naturally explains
high linear polarization of the precursor emission, its spectral and
fluctuation peculiarities as well as suggests a specific connection between the
precursor and the main pulse at widely spaced frequencies. This is believed to
stimulate multifrequency single-pulse studies of intensity modulation in
different pulsars.Comment: 5 pages, no figures. Accepted for publication in MNRAS Letter
Topological code Autotune
Many quantum systems are being investigated in the hope of building a
large-scale quantum computer. All of these systems suffer from decoherence,
resulting in errors during the execution of quantum gates. Quantum error
correction enables reliable quantum computation given unreliable hardware.
Unoptimized topological quantum error correction (TQEC), while still effective,
performs very suboptimally, especially at low error rates. Hand optimizing the
classical processing associated with a TQEC scheme for a specific system to
achieve better error tolerance can be extremely laborious. We describe a tool
Autotune capable of performing this optimization automatically, and give two
highly distinct examples of its use and extreme outperformance of unoptimized
TQEC. Autotune is designed to facilitate the precise study of real hardware
running TQEC with every quantum gate having a realistic, physics-based error
model.Comment: 13 pages, 17 figures, version accepted for publicatio
Quantum computing with nearest neighbor interactions and error rates over 1%
Large-scale quantum computation will only be achieved if experimentally
implementable quantum error correction procedures are devised that can tolerate
experimentally achievable error rates. We describe a quantum error correction
procedure that requires only a 2-D square lattice of qubits that can interact
with their nearest neighbors, yet can tolerate quantum gate error rates over
1%. The precise maximum tolerable error rate depends on the error model, and we
calculate values in the range 1.1--1.4% for various physically reasonable
models. Even the lowest value represents the highest threshold error rate
calculated to date in a geometrically constrained setting, and a 50%
improvement over the previous record.Comment: 4 pages, 8 figure
Engaging the articulators enhances perception of concordant visible speech movements
PURPOSE
This study aimed to test whether (and how) somatosensory feedback signals from the vocal tract affect concurrent unimodal visual speech perception.
METHOD
Participants discriminated pairs of silent visual utterances of vowels under 3 experimental conditions: (a) normal (baseline) and while holding either (b) a bite block or (c) a lip tube in their mouths. To test the specificity of somatosensory-visual interactions during perception, we assessed discrimination of vowel contrasts optically distinguished based on their mandibular (English /ɛ/-/æ/) or labial (English /u/-French /u/) postures. In addition, we assessed perception of each contrast using dynamically articulating videos and static (single-frame) images of each gesture (at vowel midpoint).
RESULTS
Engaging the jaw selectively facilitated perception of the dynamic gestures optically distinct in terms of jaw height, whereas engaging the lips selectively facilitated perception of the dynamic gestures optically distinct in terms of their degree of lip compression and protrusion. Thus, participants perceived visible speech movements in relation to the configuration and shape of their own vocal tract (and possibly their ability to produce covert vowel production-like movements). In contrast, engaging the articulators had no effect when the speaking faces did not move, suggesting that the somatosensory inputs affected perception of time-varying kinematic information rather than changes in target (movement end point) mouth shapes.
CONCLUSIONS
These findings suggest that orofacial somatosensory inputs associated with speech production prime premotor and somatosensory brain regions involved in the sensorimotor control of speech, thereby facilitating perception of concordant visible speech movements.
SUPPLEMENTAL MATERIAL
https://doi.org/10.23641/asha.9911846R01 DC002852 - NIDCD NIH HHSAccepted manuscrip
Experimental characterization of deployable trusses and joints
The structural dynamic properties of trusses are strongly affected by the characteristics of joints connecting the individual beam elements. Joints are particularly significant in that they are often the source of nonlinearities and energy dissipation. While the joints themselves may be physically simple, direct measurement is often necessary to obtain a mathematical description suitable for inclusion in a system model. Force state mapping is a flexible, practical test method for obtaining such a description, particularly when significant nonlinear effects are present. It involves measurement of the relationship, nonlinear or linear, between force transmitted through a joint and the relative displacement and velocity across it. An apparatus and procedure for force state mapping are described. Results are presented from tests of joints used in a lightweight, composite, deployable truss built by the Boeing Aerospace Company. The results from the joint tests are used to develop a model of a full 4-bay truss segment. The truss segment was statically and dynamically tested. The results of the truss tests are presented and compared with the analytical predictions from the model
Quantum Error Correction on Linear Nearest Neighbor Qubit Arrays
A minimal depth quantum circuit implementing 5-qubit quantum error correction
in a manner optimized for a linear nearest neighbor architecture is described.
The canonical decomposition is used to construct fast and simple gates that
incorporate the necessary swap operations. Simulations of the circuit's
performance when subjected to discrete and continuous errors are presented. The
relationship between the error rate of a physical qubit and that of a logical
qubit is investigated with emphasis on determining the concatenated error
correction threshold.Comment: 4 pages, 5 figure
Contamination of spacecraft by recontact of dumped liquids
Liquids partially freeze when dumped from spacecraft producing particles which are released into free space at various velocities. Recontact of these particles with the spacecraft is possible for specific particle sizes and velocities and, therefore, can become contamination for experiments within the spacecraft or released experiments as a result of waste and potable water dumped from Space Shuttle. An examination of dump characteristics was conducted on STS-29 using both on-board video records and ground based measurements. A preliminary analysis of data from this flight indicates particle velocities are in the range of 30 to 75 ft/sec and recontact is possible for limited particle sizes
Studies in the use of cloud type statistics in mission simulation
A study to further improve NASA's global cloud statistics for mission simulation is reported. Regional homogeneity in cloud types was examined; most of the original region boundaries defined for cloud cover amount in previous studies were supported by the statistics on cloud types and the number of cloud layers. Conditionality in cloud statistics was also examined with special emphasis on temporal and spatial dependencies, and cloud type interdependence. Temporal conditionality was found up to 12 hours, and spatial conditionality up to 200 miles; the diurnal cycle in convective cloudiness was clearly evident. As expected, the joint occurrence of different cloud types reflected the dynamic processes which form the clouds. Other phases of the study improved the cloud type statistics for several region and proposed a mission simulation scheme combining the 4-dimensional atmospheric model, sponsored by MSFC, with the global cloud model
Foraminiferal and sedimentological evidence far uplift of the deep-sea floor, Gorda Rise, northeastern Pacific
Displaced benthic Foraminifera in semiconsolidated clayey sandstone from the Gorda Rise provide evidence for uplift of the deep-sea floor. The sediment was deposited by turbidity currents on the floor of the Escanaba Trough (the axial valley of the Gorda Rise) from a sublittoral source on the continental margin to the east...
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