18,042 research outputs found
Optical Quantum Computation with Perpetually Coupled Spins
The possibility of using strongly and continuously interacting spins for
quantum computation has recently been discussed. Here we present a simple
optical scheme that achieves this goal while avoiding the drawbacks of earlier
proposals. We employ a third state, accessed by a classical laser field, to
create an effective barrier to information transfer. The mechanism proves to be
highly efficient both for continuous and pulsed laser modes; moreover it is
very robust, tolerating high decay rates for the excited states. The approach
is applicable to a broad range of systems, in particular dense structures such
as solid state self-assembled (e.g., molecular) devices. Importantly, there are
existing structures upon which `first step' experiments could be immediately
performed.Comment: 5 pages including 3 figures. Updated to published versio
X-ray Observations of XSS J12270-4859 in a New Low State: A Transformation to a Disk-Free Rotation-Powered Pulsar Binary
We present XMM-Newton and Chandra observations of the low-mass X-ray binary
XSS J12270--4859, which experienced a dramatic decline in optical/X-ray
brightness at the end of 2012, indicative of the disappearance of its accretion
disk. In this new state, the system exhibits previously absent
orbital-phase-dependent, large-amplitude X-ray modulations with a decline in
flux at superior conjunction. The X-ray emission remains predominantly
non-thermal but with an order of magnitude lower mean luminosity and
significantly harder spectrum relative to the previous high flux state. This
phenomenology is identical to the behavior of the radio millisecond pulsar
binary PSR J1023+0038 in the absence of an accretion disk, where the X-ray
emission is produced in an intra-binary shock driven by the pulsar wind. This
further demonstrates that XSS J12270-4859 no longer has an accretion disk and
has transformed to a full-fledged eclipsing "redback" system that hosts an
active rotation-powered millisecond pulsar. There is no evidence for diffuse
X-ray emission associated with the binary that may arise due to outflows or a
wind nebula. An extended source situated 1.5' from XSS J12270--4859 is unlikely
to be associated, and is probably a previously uncatalogued galaxy cluster.Comment: 8 pages, 6 figures; accepted for publication in the Astrophysical
Journa
Fastener-Based Computational Models with Application to Cold-Formed Steel Shear Walls
The objective of this paper is to validate a tool that design engineers could employ to develop mechanics-based predictions of the lateral response of wood-sheathed cold-formed steel (CFS) framed shear walls applicable in a wide variety of situations. Wood framed shear walls enjoy a variety of tools, most notably SAPWood and its predecessor CASHEW, that provide a means to predict the complete hysteretic behavior of a shear wall based on the nail fastener schedule and board selection. The existence of these tools helps engineers in unique design situations, encourages innovation in shear wall design particularly for Type I shear walls, and provides enabling modeling details critical for seismic performance-based design. Recently, as part of the CFS-NEES effort, the cyclic performance of CFS stud-to-sheathing connections has been characterized. In addition, the cyclic performance of full CFS shear walls, utilizing the same connections, has also been characterized. This paper explores an engineering model implemented in OpenSees that directly employs the fastener-based characterization as the essential nonlinearity in a CFS framed shear wall. CFS shear wall framing is modeled with beam elements, hold downs are modeled with linear springs, sheathing is modeled as a rigid diaphragm, and the stud-to-sheathing connections as zero-length springs utilizing the Pinching04 material model in OpenSees. Production, analysis, and post-processing of the model are automated with custom Matlab scripts that form the basis for a future engineering tool. The model is validated against monotonic and cyclic shear wall tests, and is shown to have good agreement. In addition to providing a mechanical means to assess shear walls, high fidelity shell finite element models are completed in ABAQUS to shed additional light on the mechanics-based OpenSees model. The long-term goal of the modelling is to provide a reliable means to predict the lateral response of any CFS framed system that relies on connection deformations, such as gravity walls or wood-sheathed floor diaphragms in addition to shear walls
Superabsorption of light via quantum engineering
Almost 60 years ago Dicke introduced the term superradiance to describe a
signature quantum effect: N atoms can collectively emit light at a rate
proportional to N^2. Even for moderate N this represents a significant increase
over the prediction of classical physics, and the effect has found applications
ranging from probing exciton delocalisation in biological systems, to
developing a new class of laser, and even in astrophysics. Structures that
super-radiate must also have enhanced absorption, but the former always
dominates in natural systems. Here we show that modern quantum control
techniques can overcome this restriction. Our theory establishes that
superabsorption can be achieved and sustained in certain simple nanostructures,
by trapping the system in a highly excited state while extracting energy into a
non-radiative channel. The effect offers the prospect of a new class of quantum
nanotechnology, capable of absorbing light many times faster than is currently
possible; potential applications of this effect include light harvesting and
photon detection. An array of quantum dots or a porphyrin ring could provide an
implementation to demonstrate this effect
WFPC2 Observations of Massive and Compact Young Star Clusters in M31
We present color magnitude diagrams of four blue massive and compact star
clusters in M31: G38, G44, G94, and G293. The diagrams of the four clusters
reveal a well-populated upper main sequence and various numbers of supergiants.
The U-B and B-V colors of the upper main sequence stars are used to determine
reddening estimates of the different lines of sight in the M31 disk. Reddening
values range from E(B-V) = 0.20 +/- 0.10 to 0.31 +/- 0.11. We statistically
remove field stars on the basis of completeness, magnitude and color. Isochrone
fits to the field-subtracted, reddening-corrected diagrams yield age estimates
ranging from 63 +/- 15 Myr to 160 +/- 60 Myr. Implications for the recent
evolution of the disk near NGC 206 are discussed.Comment: 17 pages, Latex, ApJ, in Pres
Coherence of Spin Qubits in Silicon
Given the effectiveness of semiconductor devices for classical computation
one is naturally led to consider semiconductor systems for solid state quantum
information processing. Semiconductors are particularly suitable where local
control of electric fields and charge transport are required. Conventional
semiconductor electronics is built upon these capabilities and has demonstrated
scaling to large complicated arrays of interconnected devices. However, the
requirements for a quantum computer are very different from those for classical
computation, and it is not immediately obvious how best to build one in a
semiconductor. One possible approach is to use spins as qubits: of nuclei, of
electrons, or both in combination. Long qubit coherence times are a
prerequisite for quantum computing, and in this paper we will discuss
measurements of spin coherence in silicon. The results are encouraging - both
electrons bound to donors and the donor nuclei exhibit low decoherence under
the right circumstances. Doped silicon thus appears to pass the first test on
the road to a quantum computer.Comment: Submitted to J Cond Matter on Nov 15th, 200
Global Optical Control of a Quantum Spin Chain
Quantum processors which combine the long decoherence times of spin qubits
together with fast optical manipulation of excitons have recently been the
subject of several proposals. I show here that arbitrary single- and entangling
two-qubit gates can be performed in a chain of perpetually coupled spin qubits
solely by using laser pulses to excite higher lying states. It is also
demonstrated that universal quantum computing is possible even if these pulses
are applied {\it globally} to a chain; by employing a repeating pattern of four
distinct qubit units the need for individual qubit addressing is removed. Some
current experimental qubit systems would lend themselves to implementing this
idea.Comment: 5 pages, 3 figure
X-ray Observations of High-B Radio Pulsars
The study of high-magnetic-field pulsars is important for examining the
relationships between radio pulsars, magnetars, and X-ray-isolated neutron
stars (XINSs). Here we report on X-ray observations of three such
high-magnetic-field radio pulsars. We first present the results of a deep
XMM-Newton observation of PSR J1734-3333, taken to follow up on its initial
detection in 2009. The pulsar's spectrum is well fit by a blackbody with a
temperature of 300 +/- 60 eV, with bolometric luminosity L_bb = 2.0(+2.2
-0.7)e+32 erg/s = 0.0036E_dot for a distance of 6.1 kpc. We detect no X-ray
pulsations from the source, setting a 1 sigma upper limit on the pulsed
fraction of 60% in the 0.5-3 keV band. We compare PSR J1734-3333 to other
rotation-powered pulsars of similar age and find that it is significantly
hotter, supporting the hypothesis that the magnetic field affects the observed
thermal properties of pulsars. We also report on XMM-Newton and Chandra
observations of PSRs B1845-19 and J1001-5939. We do not detect either pulsar,
setting 3 sigma upper limits on their blackbody temperatures of 48 and 56 eV,
respectively. Despite the similarities in rotational properties, these sources
are significantly cooler than all but one of the XINSs, which we attribute to
the two groups having been born with different magnetic fields and hence
evolving differently.Comment: 18 pages, 2 tables, 5 figures, accepted for publication in the
Astrophysical Journa
Holocene Earthquakes and Late Pleistocene Slip-Rate Estimates on the Wassuk Range Fault Zone, Nevada
The Wassuk Range fault zone is an 80‐km‐long, east‐dipping, high‐angle normal fault that flanks the eastern margin of the Wassuk Range in central Nevada. Observations from two alluvial fan systems truncated by the fault yield information on the vertical slip rate and Holocene earthquake history along the range front. At the apex of the Rose Creek alluvial fan, radiocarbon dating of offset stratigraphy exposed in two fault trenches shows that multiple earthquakes resulted in 7.0 m of vertical offset along the fault since ∼9400 cal B.P. These data yield a Holocene vertical slip rate of 0.7±0.1 mm/yr. The south trench exposure records at least two faulting events since ∼9400 cal B.P., with the most recent displacement postdating ∼2810 cal B.P. The north trench exposure records an ∼1 m offset between ∼610 cal B.P. and A.D. ∼1850, a 1.3‐m minimum offset prior to ∼1460 cal B.P., and one earlier undated earthquake of a similar size. Variations in stratigraphy and limited datable material preclude a unique correlation of paleoevents between the two trenches. Approximately 25 km north, the range‐front fault has truncated and uplifted a remnant of the Penrod Canyon fan by \u3e40 m since the surface was deposited ∼113 ka, based on cosmogenic dating of two large boulders. These data allow an estimate of the minimum late Pleistocene vertical slip rate at \u3e0.3–0.4 mm/yr for the Wassuk Range fault zone
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