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Search Results: 1 - 10 of 268 matches for " ANGIE; POULIN "
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The genus Basilichthys (Teleostei: Atherinopsidae) revisited along its Chilean distribution range (21° to 40° S) using variation in morphologyand mtDNA
Revista chilena de historia natural , 2012, DOI: 10.4067/S0716-078X2012000100004
Abstract: there is still doubt as to the number of species of the freshwater chilean ichthyofauna, 64 % of which have conservation problems. one of the groups is that of the silversides of the genus basilichthys. three morphological species of this genus have been described in chile with disjoint distributions: basilichthys semotilus, b. microlepidotus and b. australis; the latter two overlap in distribution only in the aconcagua river and are not easily distinguishable by morphological and meristic characters. in order to evaluate the efficacy of identification of these species by molecular techniques, we analyzed the sequence of 9 % of the mitochondrial dna (control region and coi) of individuals from the loa river (21°41' s) to the valdivia river (39°50' s), adding meristic features for b. microlepidotus and b. australis in order to study population variation to clarify the taxonomy of the native species of the genus. the phylogenetic analysis showed that the individuals of basilichthys semotilus form an haplogroup separated from the other species of the genus; however, b. australis and b. microlepidotus form a monophyletic group that shares the most common haplotypes. an analysis of meristic information showed no statistically significant differences in the number of lateral line scales or number of rays in the fins between b. microlepidotus and b. australis. these results do not support the current classification for the latter two species; there appears to be one group in the extreme north of the country (basilichthys semotilus) and a second group in central chile which should be called b. microlepidotus. this information will be useful to review the conservation status of the chilean fauna.
Reporting on first sexual experience: The importance of interviewer-respondent interaction
Michelle Poulin
Demographic Research , 2010,
Abstract: Survey methodologists typically seek to improve data on sensitive topics by standardizing surveys and avoiding the use of human interviewers. This study uses data collected from 90 never-married young adults in rural Malawi to compare reports on first sexual encounters between a standard survey and an in-depth interview. A significant fraction of young women who claimed in the survey to have never been sexually active affirmed sexual experience during the in-depth interview, fielded shortly thereafter. Two elements of the in-depth interview, flexibility and reciprocal exchange, foster trust and more truthful reporting. The findings contradict the long-standing presumption that face-to-face interviews are inherently threatening when the topic is sex.
Lieb-Robinson Bound and Locality for General Markovian Quantum Dynamics
David Poulin
Physics , 2010, DOI: 10.1103/PhysRevLett.104.190401
Abstract: The Lieb-Robinson bound shows the existence of a maximum speed of signal propagation in discrete quantum mechanical systems with local interactions. This generalizes the concept of relativistic causality beyond field theory, and provides a powerful tool in theoretical condensed matter physics and quantum information science. Here, we extend the scope of this seminal result by considering general Markovian quantum evolution, where we prove that an equivalent bound holds. In addition, we use the generalized bound to demonstrate that correlations in the stationary state of a Markov process decay on a length-scale set by the Lieb-Robinson velocity and the system's relaxation time.
Macroscopic Observables
David Poulin
Physics , 2004, DOI: 10.1103/PhysRevA.71.022102
Abstract: We study macroscopic observables defined as the total value of a physical quantity over a collection of quantum systems. We show that previous results obtained for infinite ensemble of identically prepared systems lead to incorrect conclusions for finite ensembles. In particular, exact measurement of a macroscopic observable significantly disturbs the state of any finite ensemble. However, we show how this disturbance can be made arbitrarily small when the measurement are of finite accuracy. We demonstrate a tradeoff between state disturbance and measurement coarseness as a function of the size of the ensemble. Using this tradeoff, we show that the histories generated by any sequence of finite accuracy macroscopic measurements always generate a consistent family in the absence of large scale entanglement, for sufficiently large ensembles. Hence, macroscopic observables behave "classically" provided that their accuracy is coarser than the quantum correlation length-scale of the system. The role of these observable is also discussed in the context of NMR quantum information processing and bulk ensemble quantum state tomography.
Optimal and Efficient Decoding of Concatenated Quantum Block Codes
David Poulin
Physics , 2006, DOI: 10.1103/PhysRevA.74.052333
Abstract: We consider the problem of optimally decoding a quantum error correction code -- that is to find the optimal recovery procedure given the outcomes of partial "check" measurements on the system. In general, this problem is NP-hard. However, we demonstrate that for concatenated block codes, the optimal decoding can be efficiently computed using a message passing algorithm. We compare the performance of the message passing algorithm to that of the widespread blockwise hard decoding technique. Our Monte Carlo results using the 5 qubit and Steane's code on a depolarizing channel demonstrate significant advantages of the message passing algorithms in two respects. 1) Optimal decoding increases by as much as 94% the error threshold below which the error correction procedure can be used to reliably send information over a noisy channel. 2) For noise levels below these thresholds, the probability of error after optimal decoding is suppressed at a significantly higher rate, leading to a substantial reduction of the error correction overhead.
Stabilizer Formalism for Operator Quantum Error Correction
David Poulin
Physics , 2005, DOI: 10.1103/PhysRevLett.95.230504
Abstract: Operator quantum error correction is a recently developed theory that provides a generalized framework for active error correction and passive error avoiding schemes. In this paper, we describe these codes in the stabilizer formalism of standard quantum error correction theory. This is achieved by adding a "gauge" group to the standard stabilizer definition of a code that defines an equivalence class between encoded states. Gauge transformations leave the encoded information unchanged; their effect is absorbed by virtual gauge qubits that do not carry useful information. We illustrate the construction by identifying a gauge symmetry in Shor's 9-qubit code that allows us to remove 4 of its 8 stabilizer generators, leading to a simpler decoding procedure and a wider class of logical operations without affecting its essential properties. This opens the path to possible improvements of the error threshold of fault-tolerant quantum computing.
Toy Model for a Relational Formulation of Quantum Theory
David Poulin
Physics , 2005, DOI: 10.1007/s10773-006-9052-0
Abstract: In the absence of an external frame of reference physical degrees of freedom must describe relations between systems. Using a simple model, we investigate how such a relational quantum theory naturally arises by promoting reference systems to the status of dynamical entities. Our goal is to demonstrate using elementary quantum theory how any quantum mechanical experiment admits a purely relational description at a fundamental level, from which the original "non-relational" theory emerges in a semi-classical limit. According to this thesis, the non-relational theory is therefore an approximation of the fundamental relational theory. We propose four simple rules that can be used to translate an "orthodox" quantum mechanical description into a relational description, independent of an external spacial reference frame or clock. The techniques used to construct these relational theories are motivated by a Bayesian approach to quantum mechanics, and rely on the noiseless subsystem method of quantum information science used to protect quantum states against undesired noise. The relational theory naturally predicts a fundamental decoherence mechanism, so an arrow of time emerges from a time-symmetric theory. Moreover, there is no need for a "collapse of the wave packet" in our model: the probability interpretation is only applied to diagonal density operators. Finally, the physical states of the relational theory can be described in terms of "spin networks" introduced by Penrose as a combinatorial description of geometry, and widely studied in the loop formulation of quantum gravity. Thus, our simple bottom-up approach (starting from the semi-classical limit to derive the fully relational quantum theory) may offer interesting insights on the low energy limit of quantum gravity.
Classicality of quantum information processing
David Poulin
Physics , 2001, DOI: 10.1103/PhysRevA.65.042319
Abstract: The ultimate goal of the classicality programme is to quantify the amount of quantumness of certain processes. Here, classicality is studied for a restricted type of process: quantum information processing (QIP). Under special conditions, one can force some qubits of a quantum computer into a classical state without affecting the outcome of the computation. The minimal set of conditions is described and its structure is studied. Some implications of this formalism are the increase of noise robustness, a proof of the quantumness of mixed state quantum computing and a step forward in understanding the very foundation of QIP.
Characterization of amenability by a factorization property of the group von Neumann algebra
Denis Poulin
Mathematics , 2011,
Abstract: We show that the amenability of a locally compact group $G$ is equivalent to a factorization property of $VN(G)$ which is given by $ VN(G) = $. This answer partially two problems proposed by Z. Hu and M. Neufang in their article \textit{Distinguishing properties of Arens irregularity}, Proc. Amer. Math. Soc. 137 (2009), no. 5, 1753-1761.
Weighted Paley-Wiener spaces and mountain chain axioms: a detailed exposition
Philippe Poulin
Mathematics , 2013,
Abstract: The present work reviews the second half of Lyubarskii and Seip's paper, Weighted Paley--Wiener Spaces. Axioms defining a larger class of de Branges spaces are abstracted, allowing us to state and prove their results at a higher level of generality.
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