University of Waterloo

About: University of Waterloo is a education organization based out in Waterloo, Ontario, Canada. It is known for research contribution in the topics: Population & Poison control. The organization has 36093 authors who have published 93906 publications receiving 2948139 citations. The organization is also known as: UW & uwaterloo.

Reducing elliptic curve logarithms to logarithms in a finite field

Abstract: Elliptic curve cryptosystems have the potential to provide relatively small block size, high-security public key schemes that can be efficiently implemented. As with other known public key schemes, such as RSA and discrete exponentiation in a finite field, some care must be exercised when selecting the parameters involved, in this case the elliptic curve and the underlying field. Specific classes of curves that give little or no advantage over previously known schemes are discussed. The main result of the paper is to demonstrate the reduction of the elliptic curve logarithm problem to the logarithm problem in the multiplicative group of an extension of the underlying finite field. For the class of supersingular elliptic curves, the reduction takes probabilistic polynomial time, thus providing a probabilistic subexponential time algorithm for the former problem. >

The Belief in a Just World

Abstract: The “belief in a just world” refers to those more or less articulated assumptions which underlie the way people orient themselves to their environment. These assumptions have a functional component which is tied to the image of a manageable and predictable world. These are central to the ability to engage in long-term goal-directed activity. In order to plan, work for, and obtain things they want, and avoid those which are frightening or painful, people must assume that there are manageable procedures which are effective in producing the desired end states (Erikson, 1950; Merton, 1957).

Spherical Ordered Mesoporous Carbon Nanoparticles with High Porosity for Lithium–Sulfur Batteries

Abstract: Rechargeable lithium–sulfur (Li–S) batteries are attracting increasing attention due to their high theoretical specific energy density, which is 3 to 5 times higher than that of Li-ion batteries based on intercalation chemistry. Since the electronic conductivity of sulfur is extremely low, conductive carbon materials with high accessible porosity to “wire” and contain the sulfur are an essential component of the positive electrode. During the past decades, attempts have been made to fabricate C/S composites using carbon black, activated carbons (ACs), and carbon nanotubes (CNTs). Although improvements resulted, the cathodes suffered from inhomogeneous contact between the active material and the electronic conductors. A major step forward in fabricating a uniform C/S composite was reported in 2009. Some of us employed CMK-3, an ordered mesoporous carbon (OMC) featuring high specific surface area and large pore volume as a scaffold. As much as 70 wt% sulfur was incorporated into the uniform 3–4 nm mesopores, and the cells exhibited reversible capacities up to 1350 mAhg , albeit at moderate rates. Inspired by this, another OMC, a bulk bimodal mesoporous carbon (BMC-1) was investigated as a Li-S cathode. The favorable pore dimensions and large pore volume greatly improved the rate performance. An electrode with 40 wt% S showed a high initial discharge capacity of 1135 mAhg 1 at a current rate of 1 C (defined as discharge/ charge in one hour). However, similar to other reports, the capacity is sensitive to the sulfur ratio, dropping to 718 mAhg 1 at a sulfur content of 60 wt%. These results suggest that the texture of the mesoporous carbon could be further enhanced. Recently, Archer et al. reported nanoscale hollow porous C/S spheres prepared through vapor infusion. These materials displayed good cyclability and capacity at a C/5 rate, illustrating the advantages of nanosized porous carbon in the sulfur cathodes. Here we report the synthesis of unique nanoscale spherical OMCs with extremely high bimodal porosities. The particles were investigated as a cathode material and sulfur host in Li–S batteries where they showed high initial discharge capacity and good cyclability without sacrificing rate capability. Unlike bulk porous carbons, these carbon– sulfur sphere electrodes did not display significant capacity fading with the increase of sulfur content in the cathodes. We show that the nanoscale morphology of these materials is of key importance for ensuring very efficient use of the sulfur content even at high cycling rates. Morphology control is a central issue in OMC synthesis. There are numerous examples of mesoporous bulk materials obtained either by hard-templating or soft-templating, including thin films, membranes or free fibers. Most syntheses use evaporation-induced self-assembly (EISA) followed by thermal treatment for template-removal and carbonization. It is a challenge to either create solution-based OMC nanoparticle syntheses or to adapt the established EISA methods to nanoparticles. Only few examples of OMC nanoparticles have been reported so far which are mostly unsuitable for applications in Li–S cells due to low pore volume and/or surface area. Approaches include templating with PMMA colloidal crystals or mesoporous silica nanoparticles, aerosol-assisted synthesis, ultrasonic emulsification or hydrothermal synthesis. Ordered arrays of fused mesoporous carbon spheres were reported by Liu et al. using a macroporous silica as template. Recently Lei et al. reported the synthesis of 65 nm mesoporous carbon nanospheres, with both 2.7 nm mesopores and high textural porosity (surface area of 2400 mg ). These showed promising supercapacitor properties. Our spherical OMC nanoparticles of 300 nm in diameter, prepared by a novel method, can be dispersed in water by sonification to form stable colloidal suspensions. The spherical mesoporous carbon nanoparticles were obtained in a twostep casting process. An opal structure of PMMA spheres was cast with a silica precursor solution to form a silica inverse opal. The inverse opal was then used as template for a triconstituent precursor solution containing resol as the carbon precursor, tetraethylorthosilicate (TEOS) as the silica precursor and the block copolymer Pluronic F127 as a structure-directing agent. Carbonization was followed by etching of the silica template and the silica in the carbon/silica nanocomposite, resulting in the formation of OMC with hierarchical porosity. Through the presence of silica in the [*] J. Schuster, B. Mandlmeier, Prof. Dr. T. Bein Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstrasse 5–13 (Gerhard Ertl Building), 81377 Munich (Germany) E-mail: tbein@cup.uni-muenchen.de Homepage: http://bein.cup.uni-muenchen.de G. He, T. Yim, K. T. Lee, Prof. Dr. L. F. Nazar Department of Chemistry, University of Waterloo 200 University Avenue West, Waterloo, Ontario N2L 3G1 (Canada) E-mail: lfnazar@uwaterloo.ca [] These authors contributed equally to this work.

Demonstration of two-qubit algorithms with a superconducting quantum processor

Abstract: By exploiting two key aspects of quantum mechanics — the superposition and entanglement of physical states — quantum computers may eventually outperform their classical equivalents. A team based at Yale has achieved an important step towards that goal — the demonstration of the first solid-state quantum processor, which was used to execute two quantum algorithms. Quantum processors based on a few quantum bits have been demonstrated before using nuclear magnetic resonance, cold ion traps and optical systems, all of which bear little resemblance to conventional computers. This new processor is based on superconducting quantum circuits fabricated using conventional nanofabrication technology. There is still a long way to go before quantum computers can challenge the classical type. The processor is very basic, containing just two quantum bits, and operates at a fraction of a degree above absolute zero. But the chip contains all the essential features of a miniature working quantum computer and may prove scalable to more quantum bits and more complex algorithms. Quantum computers, which harness the superposition and entanglement of physical states, hold great promise for the future. Here, the demonstration of a two-qubit superconducting processor and the implementation of quantum algorithms, represents an important step in quantum computing. Quantum computers, which harness the superposition and entanglement of physical states, could outperform their classical counterparts in solving problems with technological impact—such as factoring large numbers and searching databases1,2. A quantum processor executes algorithms by applying a programmable sequence of gates to an initialized register of qubits, which coherently evolves into a final state containing the result of the computation. Building a quantum processor is challenging because of the need to meet simultaneously requirements that are in conflict: state preparation, long coherence times, universal gate operations and qubit readout. Processors based on a few qubits have been demonstrated using nuclear magnetic resonance3,4,5, cold ion trap6,7 and optical8 systems, but a solid-state realization has remained an outstanding challenge. Here we demonstrate a two-qubit superconducting processor and the implementation of the Grover search and Deutsch–Jozsa quantum algorithms1,2. We use a two-qubit interaction, tunable in strength by two orders of magnitude on nanosecond timescales, which is mediated by a cavity bus in a circuit quantum electrodynamics architecture9,10. This interaction allows the generation of highly entangled states with concurrence up to 94 per cent. Although this processor constitutes an important step in quantum computing with integrated circuits, continuing efforts to increase qubit coherence times, gate performance and register size will be required to fulfil the promise of a scalable technology.