6,489 research outputs found
Unconditional two-mode squeezing of separated atomic ensembles
We propose schemes for the unconditional preparation of a two-mode squeezed
state of effective bosonic modes realized in a pair of atomic ensembles
interacting collectively with optical cavity and laser fields. The scheme uses
Raman transitions between stable atomic ground states and under ideal
conditions produces pure entangled states in the steady state. The scheme works
both for ensembles confined within a single cavity and for ensembles confined
in separate, cascaded cavities.Comment: 4 pages, 2 figure
Effects of Measurement back-action in the stabilization of a Bose-Einstein condensate through feedback
We apply quantum filtering and control to a particle in a harmonic trap under
continuous position measurement, and show that a simple static feedback law can
be used to cool the system. The final steady state is Gaussian and dependent on
the feedback strength and coupling between the system and probe. In the limit
of weak coupling this final state becomes the ground state. An earlier model by
Haine et. al. (PRA 69, 2004) without measurement back-action showed dark
states: states that did not display error signals, thus remaining unaffected by
the control. This paper shows that for a realistic measurement process this is
not true, which indicates that a Bose-Einstein condensate may be driven towards
the ground state from any arbitrary initial state.Comment: 1 Tex, 4 PS pictures, 1 bbl fil
Commentary: Matrix metalloproteinase-13 unlucky for the forming thrombus
Matrix metalloproteinases (MMPs) are calciumâdependent zincâcontaining endoproteases involved in extracellular matrix and nonâmatrix protein degradation.1 In the latest issue of Research and Practice in Thrombosis and Haemostasis, Howes and colleagues2 investigated the role of MMPâ13 in platelet aggregation and thrombus formation and identified that MMPâ13 could engage important platelet receptors and influence platelet function in vitro. MMPâ13 is of great cardiovascular interest as expression of this metalloproteinase is significantly upregulated in a host of atherothrombotic and inflammatory conditions.
Sympathetic cooling of trapped fermions by bosons in the presence of particle losses
We study the sympathetic cooling of a trapped Fermi gas interacting with an
ideal Bose gas below the critical temperature of the Bose-Einstein
condensation. We derive the quantum master equation, which describes the
dynamics of the fermionic component, and postulating the thermal distribution
for both gases we calculate analytically the rate at which fermions are cooled
by the bosonic atoms. The particle losses constitute an important source of
heating of the degenerate Fermi gas. We evaluate the rate of loss-induced
heating and derive analytical results for the final temperature of fermions,
which is limited in the presence of particle losses.Comment: 7 pages, 2 figures, EPL style; final versio
Three-fold way to extinction in populations of cyclically competing species
Species extinction occurs regularly and unavoidably in ecological systems.
The time scales for extinction can broadly vary and inform on the ecosystem's
stability. We study the spatio-temporal extinction dynamics of a paradigmatic
population model where three species exhibit cyclic competition. The cyclic
dynamics reflects the non-equilibrium nature of the species interactions. While
previous work focusses on the coarsening process as a mechanism that drives the
system to extinction, we found that unexpectedly the dynamics to extinction is
much richer. We observed three different types of dynamics. In addition to
coarsening, in the evolutionary relevant limit of large times, oscillating
traveling waves and heteroclinic orbits play a dominant role. The weight of the
different processes depends on the degree of mixing and the system size. By
analytical arguments and extensive numerical simulations we provide the full
characteristics of scenarios leading to extinction in one of the most
surprising models of ecology
Conformational transformations induced by the charge-curvature interaction at finite temperature
The role of thermal fluctuations on the conformational dynamics of a single
closed filament is studied. It is shown that, due to the interaction between
charges and bending degrees of freedom, initially circular aggregates may
undergo transformation to polygonal shape. The transition occurs both in the
case of hardening and softening charge-bending interaction. In the former case
the charge and curvature are smoothly distributed along the chain while in the
latter spontaneous kink formation is initiated. The transition to a
non-circular conformation is analogous to the phase transition of the second
kind.Comment: 23 pages (Latex), 10 figures (Postscript), 2 biblio file (bib-file
and bbl-file
Non-equilibrium dynamics: Studies of reflection of Bose-Einstein condensates
The study of the non-equilibrium dynamics in Bose-Einstein condensed gases
has been dominated by the zero-temperature, mean field Gross-Pitaevskii
formalism. Motivated by recent experiments on the reflection of condensates
from silicon surfaces, we revisit the so-called {\em classical field}
description of condensate dynamics, which incorporates the effects of quantum
noise and can also be generalized to include thermal effects. The noise is
included in a stochastic manner through the initial conditions. We show that
the inclusion of such noise is important in the quantitative description of the
recent reflection experiments
The stochastic Gross-Pitaevskii equation II
We provide a derivation of a more accurate version of the stochastic
Gross-Pitaevskii equation, as introduced by Gardiner et al. (J. Phys. B
35,1555,(2002). The derivation does not rely on the concept of local energy and
momentum conservation, and is based on a quasi-classical Wigner function
representation of a "high temperature" master equation for a Bose gas, which
includes only modes below an energy cutoff E_R that are sufficiently highly
occupied (the condensate band). The modes above this cutoff (the non-condensate
band) are treated as being essentially thermalized. The interaction between
these two bands, known as growth and scattering processes, provide noise and
damping terms in the equation of motion for the condensate band, which we call
the stochastic Gross-Pitaevskii equation. This approach is distinguished by the
control of the approximations made in its derivation, and by the feasibility of
its numerical implementation.Comment: 24 pages of LaTeX, one figur
Quantum Kinetic Theory VI: The Growth of a Bose-Einstein Condensate
A detailed analysis of the growth of a BEC is given, based on quantum kinetic
theory, in which we take account of the evolution of the occupations of lower
trap levels, and of the full Bose-Einstein formula for the occupations of
higher trap levels, as well as the Bose stimulated direct transfer of atoms to
the condensate level introduced by Gardiner et al. We find good agreement with
experiment at higher temperatures, but at lower temperatures the experimentally
observed growth rate is somewhat more rapid. We also confirm the picture of the
``kinetic'' region of evolution, introduced by Kagan et al., for the time up to
the initiation of the condensate. The behavior after initiation essentially
follows our original growth equation, but with a substantially increased rate
coefficient.
Our modelling of growth implicitly gives a model of the spatial shape of the
condensate vapor system as the condensate grows, and thus provides an
alternative to the present phenomenological fitting procedure, based on the sum
of a zero-chemical potential vapor and a Thomas-Fermi shaped condensate. Our
method may give substantially different results for condensate numbers and
temperatures obtained from phenomentological fits, and indicates the need for
more systematic investigation of the growth dynamics of the condensate from a
supersaturated vapor.Comment: TeX source; 29 Pages including 26 PostScript figure
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