Stochastic models for quality of service of component connectors

The intensifying need for scalable software has motivated modular development and using systems distributed over networks to implement large-scale applications. In Service-oriented Computing, distributed services are composed to provide large-scale services with a specific functionality. In this way, reusability of existing services can be increased. However, due to the heterogeneity of distributed software systems, software composition is not easy and requires additional mechanisms to impose some form of a coordination on a distributed software system. Besides functional correctness, a composed service must satisfy various quantitative requirements for its clients, which are generically called its quality of service (QoS). Particularly, it is tricky to obtain the overall QoS of a composed service even if the QoS information of its constituent distributed services is given. In this thesis, we propose Stochastic Reo to specify software composition with QoS aspects and its compositional semantic models. They are also used as intermediate models to generate their corresponding stochastic models for practical analysis. Based on this, we have implemented the tool Reo2MC. Using Reo2MC, we have modeled and analyzed an industrial software, the ASK system. Its analysis results provided the best cost-effective resource utilization and some suggestions to improve the performance of the system.UBL - phd migration 201

Simple scheme for expanding a polarization-entangled W state by adding one photon

We propose a simple scheme for expanding a polarization-entangled W state. By mixing a single photon and one of the photons in an n-photon W state at a polarization-dependent beam splitter (PDBS), we can obtain an (n+1)-photon W state after post-selection. Our scheme also opens the door for generating n-photon W states using single photons and linear optics.Comment: 3 pages, 2 figure

Generation of Three-Qubit Entangled W-State by Nonlinear Optical State Truncation

We propose an alternative scheme to generate W state via optical state truncation using quantum scissors. In particular, these states may be generated through three-mode optical state truncation in a Kerr nonlinear coupler. The more general three-qubit state may be also produced if the system is driven by external classical fields.Comment: 7 pages, 2 figur

The states of W-class as shared resources for perfect teleportation and superdense coding

As we know, the states of triqubit systems have two important classes: GHZ-class and W-class. In this paper, the states of W-class are considered for teleportation and superdense coding, and are generalized to multi-particle systems. First we describe two transformations of the shared resources for teleportation and superdense coding, which allow many new protocols from some known ones for that. As an application of these transformations, we obtain a sufficient and necessary condition for a state of W-class being suitable for perfect teleportation and superdense coding. As another application, we find that state $|W>_{123}={1/2}(|100>_{123}+|010>_{123}+\sqrt{2}|001>_{123})$ can be used to transmit three classical bits by sending two qubits, which was considered to be impossible by P. Agrawal and A. Pati [Phys. Rev. A to be published]. We generalize the states of W-class to multi-qubit systems and multi-particle systems with higher dimension. We propose two protocols for teleportation and superdense coding by using W-states of multi-qubit systems that generalize the protocols by using $|W>_{123}$ proposed by P. Agrawal and A. Pati. We obtain an optimal way to partition some W-states of multi-qubit systems into two subsystems, such that the entanglement between them achieves maximum value.Comment: 10 pages, critical comments and suggestions are welcom

Tunneling anisotropic magnetoresistance in multilayer-(Co/Pt)/AlOx/Pt structures

We report observations of tunneling anisotropic magnetoresitance (TAMR) in vertical tunnel devices with a ferromagnetic multilayer-(Co/Pt) electrode and a non-magnetic Pt counter-electrode separated by an AlOx barrier. In stacks with the ferromagnetic electrode terminated by a Co film the TAMR magnitude saturates at 0.15% beyond which it shows only weak dependence on the magnetic field strength, bias voltage, and temperature. For ferromagnetic electrodes terminated by two monolayers of Pt we observe order(s) of magnitude enhancement of the TAMR and a strong dependence on field, temperature and bias. Discussion of experiments is based on relativistic ab initio calculations of magnetization orientation dependent densities of states of Co and Co/Pt model systems.Comment: 4 pages, 5 figures, to be published in Phys. Rev. Let

Multiple Stellar Populations in the Globular Cluster omega Centauri as Tracers of a Merger Event

The discovery of the Sagittarius dwarf galaxy, which is being tidally disrupted by and merging with the Milky Way, supports the view that the halo of the Galaxy has been built up at least partially by the accretion of similar dwarf systems. The Sagittarius dwarf contains several distinct populations of stars, and includes M54 as its nucleus, which is the second most massive globular cluster associated with the Milky Way. The most massive globular cluster is omega Centauri, and here we report that omega Centauri also has several distinct stellar populations, as traced by red-giant-branch stars. The most metal-rich red-giant-branch stars are about 2 Gyr younger than the dominant metal-poor component, indicating that omega Centauri was enriched over this timescale. The presence of more than one epoch of star formation in a globular cluster is quite surprising, and suggests that omega Centauri was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were much more frequent in the early history of the Galaxy and omega Centauri appears to be a relict of this era.Comment: 7 pages, 3 figures, Latex+nature.sty (included), To appear in November 4th issue of Natur