@unpublished{574, abstract = {Vaidman, in a recent article adopts the method of 'quantum weak measurements in pre- and postselected ensembles' to ascertain whether or not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{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}, } @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}, } @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{7328, abstract = {An experimental technique for measuring the current density distribution with a resolution smaller than the channel/rib scale of the flow field in polymer electrolyte fuel cells (PEFCs) is presented. The electron conductors in a plane perpendicular to the channel direction are considered as two-dimensional resistors. Hence, the current density is obtained from the solution of Laplace's equation with the potentials at current collector and reaction layer as boundary conditions. Using ohmic drop for calculating the local current, detailed knowledge of all resistances involved is of prime importance. In particular, the contact resistance between the gas diffusion layer (GDL) and flow field rib, as well as GDL bulk conductivity, are strongly dependent on clamping pressure. They represent a substantial amount of the total ohmic drop and therefore require careful consideration. The detailed experimental setup as well as the concise procedure for quantitative data evaluation is described. Finally, the method is applied successfully to a cell operated on pure oxygen and air up to high current densities. The results show that electrical and ionic resistances seem to govern the current distribution at low current regimes, whereas mass transport limitations locally hamper the current production at high loads.}, author = {Freunberger, Stefan Alexander and Reum, Mathias and Evertz, Jörg and Wokaun, Alexander and Büchi, Felix N.}, issn = {0013-4651}, journal = {Journal of The Electrochemical Society}, number = {11}, publisher = {The Electrochemical Society}, title = {{Measuring the current distribution in PEFCs with sub-millimeter resolution}}, doi = {10.1149/1.2345591}, volume = {153}, year = {2006}, } @article{7327, abstract = {Propagation of performance changes to adjacent cells in polymer electrolyte fuel cell stacks is studied by means of voltage monitoring and local current density measurements in peripheral cells of the stack. A technical fuel cell stack has been modified by implementing two independent reactant and coolant supplies in order to deliberately change the performance of one cell (anomalous cell) and study the coupling phenomena to adjacent cells (coupling cells), while keeping the working conditions of the later cell-group unaltered. Two anomalies are studied: (i) air starvation and (ii) thermal anomaly, in a single anomalous cell in the stack and their coupling to adjacent cells. The results have shown that anomalies inducing considerable changes in the local current density of the anomalous cell (such as air starvation) propagate to adjacent cells affecting their performance. The propagation of local current density changes takes place via the common bipolar plate due to its finite thickness and in-plane conductivity. Consequently, anomalies which do not strongly influence the local current density distribution (such as a thermal anomaly under the studied working conditions) do not propagate to adjacent cells.}, author = {Santis, Marco and Freunberger, Stefan Alexander and Papra, Matthias and Wokaun, Alexander and Büchi, Felix N.}, issn = {0378-7753}, journal = {Journal of Power Sources}, number = {2}, pages = {1076--1083}, publisher = {Elsevier}, title = {{Experimental investigation of coupling phenomena in polymer electrolyte fuel cell stacks}}, doi = {10.1016/j.jpowsour.2006.06.007}, volume = {161}, year = {2006}, } @inproceedings{7326, abstract = {Often the properties of a single cell are considered as representative for a complete polymer electrolyte fuel cell stack or even a fuel cell system. In some cases this comes close, however, in many real cases differences on several scales become important. Cell interaction phenomena in fuel cell stacks that arise from inequalities between adjacent cells are investigated in detail experimentally. For that, a specialized 2-cell stack with advanced localized diagnostics was developed. The results show that inequalities propagate by electrical coupling, inhomogeneous cell polarization and inducing in-plane current in the common bipolar plate. The effects of the different loss-mechanisms are analyzed and quantified. }, author = {Büchi, Felix N. and Freunberger, Stefan Alexander and Santis, Marco}, booktitle = {ECS Transactions}, location = {Cancun, Mexico}, number = {1}, pages = {963--968}, publisher = {ECS}, title = {{What is learned beyond the scale of single cells?}}, doi = {10.1149/1.2356215}, volume = {3}, year = {2006}, }