15,288 research outputs found
Boundary of Quantum Evolution under Decoherence
Relaxation effects impose fundamental limitations on our ability to
coherently control quantum mechanical phenomena. In this letter, we establish
physical limits on how closely can a quantum mechanical system be steered to a
desired target state in the presence of relaxation. In particular, we
explicitly compute the maximum coherence or polarization that can be
transferred between coupled nuclear spins in the presence of very general
decoherence mechanisms that include cross-correlated relaxation. We give
analytical expressions for the control laws (pulse sequences) which achieve
these physical limits and provide supporting experimental evidence.
Exploitation of cross-correlation effects has recently led to the development
of powerful methods in NMR spectroscopy to study very large biomolecules in
solution. We demonstrate with experiments that the optimal pulse sequences
provide significant gains over these state of the art methods, opening new
avenues for spectroscopy of much larger proteins. Surprisingly, in spite of
very large relaxation rates, optimal control can transfer coherence without any
loss when cross-correlated relaxation rates are tuned to auto-correlated
relaxation rates
Geometric optimal control of the contrast imaging problem in Nuclear Magnetic Resonance
The objective of this article is to introduce the tools to analyze the
contrast imaging problem in Nuclear Magnetic Resonance. Optimal trajectories
can be selected among extremal solutions of the Pontryagin Maximum Principle
applied to this Mayer type optimal problem. Such trajectories are associated to
the question of extremizing the transfer time. Hence the optimal problem is
reduced to the analysis of the Hamiltonian dynamics related to singular
extremals and their optimality status. This is illustrated by using the
examples of cerebrospinal fluid / water and grey / white matter of cerebrum.Comment: 30 pages, 13 figur
Broadband Relaxation-Optimized Polarization Transfer in Magnetic Resonance
Many applications of magnetic resonance are limited by rapid loss of spin
coherence caused by large transverse relaxation rates. In nuclear magnetic
resonance (NMR) of large proteins, increased relaxation losses lead to poor
sensitivity of experiments and increased measurement time. In this paper we
develop broadband relaxation optimized pulse sequences (BB-CROP) which approach
fundamental limits of coherence transfer efficiency in the presence of very
general relaxation mechanisms that include cross-correlated relaxation. These
broadband transfer schemes use new techniques of chemical shift refocusing
(STAR echoes) that are tailored to specific trajectories of coupled spin
evolution. We present simulations and experimental data indicating significant
enhancement in the sensitivity of multi-dimensional NMR experiments of large
molecules by use of these methods
sscMap: An extensible Java application for connecting small-molecule drugs using gene-expression signatures
Background: Connectivity mapping is a process to recognize novel
pharmacological and toxicological properties in small molecules by comparing
their gene expression signatures with others in a database. A simple and robust
method for connectivity mapping with increased specificity and sensitivity was
recently developed, and its utility demonstrated using experimentally derived
gene signatures.
Results: This paper introduces sscMap (statistically significant connections'
map), a Java application designed to undertake connectivity mapping tasks using
the recently published method. The software is bundled with a default
collection of reference gene-expression profiles based on the publicly
available dataset from the Broad Institute Connectivity Map 02, which includes
data from over 7000 Affymetrix microarrays, for over 1000 small-molecule
compounds, and 6100 treatment instances in 5 human cell lines. In addition, the
application allows users to add their custom collections of reference profiles
and is applicable to a wide range of other 'omics technologies.
Conclusions: The utility of sscMap is two fold. First, it serves to make
statistically significant connections between a user-supplied gene signature
and the 6100 core reference profiles based on the Broad Institute expanded
dataset. Second, it allows users to apply the same improved method to
custom-built reference profiles which can be added to the database for future
referencing. The software can be freely downloaded from
http://purl.oclc.org/NET/sscMapComment: 3 pages, 1 table, 1 eps figur
Tube Width Fluctuations in F-Actin Solutions
We determine the statistics of the local tube width in F-actin solutions,
beyond the usually reported mean value. Our experimental observations are
explained by a segment fluid theory based on the binary collision approximation
(BCA). In this systematic generalization of the standard mean-field approach
effective polymer segments interact via a potential representing the
topological constraints. The analytically predicted universal tube width
distribution with a stretched tail is in good agreement with the data.Comment: Final version, 5 pages, 4 figure
Systematic review of grounded theory studies in physiotherapy
Aim: This systematic review aimed at appraising the methodological rigor of grounded theory research published in the field of physiotherapy to assess how the methodology is understood and applied. A secondary aim was to provide research implications drawn from the findings to guide future grounded theory methodology research. Methods: A systematic search was conducted in MEDLINE, CINHAL, SPORT Discus, Science Direct, PubMed, Scopus and Web of Science to identify studies in the field of physiotherapy that reported using grounded theory methodology and/or methods in the study title and/or abstract. The descriptive characteristics and methodological quality of eligible studies were examined using the assessment guidelines developed by Hutchison, Johnston and Breckon (2011). Findings: The review included sixty-eight studies conducted between 1998 and 2017. The findings showed that grounded theory methodology is becoming increasingly used by physiotherapy researchers. Thirty-six studies (53%) demonstrated a good understanding and appropriate application of grounded theory methodology. Thirty-two studies (47%) presented descriptive findings and were considered to be of poor methodological quality. Conclusions: There are several key tenets of grounded theory methodology that are integral to the iterative process of qualitative theorizing and need to be applied throughout all research practices including sampling, data collection and analysis
Maximising transparency in a doctoral thesis: The complexities of writing about the use of QSR*NVIVO within a grounded theory study
This paper discusses the challenges of how to provide a transparent account of the use of the software programme QSR*NVIVO (QSR 2000) within a Grounded Theory framework (Glaser and Strauss 1967; Strauss and Corbin 1998). Psychology students are increasingly pursuing qualitative research projects such to the extent that the UK Economic and Social Research Council (ESRC) advise that students should have skill in the use of computer assisted qualitative data analysis software (CAQDAS) (Economic and Social Research Council 2001). Unlike quantitative studies, rigid formulae do not exist for writing-up qualitative projects for doctoral theses. Most authors, however, agree that transparency is essential when communicating the findings of qualitative research. Sparkes (2001) recommends that evaluative criteria for qualitative research should be commensurable with the aims, objectives, and epistemological assumptions of the research project. Likewise, the use of CAQDAS should vary according to the research methodology followed, and thus researchers should include a discussion of how CAQDAS was used. This paper describes how the evolving process of coding data, writing memos, categorising, and theorising were integrated into the written thesis. The structure of the written document is described including considerations about restructuring and the difficulties of writing about an iterative process within a linear document
Time Optimal Control in Spin Systems
In this paper, we study the design of pulse sequences for NMR spectroscopy as
a problem of time optimal control of the unitary propagator. Radio frequency
pulses are used in coherent spectroscopy to implement a unitary transfer of
state. Pulse sequences that accomplish a desired transfer should be as short as
possible in order to minimize the effects of relaxation and to optimize the
sensitivity of the experiments. Here, we give an analytical characterization of
such time optimal pulse sequences applicable to coherence transfer experiments
in multiple-spin systems. We have adopted a general mathematical formulation,
and present many of our results in this setting, mindful of the fact that new
structures in optimal pulse design are constantly arising. Moreover, the
general proofs are no more difficult than the specific problems of current
interest. From a general control theory perspective, the problems we want to
study have the following character. Suppose we are given a controllable right
invariant system on a compact Lie group, what is the minimum time required to
steer the system from some initial point to a specified final point? In NMR
spectroscopy and quantum computing, this translates to, what is the minimum
time required to produce a unitary propagator? We also give an analytical
characterization of maximum achievable transfer in a given time for the two
spin system.Comment: 20 Pages, 3 figure
Assessment of economic factors affecting the satellite power system. Volume 2: The systems implications of rectenna siting issues
The feasibility was evaluated of finding potential sites for Solar Power Satellite (SPS) receiving antennas (rectennas) in the continental United States, in sufficient numbers to permit the SPS to make a major contribution to U.S. generating facilities, and to give statistical validity to an assessment of the characteristics of such sites and their implications for the design of the SPS system. It is found that the cost-optimum power output of the SPS does not depend on the particular value assigned to the cost per unit area of a rectenna and its site, as long as it is independent of rectenna area. Many characteristics of the sites chosen affect the optimum design of the rectenna itself
Lane-formation vs. cluster-formation in two dimensional square-shoulder systems: A genetic algorithm approach
Introducing genetic algorithms as a reliable and efficient tool to find
ordered equilibrium structures, we predict minimum energy configurations of the
square shoulder system for different values of corona width . Varying
systematically the pressure for different values of we obtain
complete sequences of minimum energy configurations which provide a deeper
understanding of the system's strategies to arrange particles in an
energetically optimized fashion, leading to the competing self-assembly
scenarios of cluster-formation vs. lane-formation.Comment: 5 pages, 6 figure
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