19,027 research outputs found
Degradation of a quantum reference frame
We investigate the degradation of reference frames, treated as dynamical
quantum systems, and quantify their longevity as a resource for performing
tasks in quantum information processing. We adopt an operational measure of a
reference frame's longevity, namely, the number of measurements that can be
made against it with a certain error tolerance. We investigate two distinct
types of reference frame: a reference direction, realized by a spin-j system,
and a phase reference, realized by an oscillator mode with bounded energy. For
both cases, we show that our measure of longevity increases quadratically with
the size of the reference system and is therefore non-additive. For instance,
the number of measurements that a directional reference frame consisting of N
parallel spins can be put to use scales as N^2. Our results quantify the extent
to which microscopic or mesoscopic reference frames may be used for repeated,
high-precision measurements, without needing to be reset - a question that is
important for some implementations of quantum computing. We illustrate our
results using the proposed single-spin measurement scheme of magnetic resonance
force microscopy.Comment: 9 pages plus appendices, 4 figures, published versio
X-ray fluoresced high-Z (up to Z = 82) K-x-rays produced by LiNbO3 and LiTaO3 pyroelectric crystal electron accelerators
High-energy bremsstrahlung and K X-rays were used to produce nearly
background-free K X-ray spectra of up to 87 keV (Pb) via X-ray fluorescence.
The fluorescing radiation was produced by electron accelerators, consisting of
heated and cooled cylindrical LiTaO3 and LiNbO3 crystals at mTorr pressures.
The newly discovered process of gas amplification whereby the ambient gas
pressure is optimized to maximize the electron energy was used to produce
energetic electrons which when incident on a W/Bi target gave rise to a
radiation field consisting of high-energy bremsstrahlung as well as W and Bi K
X-rays. These photons were used to fluoresce Ta and Pb K X-rays.Comment: 6 pages, 6 figures, PD
Relativistically invariant quantum information
We show that quantum information can be encoded into entangled states of
multiple indistinguishable particles in such a way that any inertial observer
can prepare, manipulate, or measure the encoded state independent of their
Lorentz reference frame. Such relativistically invariant quantum information is
free of the difficulties associated with encoding into spin or other degrees of
freedom in a relativistic context.Comment: 5 pages, published versio
Optimal measurements for relative quantum information
We provide optimal measurement schemes for estimating relative parameters of
the quantum state of a pair of spin systems. We prove that the optimal
measurements are joint measurements on the pair of systems, meaning that they
cannot be achieved by local operations and classical communication. We also
demonstrate that in the limit where one of the spins becomes macroscopic, our
results reproduce those that are obtained by treating that spin as a classical
reference direction.Comment: 6 pages, 1 figure, published versio
Constraints on the anisotropy of dark energy
If the equation of state of dark energy is anisotropic there will be
additional quadrupole anisotropy in the cosmic microwave background induced by
the time dependent anisotropic stress quantified in terms of .
Assuming that the entire amplitude of the observed quadrupole is due to this
anisotropy, we conservatively impose a limit of for any value of assuming that . This is
considerably tighter than that which comes from SNe. Stronger limits, upto a
factor of 10, are possible for specific values of and .
Since we assume this component is uncorrelated with the stochastic component
from inflation, we find that both the expectation value and the sample variance
are increased. There no improvement in the likelihood of an anomalously low
quadrupole as suggested by previous work on an elliptical universe
The Highly Oscillatory Behavior of Automorphic Distributions for SL(2)
Automorphic distributions for SL(2) arise as boundary values of modular forms
and, in a more subtle manner, from Maass forms. In the case of modular forms of
weight one or of Maass forms, the automorphic distributions have continuous
first antiderivatives. We recall earlier results of one of us on the Holder
continuity of these continuous functions and relate them to results of other
authors; this involves a generalization of classical theorems on Fourier series
by S. Bernstein and Hardy-Littlewood. We then show that the antiderivatives are
non-differentiable at all irrational points, as well as all, or in certain
cases, some rational points. We include graphs of several of these functions,
which clearly display a high degree of oscillation. Our investigations are
motivated in part by properties of "Riemann's nondifferentiable function", also
known as "Weierstrass' function".Comment: 27 pages, 6 Figures; version 2 corrects misprints and updates
reference
Response versus Chain Length of Alkanethiol-Capped Au Nanoparticle Chemiresistive Chemical Vapor Sensors
Au nanoparticles capped with a homologous series of straight chain alkanethiols (containing 4−11 carbons in length) have been investigated as chemiresistive organic vapor sensors. The series of alkanethiols was used to elucidate the mechanisms of vapor detection by such capped nanoparticle chemiresistive films and to highlight the molecular design principles that govern enhanced detection. The thiolated Au nanoparticle chemiresistors demonstrated rapid and reversible responses to a set of test vapors (n-hexane, n-heptane, n-octane, iso-octane, cyclohexane, toluene, ethyl acetate, methanol, ethanol, isopropanol, and 1-butanol) that possessed a variety of analyte physicochemical properties. The resistance sensitivity to nonpolar and aprotic polar vapors systematically increased as the chain length of the capping reagent increased. Decreases in the nanoparticle film resistances, which produced negative values of the differential resistance response, were observed upon exposure of the sensor films to alcohol vapors. The response signals became more negative with higher alcohol vapor concentrations, producing negative values of the sensor sensitivity. Sorption data measured on Au nanoparticle chemiresistor films using a quartz crystal microbalance allowed for the measurement of the partition coefficients of test vapors in the Au nanoparticle films. This measurement assumed that analyte sorption only occurred at the organic interface and not the surface of the Au core. Such an assumption produced partition coefficient values that were independent of the length of the ligand. Furthermore, the value of the partition coefficient was used to obtain the particle-to-particle interfacial effective dielectric constant of films upon exposure to analyte vapors. The values of the dielectric constant upon exposure to alcohol vapors suggested that the observed resistance response changes observed were not significantly influenced by this dielectric change, but rather were primarily influenced by morphological changes and by changes in the interparticle spacing
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Assessment of Vessel Density on Non-Contrast Computed Tomography to Detect Basilar Artery Occlusion
Introduction: Basilar artery occlusion (BAO) may be clinically occult due to variable and non-specific symptomatology. We evaluated the qualitative and quantitative determination of a hyperdense basilar artery (HDBA) on non-contrast computed tomography (NCCT) brain for the diagnosis of BAO.Methods: We conducted a case control study of patients with confirmed acute BAO vs a control group of suspected acute stroke patients without BAO. Two EM attending physicians, one third-year EM resident, and one medical student performed qualitative and quantitative assessments for the presence of a HDBA on axial NCCT images. Our primary outcome measures were sensitivity and specificity for BAO. Our secondary outcomes were inter-rater and intra-rater reliability of the qualitative and quantitative assessments.Results: We included 60 BAO and 65 control patients in our analysis. Qualitative assessment of the hyperdense basilar artery sign was poorly sensitive (54%–72%) and specific (55%–89%). Quantitative measurement improved the specificity of hyperdense basilar artery assessment for diagnosing BAO, with a threshold of 61.0–63.8 Hounsfield units demonstrating relatively high specificity of 85%–94%. There was moderate inter-rater agreement for the qualitative assessment of HDBA (Fleiss’ kappa statistic 0.508, 95% confidence interval: 0.435–0.581). Agreement improved for quantitative assessments, but still fell in the moderate range (Shrout-Fleiss intraclass correlation coefficient: 0.635). Intra-rater reliability for the quantitative assessments of the two attending physician reviewers demonstrated substantial consistency.Conclusion: Our results highlight the importance of carefully examining basilar artery density when interpreting the NCCT of patients with altered consciousness or other signs and symptoms concerning for an acute basilar artery occlusion. If the Hounsfield unit density of the basilar artery exceeds 61 Hounsfield units, BAO should be highly suspected
Light Nuclei as Quantized Skyrmions
We consider the rigid body quantization of Skyrmions with topological charges
1 to 8, as approximated by the rational map ansatz. Novel, general expressions
for the elements of the inertia tensors, in terms of the approximating rational
map, are presented and are used to determine the kinetic energy contribution to
the total energy of the ground and excited states of the quantized Skyrmions.
Our results are compared to the experimentally determined energy levels of the
corresponding nuclei, and the energies and spins of a few as yet unobserved
states are predicted.Comment: 33 pages, 16 figures, Section 13 replace
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