259 research outputs found
Effect of time delay on feedback control of a flashing ratchet
It was recently shown that the use of feedback control can improve the
performance of a flashing ratchet. We investigate the effect of a time delay in
the implementation of feedback control in a closed-loop collective flashing
ratchet, using Langevin dynamics simulations. Surprisingly, for a large
ensemble, a well-chosen delay time improves the ratchet performance by allowing
the system to synchronize into a quasi-periodic stable mode of oscillation that
reproduces the optimal average velocity for a periodically flashing ratchet.
For a small ensemble, on the other hand, finite delay times significantly
reduce the benefit of feedback control for the time-averaged velocity, because
the relevance of information decays on a time scale set by the diffusion time
of the particles. Based on these results, we establish that experimental use of
feedback control is realistic.Comment: 6 pages, 6 figures, to appear in Europhysics Letter
Closed-loop control strategy with improved current for a flashing ratchet
We show how to switch on and off the ratchet potential of a collective
Brownian motor, depending only on the position of the particles, in order to
attain a current higher than or at least equal to that induced by any periodic
flashing. Maximization of instant velocity turns out to be the optimal protocol
for one particle but is nevertheless defeated by a periodic switching when a
sufficiently large ensemble of particles is considered. The protocol presented
in this article, although not the optimal one, yields approximately the same
current as the optimal protocol for one particle and as the optimal periodic
switching for an infinite number of them.Comment: 4 pages, 4 figure
Deterministic ratchet from stationary light fields
Ratchets are dynamic systems where particle transport is induced by
zero-average forces due to the interplay between nonlinearity and asymmetry.
Generally, they rely on the effect of a strong external driving. We show that
stationary optical lattices can be designed to generate particle flow in one
direction while requiring neither noise nor driving. Such optical fields must
be arranged to yield a combination of conservative (dipole) and nonconservative
(radiation pressure) forces. Under strong friction all paths converge to a
discrete set of limit periodic trajectories flowing in the same direction.Comment: 6 pages, 4 figure
Transient fluctuation theorem in closed quantum systems
Our point of departure are the unitary dynamics of closed quantum systems as
generated from the Schr\"odinger equation. We focus on a class of quantum
models that typically exhibit roughly exponential relaxation of some observable
within this framework. Furthermore, we focus on pure state evolutions. An
entropy in accord with Jaynes principle is defined on the basis of the quantum
expectation value of the above observable. It is demonstrated that the
resulting deterministic entropy dynamics are in a sense in accord with a
transient fluctuation theorem. Moreover, we demonstrate that the dynamics of
the expectation value are describable in terms of an Ornstein-Uhlenbeck
process. These findings are demonstrated numerically and supported by
analytical considerations based on quantum typicality.Comment: 5 pages, 6 figure
Nonequilibrium potential and fluctuation theorems for quantum maps
© 2015 American Physical Society. We derive a general fluctuation theorem for quantum maps. The theorem applies to a broad class of quantum dynamics, such as unitary evolution, decoherence, thermalization, and other types of evolution for quantum open systems. The theorem reproduces well-known fluctuation theorems in a single and simplified framework and extends the Hatano-Sasa theorem to quantum nonequilibrium processes. Moreover, it helps to elucidate the physical nature of the environment that induces a given dynamics in an open quantum system.This work has been supported by grants ENFASIS (Grant No. FIS2011-22644) and TerMic (Grant No. FIS2014-52486-R) from the Spanish Government. G.M.P. acknowledges Grant No. BES-2012-054025. This work also benefited from the COST Action MP1209.Peer Reviewe
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