2,678 research outputs found
Quantum repeaters with imperfect memories: cost and scalability
Memory dephasing and its impact on the rate of entanglement generation in
quantum repeaters is addressed. For systems that rely on probabilistic schemes
for entanglement distribution and connection, we estimate the maximum
achievable rate per employed memory for our optimized partial nesting protocol.
We show that, for any given distance , the polynomial scaling of rate with
distance can only be achieved if quantum memories with coherence times on the
order of or longer, with being the speed of light in the channel, are
available. The above rate degrades as a power of
with distance when the coherence time .Comment: Extended version with 5 figure
Push recovery with stepping strategy based on time-projection control
In this paper, we present a simple control framework for on-line push
recovery with dynamic stepping properties. Due to relatively heavy legs in our
robot, we need to take swing dynamics into account and thus use a linear model
called 3LP which is composed of three pendulums to simulate swing and torso
dynamics. Based on 3LP equations, we formulate discrete LQR controllers and use
a particular time-projection method to adjust the next footstep location
on-line during the motion continuously. This adjustment, which is found based
on both pelvis and swing foot tracking errors, naturally takes the swing
dynamics into account. Suggested adjustments are added to the Cartesian 3LP
gaits and converted to joint-space trajectories through inverse kinematics.
Fixed and adaptive foot lift strategies also ensure enough ground clearance in
perturbed walking conditions. The proposed structure is robust, yet uses very
simple state estimation and basic position tracking. We rely on the physical
series elastic actuators to absorb impacts while introducing simple laws to
compensate their tracking bias. Extensive experiments demonstrate the
functionality of different control blocks and prove the effectiveness of
time-projection in extreme push recovery scenarios. We also show self-produced
and emergent walking gaits when the robot is subject to continuous dragging
forces. These gaits feature dynamic walking robustness due to relatively soft
springs in the ankles and avoiding any Zero Moment Point (ZMP) control in our
proposed architecture.Comment: 20 pages journal pape
Restricted Discrete Invariance and Self-Synchronization For Stable Walking of Bipedal Robots
Models of bipedal locomotion are hybrid, with a continuous component often
generated by a Lagrangian plus actuators, and a discrete component where leg
transfer takes place. The discrete component typically consists of a locally
embedded co-dimension one submanifold in the continuous state space of the
robot, called the switching surface, and a reset map that provides a new
initial condition when a solution of the continuous component intersects the
switching surface. The aim of this paper is to identify a low-dimensional
submanifold of the switching surface, which, when it can be rendered invariant
by the closed-loop dynamics, leads to asymptotically stable periodic gaits. The
paper begins this process by studying the well-known 3D Linear Inverted
Pendulum (LIP) model, where analytical results are much easier to obtain. A key
contribution here is the notion of \textit{self-synchronization}, which refers
to the periods of the pendular motions in the sagittal and frontal planes
tending to a common period. The notion of invariance resulting from the study
of the 3D LIP model is then extended to a 9-DOF 3D biped. A numerical study is
performed to illustrate that asymptotically stable walking may be obtained.Comment: Conferenc
Specific Heat of the Ca-Intercalated Graphite Superconductor CaC
The superconducting state of Ca-intercalated graphite CaC6 has been
investigated by specific heat measurements. The characteristic anomaly at the
superconducting transition (Tc = 11.4 K) indicates clearly the bulk nature of
the superconductivity. The temperature and magnetic field dependence of the
electronic specific heat are consistent with a fully-gapped superconducting
order parameter. The estimated electron-phonon coupling constant is lambda =
0.60 - 0.74 suggesting that the relatively high Tc of CaC6 can be explained
within the weak-coupling BCS approach.Comment: 4 pages, 4 figs, submitted to Phys. Rev. Let
Superconductivity in Heavy Alkaline-Earths Intercalated Graphites
We report the discovery of superconductivity below 1.65(6) K in
Sr-intercalated graphite SrC6, by susceptibility and specific heat (Cp)
measurements. In comparison with CaC6, we found that the anisotropy of the
upper critical fields for SrC6 is much reduced. The Cp anomaly at Tc is smaller
than the BCS prediction indicating an anisotropic superconducting gap for SrC6
similar to CaC6. The significantly lower Tc of SrC6 as compared to CaC6 can be
understood in terms of "negative" pressure effects, which decreases the
electron-phonon coupling for both in-plane intercalant and the out-of-plane C
phonon modes. We observed no superconductivity for BaC6 down to 0.3 K.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Confinement interaction in nonlinear generalizations of the Wick-Cutkosky model
We consider nonlinear-mediating-field generalizations of the Wick-Cutkosky
model. Using an iterative approach and eliminating the mediating field by means
of the covariant Green function we arrive at a Lagrangian density containing
many-point time-nonlocal interaction terms. In low-order approximations of
theory we obtain the usual two-current interaction as well as
a three-current interaction of a confining type. The same result is obtained
without approximation for a version of the dipole model. The transition to the
Hamiltonian formalism and subsequent canonical quantization is performed with
time non-locality taken into account approximately.
A relativistic three-particle wave equation is derived variationally by using
a three-particle Fock space trial state. The non-relativistic limit of this
equation is obtained and its properties are analyzed and discussed.Comment: 15 pages, 1 figure, LaTe
On the finite-volume Lattice Boltzmann modeling of thermo-hydrodynamics
AbstractIn this paper, Thermal Finite-Volume Lattice Boltzmann Method is developed. To demonstrate the temperature field, the Double Distribution Function (DDF) of thermal lattice Boltzmann equation is used. The upwind biasing factors based on pressure and temperature are defined and applied as flux corrector in the thermo-hydrodynamic lattice Boltzmann equations. A consistent open and solid boundary treatment of flow is also addressed. The unknown energy distribution at the boundary cells are decomposed into its equilibrium and non-equilibrium parts. Then the non-equilibrium part is approximated with extrapolation of the non-equilibrium part of the populations at the neighboring nodes. This treatment enlarges the domain stability and led up to faster convergence. Two test cases namely, thermo-hydrodynamic in a backward-facing step and around a circular cylinder inserted within a backward-facing step are carried out. The results are compared with the available solutions in the technical literature
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