Bi-Directional Safety Analysis for Product-Line, Multi-Agent Systems

Abstract. Safety-critical systems composed of highly similar, semi-autonomous agents are being developed in several application domains. An example of such multi-agent systems is a fleet, or “constellation ” of satellites. In constellations of satellites, each satellite is commonly treated as a distinct autonomous agent that must cooperate to achieve higher-level constellation goals. In previous work, we have shown that modeling a constellation of satellites or spacecraft as a product line of agents (where the agents have many shared commonalities and a few key differences) enables reuse of software analysis and design assets. We have also previously developed efficient safety analysis techniques for product lines. We now propose the use of Bi-Directional Safety Analysis (BDSA) to aid in system certification. We extend BDSA to product lines of multi-agent systems and show how the analysis artifacts thus produced contribute to the software’s safety case for certification purposes. The product-line approach lets us reuse portions of the safety analysis for multiple agents, significantly reducing the burden of certification. We motivate and illustrate this work through a specific application, a product-line, multi-agent satellite constellation

Entangled photon apparatus for the undergraduate laboratory

We present detailed instructions for constructing and operating an apparatus to produce and detect polarization-entangled photons. The source operates by type-I spontaneous parametric downconversion in a two-crystal geometry. Photons are detected in coincidence by single-photon counting modules and show strong angular and polarization correlations. We observe more than 100 entangled photon pairs per second. A test of a Bell inequality can be performed in an afternoon.Comment: 6 pages, 9 figure

Shear-wave vibrational directions and related fault movements in Southern California Earthquakes

Vibrational directions of direct shear waves from a number of small local earthquakes in southern California, recorded at Pasadena and Riverside, are determined and related to corresponding faulting at the source. A theoretical relationship between wave vibrational directions and fault displacements is proposed. Directions of SV and SH motions from various fault types are adduced from this relationship. Observations of initial shear wave motions indicate generally consistent SH displacements, usually less consistent SV displacements, and ratios of SV/SH which usually vary widely. Polarization of SH waves is indicated; that of SV wave, suggested. The entire shear wave is probably approximately plane-polarized. The results of this study indicate that horizontal components of faulting in southern California usually follow the same general direction, whereas vertical fault components appear to vary in direction. Comparison of observed SV and SH motions with (1) theoretical shear motions and (2) Gutenberg's (1941) observations of compressional wave impulses, provides analysis of faulting at the source. Seismic data and regional surface geology indicate a fault pattern involving, primarily, northwesterly-trending right-handed transcurrent faults in some parts of southern California, approximately east-west-trending reverse or thrust faults in other parts, and the coexistence of the two in a few localities. Simplified stress distributions in agreement with the data are discussed briefly

Entangled photons, nonlocality and Bell inequalities in the undergraduate laboratory

We use polarization-entangled photon pairs to demonstrate quantum nonlocality in an experiment suitable for advanced undergraduates. The photons are produced by spontaneous parametric downconversion using a violet diode laser and two nonlinear crystals. The polarization state of the photons is tunable. Using an entangled state analogous to that described in the Einstein-Podolsky-Rosen ``paradox,'' we demonstrate strong polarization correlations of the entanged photons. Bell's idea of a hidden variable theory is presented by way of an example and compared to the quantum prediction. A test of the Clauser, Horne, Shimony and Holt version of the Bell inequality finds $S = 2.307 \pm 0.035$, in clear contradiciton of hidden variable theories. The experiments described can be performed in an afternoon.Comment: 10 pages, 6 figure

Single hole transistor in a p-Si/SiGe quantum well

A single hole transistor is patterned in a p-Si/SiGe quantum well by applying voltages to nanostructured top gate electrodes. Gating is achieved by oxidizing the etched semiconductor surface and the mesa walls before evaporation of the top gates. Pronounced Coulomb blockade effects are observed at small coupling of the transistor island to source and drain.Comment: 3 pages, 3 figure

A physical model of quantum cascade lasers: Application to GaAs, GaN and SiGe devices

The philosophy behind this work has been to build a predictive bottom up physical model of quantum cascade lasers (QCLs) for use as a design tool, to interpret experimental results and hence improve understanding of the physical processes occurring inside working devices and as a simulator for developing new material systems. The standard model uses the envelope function and effective mass approximations to solve two complete periods of the QCL under an applied bias. Other models, such as k·p and empirical pseudopotential, have been employed in p-type systems where the more complex band structure requires it. The resulting wave functions are then used to evaluate all relevant carrier-phonon, carrier-carrier and alloy scattering rates from each quantised state to all others within the same and the neighbouring period. This information is then used to construct a rate equation for the equilibrium carrier density in each subband and this set of coupled rate equations are solved self-consistently to obtain the carrier density in each eigenstate. The latter is a fundamental description of the device and can be used to calculate the current density and gain as a function of the applied bias and temperature, which in turn yields the threshold current and expected temperature dependence of the device characteristics. A recent extension which includes a further iteration of an energy balance equation also yields the average electron (or hole) temperature over the subbands. This paper will review the method and describe its application to mid-infrared and terahertz, GaAs, GaN, SiGe cascade laser designs

Analysis of the resistance in p-SiGe over a wide temperature range

The temperature dependence of a system exhibiting a `metal-insulator transition in two dimensions at zero magnetic field' (MIT) is studied up to 90K. Using a classical scattering model we are able to simulate the non-monotonic temperature dependence of the resistivity in the metallic high density regime. We show that the temperature dependence arises from a complex interplay of metallic and insulating contributions contained in the calculation of the scattering rate 1/\td(E,T), each dominating in a limited temperature range.Comment: 4 pages with 5 figure

n-Si/SiGe quantum cascade structures for THz emission

In this work we report on modelling the electron transport in n-Si/SiGe structures. The electronic structure is calculated within the effective-mass complex-energy framework, separately for perpendicular (Xz) and in-plane (Xxy) valleys, the degeneracy of which is lifted by strain, and additionally by size quantization. The transport is described via scattering between quantized states, using the rate equations approach and tight-binding expansion, taking the coupling with two nearest-neighbour periods. The acoustic phonon, optical phonon, alloy and interface roughness scattering are taken in the model. The calculated U/I dependence and gain profiles are presented for a couple of QC structures