16,840 research outputs found
Absolute Dynamical Limit to Cooling Weakly-Coupled Quantum Systems
Cooling of a quantum system is limited by the size of the control forces that
are available (the "speed" of control). We consider the most general cooling
process, albeit restricted to the regime in which the thermodynamics of the
system is preserved (weak coupling). Within this regime, we further focus on
the most useful control regime, in which a large cooling factor, and good
ground-state cooling can be achieved. We present a control protocol for
cooling, and give clear structural arguments, as well as strong numerical
evidence, that this protocol is globally optimal. From this we obtain simple
expressions for the limit to cooling that is imposed by the speed of control.Comment: 4 pages, Revetex4-1, 2 png figure
Brownian transport in corrugated channels with inertia
The transport of suspended Brownian particles dc-driven along corrugated
narrow channels is numerically investigated in the regime of finite damping. We
show that inertial corrections cannot be neglected as long as the width of the
channel bottlenecks is smaller than an appropriate particle diffusion length,
which depends on the the channel corrugation and the drive intensity. Being
such a diffusion length inversely proportional to the damping constant,
transport through sufficiently narrow obstructions turns out to be always
sensitive to the viscosity of the suspension fluid. The inertia corrections to
the transport quantifiers, mobility and diffusivity, markedly differ for
smoothly and sharply corrugated channels.Comment: 9 pages including figures. arXiv admin note: substantial text overlap
with arXiv:1202.436
The Quantum Emergence of Chaos
The dynamical status of isolated quantum systems, partly due to the linearity
of the Schrodinger equation is unclear: Conventional measures fail to detect
chaos in such systems. However, when quantum systems are subjected to
observation -- as all experimental systems must be -- their dynamics is no
longer linear and, in the appropriate limit(s), the evolution of expectation
values, conditioned on the observations, closely approaches the behavior of
classical trajectories. Here we show, by analyzing a specific example, that
microscopic continuously observed quantum systems, even far from any classical
limit, can have a positive Lyapunov exponent, and thus be truly chaotic.Comment: 4 pages, 4 figure
Twin wall of cubic-tetragonal ferroelastics
We derive solutions for the twin wall linking two tetragonal variants of the
cubic-tetragonal ferroelastic transformation, including for the first time the
dilatational and shear energies and strains. Our solutions satisfy the
compatibility relations exactly and are obtained at all temperatures. They
require four non-vanishing strains except at the Barsch-Krumhansl temperature
TBK (where only the two deviatoric strains are needed). Between the critical
temperature and TBK, material in the wall region is dilated, while below TBK it
is compressed. In agreement with experiment and more general theory, the twin
wall lies in a cubic 110-type plane. We obtain the wall energy numerically as a
function of temperature and we derive a simple estimate which agrees well with
these values.Comment: 4 pages (revtex), 3 figure
Conditional control of quantum beats in a cavity QED system
We probe a ground-state superposition that produces a quantum beat in the
intensity correlation of a two-mode cavity QED system. We mix drive with
scattered light from an atomic beam traversing the cavity, and effectively
measure the interference between the drive and the light from the atom. When a
photon escapes the cavity, and upon detection, it triggers our feedback which
modulates the drive at the same beat frequency but opposite phase for a given
time window. This results in a partial interruption of the beat oscillation in
the correlation function, that then returns to oscillate.Comment: 9 pages, 5 figures, XVII Reuni\'on Iberoamericana de \'Optica, X
Encuentro de \'Optica, L\'aseres y Aplicaciones (RIAO-OPTILAS-2010
Signatures of Spin Glass Freezing in NiO Nanoparticles
We present a detailed study of the magnetic properties of sol-gel prepared
nickel oxide nanoparticles of different sizes. We report various measurements
such as frequency, field and temperature dependence of ac susceptibility,
temperature and field dependence of dc magnetization and time decay of
thermoremanent magnetization. Our results and analysis show that the system
behaves as a spin glass.Comment: 8 pages, 9 figure
A criterion for the number of factors
This note proposes a new criterion for the determination of the number of factors in an approximate static factor model. The criterion is strongly associated with the scree test and compares the differences between consecutive eigenvalues to a threshold. The size of the threshold is derived from a hyperbola and depends only on the sample size and the number of factors k. Monte Carlo simulations compare its properties with well-established estimators from the literature. Our criterion shows similar results as the standard implementations of these estimators, but is not prone to a lack of robustness against a too large a priori determined maximum number of factors kmax
Optimal Unravellings for Feedback Control in Linear Quantum Systems
For quantum systems with linear dynamics in phase space much of classical
feedback control theory applies. However, there are some questions that are
sensible only for the quantum case, such as: given a fixed interaction between
the system and the environment what is the optimal measurement on the
environment for a particular control problem? We show that for a broad class of
optimal (state-based) control problems (the stationary
Linear-Quadratic-Gaussian class), this question is a semi-definite program.
Moreover, the answer also applies to Markovian (current-based) feedback.Comment: 5 pages. Version published by Phys. Rev. Let
Stochastic simulations of conditional states of partially observed systems, quantum and classical
In a partially observed quantum or classical system the information that we
cannot access results in our description of the system becoming mixed even if
we have perfect initial knowledge. That is, if the system is quantum the
conditional state will be given by a state matrix and if classical
the conditional state will be given by a probability distribution
where is the result of the measurement. Thus to determine the evolution of
this conditional state under continuous-in-time monitoring requires an
expensive numerical calculation. In this paper we demonstrating a numerical
technique based on linear measurement theory that allows us to determine the
conditional state using only pure states. That is, our technique reduces the
problem size by a factor of , the number of basis states for the system.
Furthermore we show that our method can be applied to joint classical and
quantum systems as arises in modeling realistic measurement.Comment: 16 pages, 11 figure
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