2,239 research outputs found
Entanglement reduction induced by geometrical confinement in polymer thin films
We report simulation results on melts of entangled linear polymers confined
in a free-standing thin film. We study how the geometric constraints imposed by
the confinement alter the entanglement state of the system compared to the
equivalent bulk system using various observables. We find that the confinement
compresses the chain conformation uniaxially, decreasing the volume pervaded by
the chain, which in turn reduces the number of the accessible inter-chain
contact that could lead to entanglements. This local and non-uniform effect
depends on the position of the chain within the film. We also test a recently
presented theory that predicts how the number of entanglements decreases with
geometrical confinement.Comment: 28 pages, 10 figure
Calibration of piezoelectric positioning actuators using a reference voltage-to-displacement transducer based on quartz tuning forks
We use a piezoelectric quartz tuning fork to calibrate the displacement of
ceramic piezoelectric scanners which are widely employed in scanning probe
microscopy. We measure the static piezoelectric response of a quartz tuning
fork and find it to be highly linear, non-hysteretic and with negligible creep.
These performance characteristics, close to those of an ideal transducer, make
quartz transducers superior to ceramic piezoelectric actuators. Furthermore,
quartz actuators in the form of a tuning fork have the advantage of yielding
static displacements comparable to those of local probe microscope scanners. We
use the static displacement of a quartz tuning fork as a reference to calibrate
the three axis displacement of a ceramic piezoelectric scanner. Although this
calibration technique is a non-traceable method, it can be more versatile than
using calibration grids because it enables to characterize the linear and
non-linear response of a piezoelectric scanner in a broad range of
displacements, spanning from a fraction of a nanometer to hundreds of
nanometers. In addition, the creep and the speed dependent piezoelectric
response of ceramic scanners can be studied in detail.Comment: 9 pages, 3 figure
Continuous-variable quantum neural networks
We introduce a general method for building neural networks on quantum
computers. The quantum neural network is a variational quantum circuit built in
the continuous-variable (CV) architecture, which encodes quantum information in
continuous degrees of freedom such as the amplitudes of the electromagnetic
field. This circuit contains a layered structure of continuously parameterized
gates which is universal for CV quantum computation. Affine transformations and
nonlinear activation functions, two key elements in neural networks, are
enacted in the quantum network using Gaussian and non-Gaussian gates,
respectively. The non-Gaussian gates provide both the nonlinearity and the
universality of the model. Due to the structure of the CV model, the CV quantum
neural network can encode highly nonlinear transformations while remaining
completely unitary. We show how a classical network can be embedded into the
quantum formalism and propose quantum versions of various specialized model
such as convolutional, recurrent, and residual networks. Finally, we present
numerous modeling experiments built with the Strawberry Fields software
library. These experiments, including a classifier for fraud detection, a
network which generates Tetris images, and a hybrid classical-quantum
autoencoder, demonstrate the capability and adaptability of CV quantum neural
networks
Supertubes and special holonomy
We obtain a -supersymmetric 6-brane solution of IIA Supergravity by
T-dualizing the supertube recently found. The resulting electric
charge is related to the original -brane charge. The uplifted solution to
eleven dimensions results to be a purely geometrical configuration, which can
be interpreted as a bound state of a Taub-NUT space and a pp-wave. Being the
non trivial part of the metric pseudo-Riemannian, the resulting reduced
holonomy group is non-compact and locally isomorphic to a semidirect product of
an Abelian four dimensional group and SU(2).Comment: 11 pages, latex, no figure
Neutrino and axion bounds from the globular cluster M5 (NGC 5904)
The red-giant branch (RGB) in globular clusters is extended to larger
brightness if the degenerate helium core loses too much energy in "dark
channels." Based on a large set of archival observations, we provide
high-precision photometry for the Galactic globular cluster M5 (NGC 5904),
allowing for a detailed comparison between the observed tip of the RGB with
predictions based on contemporary stellar evolution theory. In particular, we
derive 95% confidence limits of on the
axion-electron coupling and (Bohr
magneton ) on a neutrino dipole moment, based on a detailed
analysis of statistical and systematic uncertainties. The cluster distance is
the single largest source of uncertainty and can be improved in the future.Comment: 5 pages, 2 figures, accepted for publication in Physical Review
Letter
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