9,424 research outputs found
Quantum Limitations on the Storage and Transmission of Information
Information must take up space, must weigh, and its flux must be limited.
Quantum limits on communication and information storage leading to these
conclusions are here described. Quantum channel capacity theory is reviewed for
both steady state and burst communication. An analytic approximation is given
for the maximum signal information possible with occupation number signal
states as a function of mean signal energy. A theorem guaranteeing that these
states are optimal for communication is proved. A heuristic "proof" of the
linear bound on communication is given, followed by rigorous proofs for signals
with specified mean energy, and for signals with given energy budget. And
systems of many parallel quantum channels are shown to obey the linear bound
for a natural channel architecture. The time--energy uncertainty principle is
reformulated in information language by means of the linear bound. The quantum
bound on information storage capacity of quantum mechanical and quantum field
devices is reviewed. A simplified version of the analytic proof for the bound
is given for the latter case. Solitons as information caches are discussed, as
is information storage in one dimensional systems. The influence of signal
self--gravitation on communication is considerd. Finally, it is shown that
acceleration of a receiver acts to block information transfer.Comment: Published relatively inaccessible review on a perennially interesting
subject. Plain TeX, 47 pages, 5 jpg figures (not embedded
Non-universal disordered Glauber dynamics
We consider the one-dimensional Glauber dynamics with coupling disorder in
terms of bilinear fermion Hamiltonians. Dynamic exponents embodied in the
spectrum gap of these latter are evaluated numerically by averaging over both
binary and Gaussian disorder realizations. In the first case, these exponents
are found to follow the non-universal values of those of plain dimerized
chains. In the second situation their values are still non-universal and
sub-diffusive below a critical variance above which, however, the relaxation
time is suggested to grow as a stretched exponential of the equilibrium
correlation length.Comment: 11 pages, 5 figures, brief addition
The shape and mechanics of curved fold origami structures
We develop recursion equations to describe the three-dimensional shape of a
sheet upon which a series of concentric curved folds have been inscribed. In
the case of no stretching outside the fold, the three-dimensional shape of a
single fold prescribes the shape of the entire origami structure. To better
explore these structures, we derive continuum equations, valid in the limit of
vanishing spacing between folds, to describe the smooth surface intersecting
all the mountain folds. We find that this surface has negative Gaussian
curvature with magnitude equal to the square of the fold's torsion. A series of
open folds with constant fold angle generate a helicoid
Uncertainty quantification for radio interferometric imaging: II. MAP estimation
Uncertainty quantification is a critical missing component in radio
interferometric imaging that will only become increasingly important as the
big-data era of radio interferometry emerges. Statistical sampling approaches
to perform Bayesian inference, like Markov Chain Monte Carlo (MCMC) sampling,
can in principle recover the full posterior distribution of the image, from
which uncertainties can then be quantified. However, for massive data sizes,
like those anticipated from the Square Kilometre Array (SKA), it will be
difficult if not impossible to apply any MCMC technique due to its inherent
computational cost. We formulate Bayesian inference problems with
sparsity-promoting priors (motivated by compressive sensing), for which we
recover maximum a posteriori (MAP) point estimators of radio interferometric
images by convex optimisation. Exploiting recent developments in the theory of
probability concentration, we quantify uncertainties by post-processing the
recovered MAP estimate. Three strategies to quantify uncertainties are
developed: (i) highest posterior density credible regions; (ii) local credible
intervals (cf. error bars) for individual pixels and superpixels; and (iii)
hypothesis testing of image structure. These forms of uncertainty
quantification provide rich information for analysing radio interferometric
observations in a statistically robust manner. Our MAP-based methods are
approximately times faster computationally than state-of-the-art MCMC
methods and, in addition, support highly distributed and parallelised
algorithmic structures. For the first time, our MAP-based techniques provide a
means of quantifying uncertainties for radio interferometric imaging for
realistic data volumes and practical use, and scale to the emerging big-data
era of radio astronomy.Comment: 13 pages, 10 figures, see companion article in this arXiv listin
Magnetoelastics of High Field Phenomena in Antiferromagnets UO2 and CeRhIn5
We use a recently developed optical fiber Bragg grating technique, in
continuous and pulsed magnetic fields in excess of 90T, to study magnetoelastic
correlations in magnetic materials at cryogenic temperatures. Both insulating
UO2 and metallic CeRhIn5 present antiferromagnetic ground states, at T_N =
30.3K and T_N = 3.85K respectively. A strong coupling of the magnetism to the
crystal lattice degrees of freedom in UO2 is found, revealing piezomagnetism as
well as the dynamics of antiferromagnetic domain switching between spin
arrangements connected by time reversal. The AFM domains become harder to
switch as the temperature is reduced, reaching a record value H_PZ(T = 4K) =
18T. The effect of strong magnetic fields is also studied in CeRhIn5, where an
anomaly in the sample crystallographic c-axis of magnitude Delta_c/c = 2 ppm is
found associated to a recently proposed electronic nematic state at H_en = 30T
applied 11o off the c-axis. Here we show that while this anomaly is absent when
the magnetic field is applied 18o off the a-axis, strong magnetoelastic quantum
oscillations attest to the high quality of the single crystal samples.Comment: 5 pages, figures include
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