3,201 research outputs found
Fast Decoders for Topological Quantum Codes
We present a family of algorithms, combining real-space renormalization
methods and belief propagation, to estimate the free energy of a topologically
ordered system in the presence of defects. Such an algorithm is needed to
preserve the quantum information stored in the ground space of a topologically
ordered system and to decode topological error-correcting codes. For a system
of linear size L, our algorithm runs in time log L compared to L^6 needed for
the minimum-weight perfect matching algorithm previously used in this context
and achieves a higher depolarizing error threshold.Comment: 4 pages, 4 figure
An NPZ Model with State-Dependent Delay due to Size-Structure in Juvenile Zooplankton
The study of planktonic ecosystems is important as they make up the bottom
trophic levels of aquatic food webs. We study a closed
Nutrient-Phytoplankton-Zooplankton (NPZ) model that includes size structure in
the juvenile zooplankton. The closed nature of the system allows the
formulation of a conservation law of biomass that governs the system. The model
consists of a system of nonlinear ordinary differential equation coupled to a
partial differential equation. We are able to transform this system into a
system of delay differential equations where the delay is of threshold type and
is state-dependent. The system of delay differential equations can be further
transformed into one with fixed delay. Using the different forms of the model
we perform a qualitative analysis of the solutions, which includes studying
existence and uniqueness, positivity and boundedness, local and global
stability, and conditions for extinction. Key parameters that are explored are
the total biomass in the system and the maturity level at which the juvenile
zooplankton reach maturity. Numerical simulations are also performed to verify
our analytical results
Relational time for systems of oscillators
Using an elementary example based on two simple harmonic oscillators, we show
how a relational time may be defined that leads to an approximate Schrodinger
dynamics for subsystems, with corrections leading to an intrinsic decoherence
in the energy eigenstates of the subsystem.Comment: Contribution to the Int. J. of Quant. Info. issue dedicated to the
memory of Asher Pere
Hybridization of Magnetism and Piezoelectricity for an Energy Scavenger based on Temporal Variation of Temperature
Autonomous microsystems are confronted today to a major challenge : the one
of energy supply. Energy scavenging, i.e. collecting energy from the ambient
environment has been developed to answer this problematic. Various sources have
already been successfully used (solar, vibration). This article presents
temporal variations of temperature as a new source of exploitable energy. A
brief review will take place at the beginning, exposing the different
approaches used in the past. Then we will focus our attention on hybridization
of magnetism and piezoelectricity. A new kind of thermal generator is proposed
and a preliminary model is exposed. Conclusions will be drawn on the
suitability of this prototype and the improvements that are needed to increase
its potential.Comment: Submitted on behalf of EDA Publishing Association
(http://irevues.inist.fr/handle/2042/16838
Interaction and flocculation of spherical colloids wetted by a surface-induced corona of paranematic order
Particles dispersed in a liquid crystal above the nematic-isotropic phase
transition are wetted by a surface-induced corona of paranematic order. Such
coronas give rise to pronounced two-particle interactions. In this article, we
report details on the analytical and numerical study of these interactions
published recently [Phys. Rev. Lett. 86, 3915 (2001)]. We especially
demonstrate how for large particle separations the asymptotic form of a Yukawa
potential arises. We show that the Yukawa potential is a surprisingly good
description for the two-particle interactions down to distances of the order of
the nematic coherence length. Based on this fact, we extend earlier studies on
a temperature induced flocculation transition in electrostatically stabilized
colloidal dispersions [Phys. Rev. E 61, 2831 (2000)]. We employ the Yukawa
potential to establish a flocculation diagram for a much larger range of the
electrostatic parameters, namely the surface charge density and the Debye
screening length. As a new feature, a kinetically stabilized dispersion close
to the nematic-isotropic phase transition is found.Comment: Revtex v4.0, 16 pages, 12 Postscript figures. Accepted for
publication in Phys. Rev.
Quantum reference frames and deformed symmetries
In the context of constrained quantum mechanics, reference systems are used
to construct relational observables that are invariant under the action of the
symmetry group. Upon measurement of a relational observable, the reference
system undergoes an unavoidable measurement "back-action" that modifies its
properties. In a quantum-gravitational setting, it has been argued that such a
back-action may produce effects that are described at an effective level as a
form of deformed (or doubly) special relativity. We examine this possibility
using a simple constrained system that has been extensively studied in the
context of quantum information. While our conclusions support the idea of a
symmetry deformation, they also reveal a host of other effects that may be
relevant to the context of quantum gravity, and could potentially conceal the
symmetry deformation.Comment: 11 pages, revtex. Comments are welcom
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