16,977 research outputs found
Star-triangle equivalence in soap froths
In two dimensional foams at equilibrium, triangular bubbles can be freely
exchanged with 3-fold stars --three edges ending at a central vertex. This
theorem is deduced here from Moukarzel's duality. Moreover, to probe the
method, a few related properties are established: under slow gas diffusion, T2
processes are continuous for triangles but not for other types of bubbles. In
general, the gas flow results in different configurations in the presence of a
triangle than in the presence of a star.Comment: 7 pages, 4 figures (6 eps files
Foams in contact with solid boundaries: equilibrium conditions and conformal invariance
A liquid foam in contact with a solid surface forms a two-dimensional foam on
the surface. We derive the equilibrium equations for this 2D foam when the
solid surface is curved and smooth, generalising the standard case of flat Hele
Shaw cells. The equilibrium conditions at the vertices in 2D, at the edges in
3D, are invariant by conformal transformations. Regarding the films, conformal
invariance only holds with restrictions, which we explicit for 3D and flat 2D
foams. Considering foams confined in thin interstices between two non parallel
plates, normal incidence and Laplace's law lead to an approximate equation
relating the plate profile to the conformal map. Solutions are given for the
logarithm and power laws in the case of constant pressure. The paper concludes
on a comparison with available experimental data.Comment: 10 pages, 7 figure
Integration of the VIMOS control system
The VIRMOS consortium of French and Italian Institutes (PI: O. Le Fevre,
co-PI: G. Vettolani) is manufacturing two wide field imaging multi-object
spectrographs for the European Southern Observatory Very Large Telescope (VLT),
with emphasis on the ability to carry over spectroscopic surveys of large
numbers of sources: the VIsible Multi-Object Spectrograph, VIMOS, and the Near
InfraRed Multi-Object Spectrograph, NIRMOS. There are 52 motors to be
controlled in parallel in the spectrograph, making VIMOS a complex machine to
be handled. This paper will focus on the description of the control system,
designed in the ESO VLT standard control concepts, and on some integration
issues and problem solving strategies.Comment: 3 pages, 3 figures, ICALEPCS 2001 Conference, PSN#TUBT00
Optomechanical tailoring of quantum fluctuations
We propose the use of feedback mechanism to control the level of quantum
noise in a radiation field emerging from a pendular Fabry-Perot cavity. It is
based on the possibility to perform quantum nondemolition measurements by means
of optomechanical coupling.Comment: ReVTeX file, 8 pages, 1 Postscript figure. to appear in J. Opt. B:
Quant. Semiclass. Op
The half-filled Hubbard chain in the Composite Operator Method: A comparison with Bethe Ansatz
The one-dimensional Hubbard model at half-filling is studied in the framework
of the Composite Operator Method using a static approximation. A solution
characterized by strong antiferromagnetic correlations and a gap for any
nonzero on-site interaction U is found. The corresponding ground-state energy,
double occupancy and specific heat are in excellent agreement with those
obtained within the Bethe Ansatz. These results show that the Composite
Operator Method is an appropriate framework for the half-filled Hubbard chain
and can be applied to evaluate properties, like the correlation functions,
which cannot be obtained by means of the Bethe Ansatz, except for some limiting
cases.Comment: 7 pages, 3 embedded Postscript figures, EuroTeX, submitted to
EuroPhysics Letter
Advanced vehicle separation apparatus
A method of obtaining test data from two independent models or bodies in a conventional wind tunnel is described. The system makes efficient use of wind tunnel test time with computer control performing complex coordinate transformations necessary for model positioning. The apparatus is designed to be used in any of the three Unitary Wind Tunnels at NASA-Ames Research Center. Mechanical design details and a brief description of the control system for the separation apparatus are presented
Using qualitative models to define sustainable management for the commons in data poor conditions
Acknowledgments This work was funded by the University of Aberdeen and Scottish Natural Heritage (SNH) and their support is gratefully acknowledged. We thank MASTS (the Marine Alliance for Science and Technology for Scotland) for their role in funding this work and B. Leyshon and F. Manson (SNH) for fruitful discussion.Peer reviewedPostprin
The entangling power of a "glocal" dissipative map
We consider a model of two qubits dissipating into both local and global
environments (generally at non-zero temperatures), with the possibility of
interpolating between purely local dissipation and purely global one. The
corresponding dissipative dynamical map is characterized in terms of its Kraus
operators focusing on the stationary regime. We then determine conditions under
which entanglement can be induced by the action of such a map. It results
(rather counterintuitively) that in order to have entanglement in the presence
of local environment, this latter must be at nonzero temperature.Comment: 13 pages, 6 figure
Bayesian feedback versus Markovian feedback in a two-level atom
We compare two different approaches to the control of the dynamics of a
continuously monitored open quantum system. The first is Markovian feedback as
introduced in quantum optics by Wiseman and Milburn [Phys. Rev. Lett. {\bf 70},
548 (1993)]. The second is feedback based on an estimate of the system state,
developed recently by Doherty {\em et al.} [Phys. Rev. A {\bf 62}, 012105
(2000)]. Here we choose to call it, for brevity, {\em Bayesian feedback}. For
systems with nonlinear dynamics, we expect these two methods of feedback
control to give markedly different results. The simplest possible nonlinear
system is a driven and damped two-level atom, so we choose this as our model
system. The monitoring is taken to be homodyne detection of the atomic
fluorescence, and the control is by modulating the driving. The aim of the
feedback in both cases is to stabilize the internal state of the atom as close
as possible to an arbitrarily chosen pure state, in the presence of inefficient
detection and other forms of decoherence. Our results (obtain without recourse
to stochastic simulations) prove that Bayesian feedback is never inferior, and
is usually superior, to Markovian feedback. However it would be far more
difficult to implement than Markovian feedback and it loses its superiority
when obvious simplifying approximations are made. It is thus not clear which
form of feedback would be better in the face of inevitable experimental
imperfections.Comment: 10 pages, including 3 figure
Preserving Information from the Beginning to the End of time in a Robertson-Walker Spacetime
Preserving information stored in a physical system subjected to noise can be
modeled in a communication-theoretic paradigm, in which storage and retrieval
correspond to an input encoding and output decoding, respectively. The encoding
and decoding are then constructed in such a way as to protect against the
action of a given noisy quantum channel. This paper considers the situation in
which the noise is not due to technological imperfections, but rather to the
physical laws governing the evolution of the universe. In particular, we
consider the dynamics of quantum systems under a 1+1 Robertson-Walker spacetime
and find that the noise imparted to them is equivalent to the well known
amplitude damping channel. Since one might be interested in preserving both
classical and quantum information in such a scenario, we study trade-off coding
strategies and determine a region of achievable rates for the preservation of
both kinds of information. For applications beyond the physical setting studied
here, we also determine a trade-off between achievable rates of classical and
quantum information preservation when entanglement assistance is available.Comment: 19 pages, 3 figures. Presentation updated, matches the published
versio
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