659 research outputs found
Reactive conformations and non-Markovian reaction kinetics of a Rouse polymer searching for a target in confinement
We investigate theoretically a diffusion-limited reaction between a reactant
attached to a Rouse polymer and an external fixed reactive site in confinement.
The present work completes and goes beyond a previous study [T. Gu\'erin, O.
B\'enichou and R. Voituriez, Nat. Chem., 4, 268 (2012)] that showed that the
distribution of the polymer conformations at the very instant of reaction plays
a key role in the reaction kinetics, and that its determination enables the
inclusion of non-Markovian effects in the theory. Here, we describe in detail
this non-Markovian theory and we compare it with numerical stochastic
simulations and with a Markovian approach, in which the reactive conformations
are approximated by equilibrium ones. We establish the following new results.
Our analysis reveals a strongly non-Markovian regime in 1D, where the Markovian
and non-Markovian dependance of the relation time on the initial distance are
different. In this regime, the reactive conformations are so different from
equilibrium conformations that the Markovian expressions of the reaction time
can be overestimated by several orders of magnitudes for long chains. We also
show how to derive qualitative scaling laws for the reaction time in a
systematic way that takes into account the different behaviors of monomer
motion at all time and length scales. Finally, we also give an analytical
description of the average elongated shape of the polymer at the instant of the
reaction and we show that its spectrum behaves a a slow power-law for large
wave numbers
Universal first-passage statistics of aging processes
Many out of equilibrium phenomena, such as diffusion-limited reactions or
target search processes, are controlled by first-passage events. So far the
general determination of the mean first-passage time (FPT) to a target in
confinement has left aside aging processes, involved in contexts as varied as
glassy dynamics, tracer diffusion in biological membranes or transport of cold
atoms in optical lattices. Here we consider general non-Markovian
scale-invariant processes in arbitrary dimension, displaying aging, and
demonstrate that all the moments of the FPT obey universal scalings with the
confining volume with non trivial exponents. Our analysis shows that a
nonlinear scaling of the mean FPT with the volume is the hallmark of aging and
provides a general tool to quantify its impact on first-passage kinetics in
confinement
Field-free two-direction alignment alternation of linear molecules by elliptic laser pulses
We show that a linear molecule subjected to a short specific elliptically
polarized laser field yields postpulse revivals exhibiting alignment
alternatively located along the orthogonal axis and the major axis of the
ellipse. The effect is experimentally demonstrated by measuring the optical
Kerr effect along two different axes. The conditions ensuring an optimal
field-free alternation of high alignments along both directions are derived.Comment: 5 pages, 4 color figure
Acute Abdomen: A Rare Presentation of Lung Cancer Metastasis
Surgical emergencies caused by bowel metastases from carcinoma of the lung are very rare. We describe two cases of symptomatic gastrointestinal metastatic small cell carcinoma: the first one concerns a 69-year-old man with an acute abdomen and the second is a 72-year-old man complaining of a gastric ulcer symptoms. We also discuss the current management and the prognosis of these patients
Ăvolutions rĂ©centes de la structure des placements des assureurs.
Lâanalyse des portefeuilles des assureurs en 2009 confirme leur forte diversification gĂ©ographique, avec une prĂ©pondĂ©rance des titres dâĂ©metteurs de la zone euro, ainsi que lâallongement de leurs placements obligataires en rĂ©ponse aux Ă©volutions de la courbe des taux.organismes dâassurance, assureurs vie, assureurs vie-mixte, assureurs non-vie, provisions techniques, contrats en euros, contrats en unitĂ©s de compte, placements financiers, mise en transparence, Ă©pargne des mĂ©nages, circuits de financement, titres de crĂ©ance, obligations, actions.
Reaching optimally oriented molecular states by laser kicks
We present a strategy for post-pulse orientation aiming both at efficiency
and maximal duration within a rotational period. We first identify the
optimally oriented states which fulfill both requirements. We show that a
sequence of half-cycle pulses of moderate intensity can be devised for reaching
these target states.Comment: 4 pages, 3 figure
Optimized time-dependent perturbation theory for pulse-driven quantum dynamics in atomic or molecular systems
We present a time-dependent perturbative approach adapted to the treatment of
intense pulsed interactions. We show there is a freedom in choosing secular
terms and use it to optimize the accuracy of the approximation. We apply this
formulation to a unitary superconvergent technique and improve the accuracy by
several orders of magnitude with respect to the Magnus expansion.Comment: 4 pages, 2 figure
Mean first-passage times of non-Markovian random walkers in confinement
The first-passage time (FPT), defined as the time a random walker takes to
reach a target point in a confining domain, is a key quantity in the theory of
stochastic processes. Its importance comes from its crucial role to quantify
the efficiency of processes as varied as diffusion-limited reactions, target
search processes or spreading of diseases. Most methods to determine the FPT
properties in confined domains have been limited to Markovian (memoryless)
processes. However, as soon as the random walker interacts with its
environment, memory effects can not be neglected. Examples of non Markovian
dynamics include single-file diffusion in narrow channels or the motion of a
tracer particle either attached to a polymeric chain or diffusing in simple or
complex fluids such as nematics \cite{turiv2013effect}, dense soft colloids or
viscoelastic solution. Here, we introduce an analytical approach to calculate,
in the limit of a large confining volume, the mean FPT of a Gaussian
non-Markovian random walker to a target point. The non-Markovian features of
the dynamics are encompassed by determining the statistical properties of the
trajectory of the random walker in the future of the first-passage event, which
are shown to govern the FPT kinetics.This analysis is applicable to a broad
range of stochastic processes, possibly correlated at long-times. Our
theoretical predictions are confirmed by numerical simulations for several
examples of non-Markovian processes including the emblematic case of the
Fractional Brownian Motion in one or higher dimensions. These results show, on
the basis of Gaussian processes, the importance of memory effects in
first-passage statistics of non-Markovian random walkers in confinement.Comment: Submitted version. Supplementary Information can be found on the
Nature website :
http://www.nature.com/nature/journal/v534/n7607/full/nature18272.htm
Laser control for the optimal evolution of pure quantum states
Starting from an initial pure quantum state, we present a strategy for
reaching a target state corresponding to the extremum (maximum or minimum) of a
given observable. We show that a sequence of pulses of moderate intensity,
applied at times when the average of the observable reaches its local or global
extremum, constitutes a strategy transferable to different control issues.
Among them, post-pulse molecular alignment and orientation are presented as
examples. The robustness of such strategies with respect to experimentally
relevant parameters is also examined.Comment: 16 pages, 9 figure
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