Optoelectronics of Inverted Type-I CdS/CdSe Core/Crown Quantum Ring

Inverted type-I heterostructure core/crown quantum rings (QRs) are quantum-efficient luminophores, whose spectral characteristics are highly tunable. Here, we study the optoelectronic properties of type-I core/crown CdS/CdSe QRs in the zincblende phase - over contrasting lateral size and crown width. For this we inspect their strain profiles, transition energies, transition matrix elements, spatial charge densities, electronic bandstructure, band-mixing probabilities, optical gain spectra, maximum optical gains and differential optical gains. Our framework uses an effective-mass envelope function theory based on the 8-band k$\cdot$p method employing the valence force field model for calculating the atomic strain distributions. The gain calculations are based on the density-matrix equation and take into consideration the excitonic effects with intraband scattering. Variations in the QR lateral size and relative widths of core and crown (ergo the composition) affect their energy levels, band-mixing probabilities, optical transition matrix elements, emission wavelengths/intensity, etc. The optical gain of QRs is also strongly dimension and composition dependent with further dependency on the injection carrier density causing band-filling effect. They also affect the maximum and differential gain at varying dimensions and compositions.Comment: Published in AIP Journal of Applied Physics (11 pages, 7 figures

Spin systems with dimerized ground states

In view of the numerous examples in the literature it is attempted to outline a theory of Heisenberg spin systems possessing dimerized ground states (``DGS systems") which comprises all known examples. Whereas classical DGS systems can be completely characterized, it was only possible to provide necessary or sufficient conditions for the quantum case. First, for all DGS systems the interaction between the dimers must be balanced in a certain sense. Moreover, one can identify four special classes of DGS systems: (i) Uniform pyramids, (ii) systems close to isolated dimer systems, (iii) classical DGS systems, and (iv), in the case of $s=1/2$, systems of two dimers satisfying four inequalities. Geometrically, the set of all DGS systems may be visualized as a convex cone in the linear space of all exchange constants. Hence one can generate new examples of DGS systems by positive linear combinations of examples from the above four classes.Comment: With corrections of proposition 4 and other minor change

Spanning Properties of Theta-Theta Graphs

We study the spanning properties of Theta-Theta graphs. Similar in spirit with the Yao-Yao graphs, Theta-Theta graphs partition the space around each vertex into a set of k cones, for some fixed integer k > 1, and select at most one edge per cone. The difference is in the way edges are selected. Yao-Yao graphs select an edge of minimum length, whereas Theta-Theta graphs select an edge of minimum orthogonal projection onto the cone bisector. It has been established that the Yao-Yao graphs with parameter k = 6k' have spanning ratio 11.67, for k' >= 6. In this paper we establish a first spanning ratio of $7.82$ for Theta-Theta graphs, for the same values of $k$. We also extend the class of Theta-Theta spanners with parameter 6k', and establish a spanning ratio of $16.76$ for k' >= 5. We surmise that these stronger results are mainly due to a tighter analysis in this paper, rather than Theta-Theta being superior to Yao-Yao as a spanner. We also show that the spanning ratio of Theta-Theta graphs decreases to 4.64 as k' increases to 8. These are the first results on the spanning properties of Theta-Theta graphs.Comment: 20 pages, 6 figures, 3 table

Quantum Walk on a Line with Two Entangled Particles

We introduce the concept of a quantum walk with two particles and study it for the case of a discrete time walk on a line. A quantum walk with more than one particle may contain entanglement, thus offering a resource unavailable in the classical scenario and which can present interesting advantages. In this work, we show how the entanglement and the relative phase between the states describing the coin degree of freedom of each particle will influence the evolution of the quantum walk. In particular, the probability to find at least one particle in a certain position after $N$ steps of the walk, as well as the average distance between the two particles, can be larger or smaller than the case of two unentangled particles, depending on the initial conditions we choose. This resource can then be tuned according to our needs, in particular to enhance a given application (algorithmic or other) based on a quantum walk. Experimental implementations are briefly discussed

An associative memory for the on-line recognition and prediction of temporal sequences

This paper presents the design of an associative memory with feedback that is capable of on-line temporal sequence learning. A framework for on-line sequence learning has been proposed, and different sequence learning models have been analysed according to this framework. The network model is an associative memory with a separate store for the sequence context of a symbol. A sparse distributed memory is used to gain scalability. The context store combines the functionality of a neural layer with a shift register. The sensitivity of the machine to the sequence context is controllable, resulting in different characteristic behaviours. The model can store and predict on-line sequences of various types and length. Numerical simulations on the model have been carried out to determine its properties.Comment: Published in IJCNN 2005, Montreal, Canad

Electronic bandstructure and optical gain of lattice matched III-V dilute nitride bismide quantum wells for 1.55 μ\mum optical communication systems

Dilute nitride bismide GaNBiAs is a potential semiconductor alloy for near- and mid-infrared applications, particularly in 1.55 $\mu$m optical communication systems. Incorporating dilute amounts of Bismuth (Bi) into GaAs reduces the effective bandgap rapidly, while significantly increasing the spin-orbit-splitting energy. Additional incorporation of dilute amounts of Nitrogen (N) helps to attain lattice matching with GaAs, while providing a route for flexible bandgap tuning. Here we present a study of the electronic bandstructure and optical gain of the lattice matched GaN$_x$Bi$_y$As$_{1-x-y}$/GaAs quaternary alloy quantum well (QW) based on the 16-band k$\cdot$p model. We have taken into consideration the interactions between the N and Bi impurity states with the host material based on the band anticrossing (BAC) and valence band anticrossing (VBAC) model. The optical gain calculation is based on the density matrix theory. We have considered different lattice matched GaNBiAs QW cases and studied their energy dispersion curves, optical gain spectrum, maximum optical gain and differential gain; and compared their performances based on these factors. The thickness and composition of these QWs were varied in order to keep the emission peak fixed at 1.55 $\mu$m. The well thickness has an effect on the spectral width of the gain curves. On the other hand, a variation in the injection carrier density has different effects on the maximum gain and differential gain of QWs of varying thicknesses. Among the cases studied, we found that the 6.3 nm thick GaN$_3$Bi$_{5.17}$As$_{91.83}$ lattice matched QW was most suited for 1.55 $\mu$m (0.8 eV) GaAs-based photonic applications.Comment: Accepted in AIP Journal of Applied Physic

Quantum Walks with Entangled Coins

We present a mathematical formalism for the description of unrestricted quantum walks with entangled coins and one walker. The numerical behaviour of such walks is examined when using a Bell state as the initial coin state, two different coin operators, two different shift operators, and one walker. We compare and contrast the performance of these quantum walks with that of a classical random walk consisting of one walker and two maximally correlated coins as well as quantum walks with coins sharing different degrees of entanglement. We illustrate that the behaviour of our walk with entangled coins can be very different in comparison to the usual quantum walk with a single coin. We also demonstrate that simply by changing the shift operator, we can generate widely different distributions. We also compare the behaviour of quantum walks with maximally entangled coins with that of quantum walks with non-entangled coins. Finally, we show that the use of different shift operators on 2 and 3 qubit coins leads to different position probability distributions in 1 and 2 dimensional graphs.Comment: Two new sections and several changes from referees' comments. 12 pages and 12 (colour) figure

Rice research, technological progress, and impacts on the poor: the Bangladesh case (summary report)

"This case study builds on an ongoing large-scale quantitative research project undertaken by BIDS/IRRI since 1987 originally in 64 unions from 57 districts of the country. It adds a qualitative research component to examine the impact of modern rice varieties (MVs) on livelihoods in a structured sample of eight of these villages across a range of favorable and unfavorable contexts..... The quantitative research shows that for households with access to land there have been direct adoption impacts in the form of increased yields and higher profits. However, since rice now only represents around 20 percent of most households' overall income, nonagricultural income is found to have gained dramatically in importance for rural households. While the profitability has declined over time, rice contributes to improved food security and provides a “springboard” for both rich and poor farm households moving into nonfarm income generation and employment... The qualitative research component generally confirmed these general findings, highlighting other factors such as the improved status associated with fixed-rent tenancy and “contract” labor arrangements. The qualitative research also shows negative adoption impacts such as shrinking common property resources (wild fish, vegetables, etc., and declining soil fertility, both of which may increase the long-term vulnerability of the poor. It also throws light on the processes of technology dissemination.... It was found that the linking of qualitative and quantitative research methodologies was useful in (a) generating complementary data of different kinds on similar issues and (b) generating new data missed within a purely quantitative approach. The sustainable livelihoods framework was a useful, flexible tool for structuring the qualitative data collection and analysis. However, the research study as a whole was limited by the fact that the qualitative component was “bolted onto” a quantitative study already underway. Therefore the framework, and the various data collection methodologies, were not systematically integrated across both components of the study. In conclusion, future agricultural research on rice may need to further address the question of MV adoption potential on risk-prone lands, the relevance of existing technology dissemination systems, the relationship between MV adoption and crop diversification, and the challenges of more sustainable crop management techniques." Authors' AbstractRice Bangladesh,