Decomposition Algorithms for Solving NP-hard Problems on a Quantum Annealer

NP-hard problems such as the maximum clique or minimum vertex cover problems, two of Karp’s 21 NP-hard problems, have several applications in computational chemistry, biochemistry and computer network security. Adiabatic quantum annealers can search for the optimum value of such NP-hard optimization problems, given the problem can be embedded on their hardware. However, this is often not possible due to certain limitations of the hardware connectivity structure of the annealer. This paper studies a general framework for a decomposition algorithm for NP-hard graph problems aiming to identify an optimal set of vertices. Our generic algorithm allows us to recursively divide an instance until the generated subproblems can be embedded on the quantum annealer hardware and subsequently solved. The framework is applied to the maximum clique and minimum vertex cover problems, and we propose several pruning and reduction techniques to speed up the recursive decomposition. The performance of both algorithms is assessed in a detailed simulation study.

     

Translinear circuits in subthreshold MOS

In this paper we provide an overview of translinear circuit design using MOS transistors operating in subthreshold region. We contrast the bipolar and MOS subthreshold characteristics and extend the translinear principle to the subthreshold MOS ohmic region through a drain/source current decomposition. A front/back-gate current decomposition is adopted; this facilitates the analysis of translinear loops, including multiple input floating gate MOS transistors. Circuit examples drawn from working systems designed and fabricated in standard digital CMOS oriented process are used as vehicles to illustrate key design considerations, systematic analysis procedures, and limitations imposed by the structure and physics of MOS transistors. Finally, we present the design of an analog VLSI translinear system with over 590,000 transistors in subthreshold CMOS. This performs phototransduction, amplification, edge enhancement and local gain control at the pixel level.     

Influence of plasmonic Au nanoparticles on the photoactivity of Fe₂O₃ electrodes for water splitting

An experimental study of the influence of gold nanoparticles on α-Fe(2)O(3) photoanodes for photoelectrochemical water splitting is described. A relative enhancement in the water splitting efficiency at photon frequencies corresponding to the plasmon resonance in gold was observed. This relative enhancement was observed only for electrode geometries with metal particles that were localized at the semiconductor-electrolyte interface, consistent with the observation that minority carrier transport to the electrolyte is the most significant impediment to achieving high efficiencies in this system.     

A three-parameter mechanical property reconstruction method for MR-based elastic property imaging

A reconstruction process featuring full parameterization of the three dimensional, time-harmonic equations of linear elasticity is developed and reconstructed property images are presented from simulation-based investigation. While interesting in its own right through the potential for increased adaptability of these reconstructive elastic imaging techniques, this study also presents a set of analysis tools used to study the poor convergence behavior found in the case of tissue like conditions (i.e. nearly incompressible materials). The choice of elastic properties for imaging in elastography research remains an open question at this point; the use of the stability and sensitivity-based analytical methods described here will help to predict and understand the value and reliability of different parameterizations of elasticity imaging. Additionally, though results indicate significant work needs to be done to achieve effective multiparameter reconstructive imaging, the methods detailed here offer the promise of increased flexibility and sophistication in elastographic imaging techniques.     

Spatial and seasonal variations in phytoplankton community structure in alkaline–saline Lake Nakuru,Kenya

Spatial and seasonal variations of phytoplankton, expressed in terms of species composition, cell density, biovolumes and biomass, collected at 10 sampling sites in alkaline–saline Lake Nakuru, Kenya, were investigated monthly from March 2004 to February 2005, in relation to selected physical and chemical water quality parameters. A total of eight species, belonging to five genera and three classes, were recorded. The Cyanophyceae comprised the bulk of the phytoplankton, followed by Euglenophyceae, while Bacillariophyceae were the least abundant. Arthrospira fusiformis was the most dominant species, in terms of density, among the Cyanophyceae, while Euglena spp. and Navicula elkabs dominated the Euglenophyceae and Bacillariophyceae density, respectively. Seasonal variations in phytoplankton species composition, density and biomass were significant (P < 0.05) with minimum cell density and biomass occurring during the dry season, and being maximum following the end of the two rainy seasons from May to November 2004, suggesting the possible influence of various environmental factors on the lake. Sampling sites located close to the inlets of inflowing rivers exhibited significantly (P < 0.05) higher phytoplankton cell density and biomass than those by the inshore sites. Soluble reactive phosphorus, ammonia‐nitrogen, conductivity and total alkalinity accounted for most of the variations in the cell densities of phytoplankton species.     

Sediment build-up on roads and footpaths of a residential area

This paper presents results on the characterisation of sediment build-up on roads and footpaths of a residential catchment in Dublin, Ireland. Compared to power and exponential functions, the linear function provided the best fit of the road sediment build-up in terms of total and size-fractionated loads. Significant road sediment build-up rates were recorded for the majority of size fractions on the three street zones sampled, however it was not possible to separate build-up rates between size fractions due to random variation in sediment loads. There was a significant difference in total road sediment build-up between two of the sampling zones, which was likely due to the proximity to a highly trafficked road. Build-up rates on footpaths could not be separated from random variation. Footpath sediment loads were between 6% and 22% of road sediment loads, with texture depth identified as an important difference between these surfaces.     

The effect of carbon nanotube agglomeration on the thermal and mechanical properties of polyethylene oxide

The effects of agglomerated versus deagglomerated multiwalled carbon nanotubes on the thermal and mechanical characteristics of polyethylene oxide were studied. The specimens were prepared via solution‐sonication method, using two procedures. In the first procedure, CNT‐PEO solutions were prepared with as‐received MWCNTs, before sonication using an ultrasonic bath; in the second procedure, aggregated MWCNTs were first deagglomerated using high‐intensity ultrasonic processor, before mixing with the PEO. The two solution preparation procedures did result in significant thermal property differences such as increased transition temperature and crystallinity, but only one mechanical property can be conclusively associated with the sample preparation method—the breaking strain which was marginally higher for nanocomposites containing deagglomerated CNTs, although in all cases the breaking strains were much lower for nanocomposites than for unfilled polymer. However, the overall results confirm the effectiveness of CNTs both as nucleation agents for semicrystalline polymers, and as fillers for enhancement of mechanical properties of the matrix polymer. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers