@article{7330, abstract = {Polymer electrolyte fuel cells (PE fuel cells) working with air at low stoichiometries (<2.0) and standard electrochemical components show a high degree of inhomogeneity in the current density distribution over the active area. An inhomogeneous current density distribution leads to a non-uniform utilization of the active area, which could negatively affect the time of life of the cells. Furthermore, it is also believed to lower cell performance. In this work, the homogenization of the current density, realized by means of tailored cathodes with along-the-air-channel redistributed catalyst loadings, is investigated. The air stoichiometry range for which a homogenization of the current density is achieved depends upon the gradient with which the catalyst is redistributed along the air channel. A gentle increasing catalyst loading profile homogenizes the current density at relatively higher air stoichiometries, while a steeper profile is suited better for lower air stoichiometries. The results show that a homogenization of the current density by means of redistributed catalyst loading has negative effects on cell performance. Model calculations corroborate the experimental findings on homogenization of the current density and deliver an explanation for the decrease in cell performance.}, author = {Santis, M. and Freunberger, Stefan Alexander and Reiner, A. and Büchi, F.N.}, issn = {0013-4686}, journal = {Electrochimica Acta}, number = {25}, pages = {5383--5393}, publisher = {Elsevier}, title = {{Homogenization of the current density in polymer electrolyte fuel cells by in-plane cathode catalyst gradients}}, doi = {10.1016/j.electacta.2006.02.008}, volume = {51}, year = {2006}, } @article{7331, abstract = {A previously developed mathematical model for water management and current density distribution in a polymer electrolyte fuel cell (PEFCs) is employed to investigate the effects of cooling strategies on cell performance. The model describes a two-dimensional slice through the cell along the channels and through the entire cell sandwich including the coolant channels and the bipolar plate. Arbitrary flow arrangements of fuel, oxidant, and coolant stream directions can be described. Due to the serious impact of temperature on all processes in the PEFC, both the relative direction of the coolant stream to the gas streams and its mass flow turns out to significantly affect the cell performance. Besides influencing the electrochemical reaction and all kinds of mass transfer temperature, variations predominantly alter the local membrane hydration distribution and subseqently its conductivity.}, author = {Freunberger, Stefan Alexander and Wokaun, Alexander and Büchi, Felix N.}, issn = {0013-4651}, journal = {Journal of The Electrochemical Society}, number = {5}, publisher = {The Electrochemical Society}, title = {{In-plane effects in large-scale PEFCs: II. The influence of cooling strategy on cell performance}}, doi = {10.1149/1.2185282}, volume = {153}, year = {2006}, } @article{7332, abstract = {A quasi-two-dimensional, along-the-channel mass and heat-transfer model for a proton exchange membrane fuel cell (PEFC) is described and validated against experimental current distribution data. The model is formulated in a dimensional manner, i.e., local transport phenomena are treated one-dimensional in through-plane direction and coupled in-plane by convective transport in the gas and coolant channels. Thus, a two-dimensional slice running through the repetitive unit of a cell from the anode channel via membrane-electrode assembly (MEA) and cathode channel to the coolant channel and from inlet to outlet is modeled. The aim of the work is to elucidate the influence of operating conditions such as feed gas humidities and stoichiometric ratios on the along-the-channel current density distribution and to identify the distinct underlying voltage loss mechanisms. Furthermore, a complicated technical flow field is modeled by a combination of co- and counterflow subdomains and compared with experimental current densities.}, author = {Freunberger, Stefan Alexander and Santis, Marco and Schneider, Ingo A. and Wokaun, Alexander and Büchi, Felix N.}, issn = {0013-4651}, journal = {Journal of The Electrochemical Society}, number = {2}, publisher = {The Electrochemical Society}, title = {{In-plane effects in large-scale PEMFCs}}, doi = {10.1149/1.2150150}, volume = {153}, year = {2006}, } @inproceedings{4574, abstract = {Many software model checkers are based on predicate abstraction. If the verification goal depends on pointer structures, the approach does not work well, because it is difficult to find adequate predicate abstractions for the heap. In contrast, shape analysis, which uses graph-based heap abstractions, can provide a compact representation of recursive data structures. We integrate shape analysis into the software model checker Blast. Because shape analysis is expensive, we do not apply it globally. Instead, we ensure that, like predicates, shape graphs are computed and stored locally, only where necessary for proving the verification goal. To achieve this, we extend lazy abstraction refinement, which so far has been used only for predicate abstractions, to three-valued logical structures. This approach does not only increase the precision of model checking, but it also increases the efficiency of shape analysis. We implemented the technique by extending Blast with calls to Tvla.}, author = {Beyer, Dirk and Thomas Henzinger and Théoduloz, Grégory}, pages = {532 -- 546}, publisher = {Springer}, title = {{Lazy shape analysis}}, doi = {10.1007/11817963_48}, volume = {4144}, year = {2006}, } @unpublished{573, abstract = {Mitchison and Jozsa recently suggested that the "chained-Zeno" counterfactual computation protocol recently proposed by Hosten et al. is counterfactual for only one output of the computer. This claim was based on the existing abstract algebraic definition of counterfactual computation, and indeed according to this definition, their argument is correct. However, a more general definition (physically adequate) for counterfactual computation is implicitly assumed by Hosten et. al. Here we explain in detail why the protocol is counterfactual and how the "history tracking" method of the existing description inadequately represents the physics underlying the protocol. Consequently, we propose a modified definition of counterfactual computation. Finally, we comment on one of the most interesting aspects of the error-correcting protocol. }, author = {Hosten, Onur and Rakher, Matthew and Barreiro, Julio and Peters, Nicholas and Kwiat, Paul}, pages = {12}, publisher = {ArXiv}, title = {{Counterfactual computation revisited}}, year = {2006}, } @unpublished{574, abstract = {Vaidman, in a recent article adopts the method of 'quantum weak measurements in pre- and postselected ensembles' to ascertain whether or not the chained-Zeno counterfactual computation scheme proposed by Hosten et al. is counterfactual; which has been the topic of a debate on the definition of counterfactuality. We disagree with his conclusion, which brings up some interesting aspects of quantum weak measurements and some concerns about the way they are interpreted. }, author = {Hosten, Onur and Kwiat, Paul}, pages = {2}, publisher = {ArXiv}, title = {{Weak measurements and counterfactual computation}}, year = {2006}, } @inproceedings{577, abstract = {Visible light photon counters (VLPCs) and solid-state photomultipliers (SSPMs) are high-efficiency single-photon detectors which have multi-photon counting capability. While both the VLPCs and the SSPMs have inferred internal quantum efficiencies above 93%, the actual measured values for both the detectors were in fact limited to less than 88%, attributed to in-coupling losses. We are currently improving this overall detection efficiency via a) custom anti-reflection coating the detectors and the in-coupling fibers, b) implementing a novel cryogenic design to reduce transmission losses and, c) using low-noise electronics to obtain a better signal-to-noise ratio.}, author = {Rangarajan, Radhika and Altepeter, Joseph B and Jeffrey, Evan R and Stoutimore, Micah J and Peters, Nicholas A and Onur Hosten and Kwiat, Paul G}, publisher = {SPIE}, title = {{High-efficiency single-photon detectors}}, doi = {10.1117/12.686117}, volume = {6372}, year = {2006}, } @inproceedings{578, abstract = {A source of single photons allows secure quantum key distribution, in addition, to being a critical resource for linear optics quantum computing. We describe our progress on deterministically creating single photons from spontaneous parametric downconversion, an extension of the Pittman, Jacobs and Franson scheme [Phys. Rev A, v66, 042303 (2002)]. Their idea was to conditionally prepare single photons by measuring one member of a spontaneously emitted photon pair and storing the remaining conditionally prepared photon until a predetermined time, when it would be "deterministically" released from storage. Our approach attempts to improve upon this by recycling the pump pulse in order to decrease the possibility of multiple-pair generation, while maintaining a high probability of producing a single pair. Many of the challenges we discuss are central to other quantum information technologies, including the need for low-loss optical storage, switching and detection, and fast feed-forward control.}, author = {Peters, Nicholas A and Arnold, Keith J and VanDevender, Aaron P and Jeffrey, Evan R and Rangarajan, Radhika and Onur Hosten and Barreiro, Julio T and Altepeter, Joseph B and Kwiat, Paul G}, publisher = {SPIE}, title = {{Towards a quasi-deterministic single-photon source}}, doi = {10.1117/12.684702}, volume = {6305}, year = {2006}, } @article{579, abstract = {The logic underlying the coherent nature of quantum information processing often deviates from intuitive reasoning, leading to surprising effects. Counterfactual computation constitutes a striking example: the potential outcome of a quantum computation can be inferred, even if the computer is not run 1. Relying on similar arguments to interaction-free measurements 2 (or quantum interrogation3), counterfactual computation is accomplished by putting the computer in a superposition of 'running' and 'not running' states, and then interfering the two histories. Conditional on the as-yet-unknown outcome of the computation, it is sometimes possible to counterfactually infer information about the solution. Here we demonstrate counterfactual computation, implementing Grover's search algorithm with an all-optical approach4. It was believed that the overall probability of such counterfactual inference is intrinsically limited1,5, so that it could not perform better on average than random guesses. However, using a novel 'chained' version of the quantum Zeno effect6, we show how to boost the counterfactual inference probability to unity, thereby beating the random guessing limit. Our methods are general and apply to any physical system, as illustrated by a discussion of trapped-ion systems. Finally, we briefly show that, in certain circumstances, counterfactual computation can eliminate errors induced by decoherence. }, author = {Onur Hosten and Rakher, Matthew T and Barreiro, Julio T and Peters, Nicholas A and Kwiat, Paul G}, journal = {Nature}, number = {7079}, pages = {949 -- 952}, publisher = {Nature Publishing Group}, title = {{Counterfactual quantum computation through quantum interrogation}}, doi = {10.1038/nature04523}, volume = {439}, year = {2006}, } @inproceedings{583, abstract = {Visible light photon counters (VLPCs) and solid-state photomultipliers (SSPMs) facilitate efficient single-photon detection. We are attempting to improve their efficiency, previously limited to < 88% by coupling losses, via anti-reflection coatings, better electronics and cryogenics.}, author = {Rangarajan, Radhika and Peters, Nicholas A and Onur Hosten and Altepeter, Joseph B and Jeffrey, Evan R and Kwiat, Paul G}, publisher = {IEEE}, title = {{Improved single-photon detection}}, doi = {10.1109/CLEO.2006.4628641}, year = {2006}, } @article{6151, author = {Salecker, Iris and Häusser, Michael and de Bono, Mario}, issn = {1469-221X}, journal = {EMBO reports}, number = {6}, pages = {585--589}, publisher = {Wiley}, title = {{On the axonal road to circuit function and behaviour: Workshop on the assembly and function of neuronal circuits}}, doi = {10.1038/sj.embor.7400713}, volume = {7}, year = {2006}, } @article{6152, author = {Rogers, Candida and Persson, Annelie and Cheung, Benny and de Bono, Mario}, issn = {0960-9822}, journal = {Current Biology}, number = {7}, pages = {649--659}, publisher = {Elsevier}, title = {{Behavioral motifs and neural pathways coordinating O2 responses and aggregation in C. elegans}}, doi = {10.1016/j.cub.2006.03.023}, volume = {16}, year = {2006}, } @article{1461, abstract = {This note proves combinatorially that the intersection pairing on the middle-dimensional compactly supported cohomology of a toric hyperkähler variety is always definite, providing a large number of non-trivial L 2 harmonic forms for toric hyperkähler metrics on these varieties. This is motivated by a result of Hitchin about the definiteness of the pairing of L 2 harmonic forms on complete hyperkähler manifolds of linear growth.}, author = {Tamas Hausel and Swartz, Edward}, journal = {Proceedings of the American Mathematical Society}, number = {8}, pages = {2403 -- 2409}, publisher = {American Mathematical Society}, title = {{Intersection forms of toric hyperkähler varieties}}, doi = {10.1090/S0002-9939-06-08248-7}, volume = {134}, year = {2006}, } @article{1462, abstract = {A Fourier transform technique is introduced for counting the number of solutions of holomorphic moment map equations over a finite field. This technique in turn gives information on Betti numbers of holomorphic symplectic quotients. As a consequence, simple unified proofs are obtained for formulas of Poincaré polynomials of toric hyperkähler varieties (recovering results of Bielawski-Dancer and Hausel-Sturmfels), Poincaré polynomials of Hubert schemes of points and twisted Atiyah-Drinfeld-Hitchin-Manin (ADHM) spaces of instantons on ℂ2 (recovering results of Nakajima-Yoshioka), and Poincaré polynomials of all Nakajima quiver varieties. As an application, a proof of a conjecture of Kac on the number of absolutely indecomposable representations of a quiver is announced.}, author = {Tamas Hausel}, journal = {PNAS}, number = {16}, pages = {6120 -- 6124}, publisher = {National Academy of Sciences}, title = {{Betti numbers of holomorphic symplectic quotients via arithmetic Fourier transform}}, doi = {10.1073/pnas.0601337103}, volume = {103}, year = {2006}, } @article{13428, abstract = {Mixtures of oppositely charged nanoparticles of various sizes and charge ratios precipitate only at the point of electroneutrality. This phenomenonspecific to the nanoscale and reminiscent of threshold precipitation of ionsis a consequence of the formation of core-and-shell nanoparticle aggregates, in which the shells are composed of like-charged particles and are stabilized by efficient electrostatic screening.}, author = {Kalsin, Alexander M. and Kowalczyk, Bartlomiej and Smoukov, Stoyan K. and Klajn, Rafal and Grzybowski, Bartosz A.}, issn = {1520-5126}, journal = {Journal of the American Chemical Society}, keywords = {Colloid and Surface Chemistry, Biochemistry, General Chemistry, Catalysis}, number = {47}, pages = {15046--15047}, publisher = {American Chemical Society}, title = {{Ionic-like behavior of oppositely charged nanoparticles}}, doi = {10.1021/ja0642966}, volume = {128}, year = {2006}, } @article{13429, abstract = {The fruitful core: Organic syntheses reported in the literature from 1850 to 2004 are analyzed with mathematical tools from network theory and statistical physics. There is a set of substances (the core) from which the majority of other organic compounds can be made (see picture; red: core, blue: periphery, green: islands). Search algorithms are used to identify small optimal sets of maximally useful chemicals.}, author = {Bishop, Kyle J. M. and Klajn, Rafal and Grzybowski, Bartosz A.}, issn = {1521-3773}, journal = {Angewandte Chemie International Edition}, keywords = {General Chemistry, Catalysis}, number = {32}, pages = {5348--5354}, publisher = {Wiley}, title = {{The core and most useful molecules in organic chemistry}}, doi = {10.1002/anie.200600881}, volume = {45}, year = {2006}, } @article{13430, abstract = {Dynamic self-assembly (DySA) processes occurring outside of thermodynamic equilibrium underlie many forms of adaptive and intellligent behaviors in natural systems. Relatively little, however, is known about the principles that govern DySA and the ways in which it can be extended to artificial ensembles. This article discusses recent advances in both the theory and the practice of nonequilibrium self-assembly. It is argued that a union of ideas from thermodynamics and dynamic systems' theory can provide a general description of DySA. In parallel, heuristic design rules can be used to construct DySA systems of increasing complexities based on a variety of suitable interactions/potentials on length scales from nanoscopic to macroscopic. Applications of these rules to magnetohydrodynamic DySA are also discussed.}, author = {Fialkowski, Marcin and Bishop, Kyle J. M. and Klajn, Rafal and Smoukov, Stoyan K. and Campbell, Christopher J. and Grzybowski, Bartosz A.}, issn = {1520-6106}, journal = {The Journal of Physical Chemistry B}, keywords = {Materials Chemistry, Surfaces, Coatings and Films, Physical and Theoretical Chemistry}, number = {6}, pages = {2482--2496}, publisher = {American Chemical Society}, title = {{Principles and implementations of dissipative (dynamic) self-assembly}}, doi = {10.1021/jp054153q}, volume = {110}, year = {2006}, } @article{903, abstract = {Background: Carcinogenesis typically involves multiple somatic mutations in caretaker (DNA repair) and gatekeeper (tumor suppressors and oncogenes) genes. Analysis of mutation spectra of the tumor suppressor that is most commonly mutated in human cancers, p53, unexpectedly suggested that somatic evolution of the p53 gene during tumorigenesis is dominated by positive selection for gain of function. This conclusion is supported by accumulating experimental evidence of evolution of new functions of p53 in tumors. These findings prompted a genome-wide analysis of possible positive selection during tumor evolution. Methods: A comprehensive analysis of probable somatic mutations in the sequences of Expressed Sequence Tags (ESTs) from malignant tumors and normal tissues was performed in order to access the prevalence of positive selection in cancer evolution. For each EST, the numbers of synonymous and non-synonymous substitutions were calculated. In order to identify genes with a signature of positive selection in cancers, these numbers were compared to: i) expected numbers and ii) the numbers for the respective genes in the ESTs from normal tissues. Results: We identified 112 genes with a signature of positive selection in cancers, i.e., a significantly elevated ratio of non-synonymous to synonymous substitutions, in tumors as compared to 37 such genes in an approximately equal-sized EST collection from normal tissues. A substantial fraction of the tumor-specific positive-selection candidates have experimentally demonstrated or strongly predicted links to cancer. Conclusion: The results of EST analysis should be interpreted with extreme caution given the noise introduced by sequencing errors and undetected polymorphisms. Furthermore, an inherent limitation of EST analysis is that multiple mutations amenable to statistical analysis can be detected only in relatively highly expressed genes. Nevertheless, the present results suggest that positive selection might affect a substantial number of genes during tumorigenic somatic evolution.}, author = {Babenko, Vladimir N and Basu, Malay K and Fyodor Kondrashov and Rogozin, Igor B and Koonin, Eugene V}, journal = {BMC Cancer}, publisher = {BioMed Central}, title = {{Signs of positive selection of somatic mutations in human cancers detected by EST sequence analysis}}, doi = {10.1186/1471-2407-6-36}, volume = {6}, year = {2006}, } @article{9505, abstract = {Cytosine methylation, a common form of DNA modification that antagonizes transcription, is found at transposons and repeats in vertebrates, plants and fungi. Here we have mapped DNA methylation in the entire Arabidopsis thaliana genome at high resolution. DNA methylation covers transposons and is present within a large fraction of A. thaliana genes. Methylation within genes is conspicuously biased away from gene ends, suggesting a dependence on RNA polymerase transit. Genic methylation is strongly influenced by transcription: moderately transcribed genes are most likely to be methylated, whereas genes at either extreme are least likely. In turn, transcription is influenced by methylation: short methylated genes are poorly expressed, and loss of methylation in the body of a gene leads to enhanced transcription. Our results indicate that genic transcription and DNA methylation are closely interwoven processes.}, author = {Zilberman, Daniel and Gehring, Mary and Tran, Robert K. and Ballinger, Tracy and Henikoff, Steven}, issn = {1546-1718}, journal = {Nature Genetics}, number = {1}, pages = {61--69}, publisher = {Nature Publishing Group}, title = {{Genome-wide analysis of Arabidopsis thaliana DNA methylation uncovers an interdependence between methylation and transcription}}, doi = {10.1038/ng1929}, volume = {39}, year = {2006}, } @article{1033, abstract = {Systems of three interacting particles are notorious for their complex physical behaviour. A landmark theoretical result in few-body quantum physics is Efimov\'s prediction1,2 of a universal set of bound trimer states appearing for three identical bosons with a resonant two-body interaction. Counterintuitively, these states even exist in the absence of a corresponding two-body bound state. Since the formulation of Efimov\'s problem in the context of nuclear physics 35 years ago, it has attracted great interest in many areas of physics3-8. However, the observation of Efimov quantum states has remained an elusive goal3,5. Here we report the observation of an Efimov resonance in an ultracold gas of caesium atoms. The resonance occurs in the range of large negative two-body scattering lengths, arising from the coupling of three free atoms to an Efimov trimer. Experimentally, we observe its signature as a giant three-body recombination loss9,10 when the strength of the two-body interaction is varied. We also detect a minimum 9,11,12 in the recombination loss for positive scattering lengths, indicating destructive interference of decay pathways. Our results confirm central theoretical predictions of Efimov physics and represent a starting point with which to explore the universal properties of resonantly interacting few-body systems7. While Feshbach resonances13,14 have provided the key to control quantum-mechanical interactions on the two-body level, Efimov resonances connect ultracold matter15 to the world of few-body quantum phenomena.}, author = {Kraemer, Tobias and Mark, Michael and Waldburger, Philipp and Danzl, Johann G and Chin, Cheng and Engeser, Bastian and Lange, Adam and Pilch, Karl and Jaakkola, Antti and Nägerl, Hanns and Grimm, Rudolf}, journal = {Nature}, number = {7082}, pages = {315 -- 318}, publisher = {Nature Publishing Group}, title = {{Evidence for Efimov quantum states in an ultracold gas of caesium atoms}}, doi = {10.1038/nature04626}, volume = {440}, year = {2006}, }