17,496 research outputs found
Roche volume filling and the dissolution of open star clusters
From direct N-body simulations we find that the dynamical evolution of star
clusters is strongly influenced by the Roche volume filling factor. We present
a parameter study of the dissolution of open star clusters with different Roche
volume filling factors and different particle numbers. We study both Roche
volume underfilling and overfilling models and compare with the Roche volume
filling case. We find that in the Roche volume overfilling limit of our
simulations two-body relaxation is no longer the dominant dissolution mechanism
but the changing cluster potential. We call this mechnism "mass-loss driven
dissolution" in contrast to "two-body relaxation driven dissolution" which
occurs in the Roche volume underfilling regime. We have measured scaling
exponents of the dissolution time with the two-body relaxation time. In this
experimental study we find a decreasing scaling exponent with increasing Roche
volume filling factor. The evolution of the escaper number in the Roche volume
overfilling limit can be described by a log-logistic differential equation. We
report the finding of a resonance condition which may play a role for the
evolution of star clusters and may be calibrated by the main periodic orbit in
the large island of retrograde quasiperiodic orbits in the Poincar\'e surfaces
of section. We also report on the existence of a stability curve which may be
of relevance with respect to the structure of star clusters.Comment: 14 pages, 10+1 figures, accepted by Astronomische Nachrichte
Velocity Tails for Inelastic Maxwell Models
We study the velocity distribution function for inelastic Maxwell models,
characterized by a Boltzmann equation with constant collision rate, independent
of the energy of the colliding particles. By means of a nonlinear analysis of
the Boltzmann equation, we find that the velocity distribution function decays
algebraically for large velocities, with exponents that are analytically
calculated.Comment: 4 pages, 2 figure
Correlated errors can lead to better performance of quantum codes
A formulation for evaluating the performance of quantum error correcting
codes for a general error model is presented. In this formulation, the
correlation between errors is quantified by a Hamiltonian description of the
noise process. We classify correlated errors using the system-bath interaction:
local versus nonlocal and two-body versus many-body interactions. In
particular, we consider Calderbank-Shor-Steane codes and observe a better
performance in the presence of correlated errors depending on the timing of the
error recovery. We also find this timing to be an important factor in the
design of a coding system for achieving higher fidelities.Comment: 5 pages, 3 figures. Replaced by the published version. Title change
Advances in decoherence control
I address the current status of dynamical decoupling techniques in terms of
required control resources and feasibility. Based on recent advances in both
improving the theoretical design and assessing the control performance for
specific noise models, I argue that significant progress may still be possible
on the road of implementing decoupling under realistic constraints.Comment: 14 pages, 3 encapsulated eps figures. To appear in Journal of Modern
Optics, Special Proceedings Volume of the XXXIV Winter Colloquium on the
Physics of Quantum Electronics, Snowbird, Jan 200
MEXIT: Maximal un-coupling times for stochastic processes
Classical coupling constructions arrange for copies of the \emph{same} Markov
process started at two \emph{different} initial states to become equal as soon
as possible. In this paper, we consider an alternative coupling framework in
which one seeks to arrange for two \emph{different} Markov (or other
stochastic) processes to remain equal for as long as possible, when started in
the \emph{same} state. We refer to this "un-coupling" or "maximal agreement"
construction as \emph{MEXIT}, standing for "maximal exit". After highlighting
the importance of un-coupling arguments in a few key statistical and
probabilistic settings, we develop an explicit \MEXIT construction for
stochastic processes in discrete time with countable state-space. This
construction is generalized to random processes on general state-space running
in continuous time, and then exemplified by discussion of \MEXIT for Brownian
motions with two different constant drifts.Comment: 28 page
Efficient decoupling schemes with bounded controls based on Eulerian orthogonal arrays
The task of decoupling, i.e., removing unwanted interactions in a system
Hamiltonian and/or couplings with an environment (decoherence), plays an
important role in controlling quantum systems. There are many efficient
decoupling schemes based on combinatorial concepts like orthogonal arrays,
difference schemes and Hadamard matrices. So far these (combinatorial)
decoupling schemes have relied on the ability to effect sequences of
instantaneous, arbitrarily strong control Hamiltonians (bang-bang controls). To
overcome the shortcomings of bang-bang control Viola and Knill proposed a
method called Eulerian decoupling that allows the use of bounded-strength
controls for decoupling. However, their method was not directly designed to
take advantage of the composite structure of multipartite quantum systems. In
this paper we define a combinatorial structure called an Eulerian orthogonal
array. It merges the desirable properties of orthogonal arrays and Eulerian
cycles in Cayley graphs (that are the basis of Eulerian decoupling). We show
that this structure gives rise to decoupling schemes with bounded-strength
control Hamiltonians that can be applied to composite quantum systems with few
body Hamiltonians and special couplings with the environment. Furthermore, we
show how to construct Eulerian orthogonal arrays having good parameters in
order to obtain efficient decoupling schemes.Comment: 8 pages, revte
Ginzburg-Landau Vortex Lattice in Superconductor Films of Finite Thickness
The Ginzburg-Landau equations are solved for ideally periodic vortex lattices
in superconducting films of arbitrary thickness in a perpendicular magnetic
field. The order parameter, current density, magnetic moment, and the
3-dimensional magnetic field inside and outside the film are obtained in the
entire ranges of the applied magnetic field, Ginzburg Landau parameter kappa,
and film thickness. The superconducting order parameter varies very little near
the surface (by about 0.01) and the energy of the film surface is small. The
shear modulus c66 of the triangular vortex lattice in thin films coincides with
the bulk c66 taken at large kappa. In thin type-I superconductor films with
kappa < 0.707, c66 can be positive at low fields and negative at high fields.Comment: 12 pages including 14 Figures, corrected, Fig.14 added, appears in
Phys. Rev. B 71, issue 1 (2005
Evidence of secondary relaxations in the dielectric spectra of ionic liquids
We investigated the dynamics of a series of room temperature ionic liquids
based on the same 1-butyl-3-methyl imidazolium cation and different anions by
means of broadband dielectric spectroscopy covering 15 decades in frequency
(10^(-6)-10^9 Hz), and in the temperature range from 400 K down to 35 K. An
ionic conductivity is observed above the glass transition temperature T_{g}
with a relaxation in the electric modulus representation. Below T_{g}, two
relaxation processes appear, with the same features as the secondary
relaxations typically observed in molecular glasses. The activation energy of
the secondary processes and their dependence on the anion are different. The
slower process shows the characteristics of an intrinsic Johari-Goldstein
relaxation, in particular an activation energy E_{beta}=24k_{B}T_{g} is found,
as observed in molecular glasses.Comment: Major revision, submitted to Phys. Rev. Let
Nontrivial Velocity Distributions in Inelastic Gases
We study freely evolving and forced inelastic gases using the Boltzmann
equation. We consider uniform collision rates and obtain analytical results
valid for arbitrary spatial dimension d and arbitrary dissipation coefficient
epsilon. In the freely evolving case, we find that the velocity distribution
decays algebraically, P(v,t) ~ v^{-sigma} for sufficiently large velocities. We
derive the exponent sigma(d,epsilon), which exhibits nontrivial dependence on
both d and epsilon, exactly. In the forced case, the velocity distribution
approaches a steady-state with a Gaussian large velocity tail.Comment: 4 pages, 1 figur
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