587 research outputs found
Stimulated Raman adiabatic passage into continuum
We propose a technique which produces nearly complete ionization of the
population of a discrete state coupled to a continuum by a two-photon
transition via a lossy intermediate state whose lifetime is much shorter than
the interaction duration. We show that using counterintuitively ordered pulses,
as in stimulated Raman adiabatic passage (STIRAP), wherein the pulse coupling
the intermediate state to the continuum precedes and partly overlaps the pulse
coupling the initial and intermediate states, greatly increases the ionization
signal and strongly reduces the population loss due to spontaneous emission
through the lossy state. For strong spontaneous emission from that state,
however, the ionization is never complete because the dark state required for
STIRAP does not exist. We demonstrate that this drawback can be eliminated
almost completely by creating a laser-induced continuum structure (LICS) by
embedding a third discrete state into the continuum with a third control laser.
This LICS introduces some coherence into the continuum, which enables a
STIRAP-like population transfer into the continuum. A highly accurate analytic
description is developed and numerical results are presented for Gaussian pulse
shapes
On the Sets of Real Numbers Recognized by Finite Automata in Multiple Bases
This article studies the expressive power of finite automata recognizing sets
of real numbers encoded in positional notation. We consider Muller automata as
well as the restricted class of weak deterministic automata, used as symbolic
set representations in actual applications. In previous work, it has been
established that the sets of numbers that are recognizable by weak
deterministic automata in two bases that do not share the same set of prime
factors are exactly those that are definable in the first order additive theory
of real and integer numbers. This result extends Cobham's theorem, which
characterizes the sets of integer numbers that are recognizable by finite
automata in multiple bases.
In this article, we first generalize this result to multiplicatively
independent bases, which brings it closer to the original statement of Cobham's
theorem. Then, we study the sets of reals recognizable by Muller automata in
two bases. We show with a counterexample that, in this setting, Cobham's
theorem does not generalize to multiplicatively independent bases. Finally, we
prove that the sets of reals that are recognizable by Muller automata in two
bases that do not share the same set of prime factors are exactly those
definable in the first order additive theory of real and integer numbers. These
sets are thus also recognizable by weak deterministic automata. This result
leads to a precise characterization of the sets of real numbers that are
recognizable in multiple bases, and provides a theoretical justification to the
use of weak automata as symbolic representations of sets.Comment: 17 page
Streptozotocin and Alloxan-based Selection Improves Toxin Resistance of Insulin-Producing RINm Cells
The aim of our study was to develop a method for
selection of subpopulations of insulin producing
RINm cells with higher resistance to beta cell toxins.
Cells, resistant to streptozotocin (RINmS) and
alloxan (RINmA), were obtained by repeated exposure
of parental RINm cells to these two toxins,
while the defense capacity, was estimated by the
MTT colorimetric method, and [3H]-thymidine incorporation
assay. We found that RINmS and
RINmA displayed higher resistance to both streptozotocin
(STZ) and alloxan (AL) when compared to
the parental RINm cells. In contrast, no differences
in sensitivity to hydrogen peroxide were found
between toxin selected and parental cells. Partial
protection from the toxic effect of STZ and AL was
obtained only in the parental RINm cells after
preincubation of cells with the unmetabolizable 3-
O-methyl-glucose. The possibility that GLUT-2 is
involved in cell sensitivity to toxins was confirmed
by Western blot analysis, which showed higher
expression of GLUT-2 in parental RINm compared
to RINmS and RINmA cells. In addition to the
higher cell defense property evidenced in the
selected cells, we also found higher insulin content
and insulin secretion in both RINmS and RINmA
cells when compared to the parental RINm cells. In
conclusion, STZ and AL treatment can be used for
selection of cell sub-populations with higher cell defense properties and hormone production. The
different GLUT-2 expression in parental and re
sistant cells suggest involvement of GLUT-2 in
mechanisms of cell response to different toxins
Qualitative Analysis of Partially-observable Markov Decision Processes
We study observation-based strategies for partially-observable Markov
decision processes (POMDPs) with omega-regular objectives. An observation-based
strategy relies on partial information about the history of a play, namely, on
the past sequence of observations. We consider the qualitative analysis
problem: given a POMDP with an omega-regular objective, whether there is an
observation-based strategy to achieve the objective with probability~1
(almost-sure winning), or with positive probability (positive winning). Our
main results are twofold. First, we present a complete picture of the
computational complexity of the qualitative analysis of POMDP s with parity
objectives (a canonical form to express omega-regular objectives) and its
subclasses. Our contribution consists in establishing several upper and lower
bounds that were not known in literature. Second, we present optimal bounds
(matching upper and lower bounds) on the memory required by pure and randomized
observation-based strategies for the qualitative analysis of POMDP s with
parity objectives and its subclasses
Exact solution, scaling behaviour and quantum dynamics of a model of an atom-molecule Bose-Einstein condensate
We study the exact solution for a two-mode model describing coherent coupling
between atomic and molecular Bose-Einstein condensates (BEC), in the context of
the Bethe ansatz. By combining an asymptotic and numerical analysis, we
identify the scaling behaviour of the model and determine the zero temperature
expectation value for the coherence and average atomic occupation. The
threshold coupling for production of the molecular BEC is identified as the
point at which the energy gap is minimum. Our numerical results indicate a
parity effect for the energy gap between ground and first excited state
depending on whether the total atomic number is odd or even. The numerical
calculations for the quantum dynamics reveals a smooth transition from the
atomic to the molecular BEC.Comment: 5 pages, 4 figure
Randomness for Free
We consider two-player zero-sum games on graphs. These games can be
classified on the basis of the information of the players and on the mode of
interaction between them. On the basis of information the classification is as
follows: (a) partial-observation (both players have partial view of the game);
(b) one-sided complete-observation (one player has complete observation); and
(c) complete-observation (both players have complete view of the game). On the
basis of mode of interaction we have the following classification: (a)
concurrent (both players interact simultaneously); and (b) turn-based (both
players interact in turn). The two sources of randomness in these games are
randomness in transition function and randomness in strategies. In general,
randomized strategies are more powerful than deterministic strategies, and
randomness in transitions gives more general classes of games. In this work we
present a complete characterization for the classes of games where randomness
is not helpful in: (a) the transition function probabilistic transition can be
simulated by deterministic transition); and (b) strategies (pure strategies are
as powerful as randomized strategies). As consequence of our characterization
we obtain new undecidability results for these games
Non-transitive linear temporal logic and logical knowledge operations
© 2015 The Author, 2015. Published by Oxford University Press. All rights reserved.We study a linear temporal logic LTLNT with non-transitive time (with NEXT and UNTIL) and possible interpretations for logical knowledge operations in this approach. We assume time to be non-transitive, linear and discrete, it is a major innovative part of our article. Motivation for our approach that time might be non-transitive and comments on possible interpretations of logical knowledge operations are given. The main result of Section 5 is a solution of the decidability problem for LTLNT, we find and describe in details the decision algorithm. In Section 6 we introduce non-transitive linear temporal logic LTLNT(m) with uniform bound (m) for non-transitivity. We compare it with standard linear temporal logic LTL and the logic LTLNT - where non-transitivity has no upper bound - and show that LTLNT may be approximated by logics LTLNT(m). Concluding part of the article contains a list of open interesting problems
Efficient formation of ground state ultracold molecules via STIRAP from the continuum at a Feshbach resonance
We develop a complete theoretical description of photoassociative Stimulated
Raman Adiabatic Passage (STIRAP) near a Feshbach resonance in a thermal atomic
gas. We show that it is possible to use low intensity laser pulses to directly
excite the continuum at a Feshbach resonance and transfer nearly the entire
atomic population to the lowest rovibrational level in the molecular ground
state. In case of a broad resonance, commonly found in several diatomic alkali
molecules, our model predicts a transfer efficiency up to 97% for a
given atom pair, and up to 70% when averaged over an atomic ensemble. The laser
intensities and pulse durations needed for optimal transfer are
W/cm and several s. Such efficiency compares to or surpasses currently
available techniques for creating stable diatomic molecules, and the
versatility of this approach simplifies its potential use for many molecular
species
Quantum effects on the dynamics of a two-mode atom-molecule Bose-Einstein condensate
We study the system of coupled atomic and molecular condensates within the
two-mode model and beyond mean-field theory (MFT). Large amplitude
atom-molecule coherent oscillations are shown to be damped by the rapid growth
of fluctuations near the dynamically unstable molecular mode. This result
contradicts earlier predictions about the recovery of atom-molecule
oscillations in the two-mode limit. The frequency of the damped oscillation is
also shown to scale as with the total number of atoms ,
rather than the expected pure scaling. Using a linearized model, we
obtain analytical expressions for the initial depletion of the molecular
condensate in the vicinity of the instability, and show that the important
effect neglected by mean field theory is an initially non-exponential
`spontaneous' dissociation into the atomic vacuum. Starting with a small
population in the atomic mode, the initial dissociation rate is sensitive to
the exact atomic amplitudes, with the fastest (super-exponential) rate observed
for the entangled state, formed by spontaneous dissociation.Comment: LaTeX, 5 pages, 3 PostScript figures, uses REVTeX and epsfig,
submitted to Physical Review A, Rapid Communication
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