Solving constrained combinatorial optimization problems via importance sampling in the grand canonical ensemble

Combinatorial optimization problems are usually NP-hard. These problems are generally tackled by heuristic or branch-and-bound methods. The aim of this paper is to tackle constrained combinatorial optimization problems by importance Monte Carlo sampling. For this, we show that every constrained combinatorial optimization problem can be represented by a thermodynamical system in a suitable grand canonical ensemble given by the quantities of temperature, volume, and chemical potential. In order to find optimum solutions of the optimization problem, the grand canonical Monte Carlo method can be applied to the corresponding thermodynamical system. This method will amount to importance sampling, i.e. good feasible solutions of the optimization problem will be preferably sampled, provided that the intensive quantities of temperature and chemical potential are appropriately chosen. Our approach extends the standard importance sampling approach in the canonical ensemble to tackle unconstrained combinatorial optimization problems. The knapsack problem is considered as a prototype example.     

State-of-the-art insulator and electrode materials for use in highcurrent high energy switching

An investigation into the failure of ceramic insulators that are used in a surface discharge switch (SDS) was conducted. The materials analyzed are Al₂O₃-25% SiC, MTF (modified alumina titanate), and CZA 500 (zirconia-alumina composite) ceramics. These insulators were subjected to high-current (~300 kA) surface discharges in atmospheric air and nitrogen. Energy-dispersive X-ray surface analysis was performed on the insulator surfaces in order to determine the contaminants that are present and the possible failure modes associated with the plasma arc environments mentioned above. Electrode erosion rates have been measured as a function of total charge transfer (up to 50 C/shot) for several in-situ materials including Cu-Nb, Cu-Nb+LaB₆, and Cu-Ta. Results from comparisons with standard Cu and CuW materials indicate that the in-situ materials represent an efficient method of retaining the copper in the bulk until it vaporizes and thus yield significantly lower erosion rates at high Coulomb transfer rates     

Controlling Spin Qubits in Quantum Dots

We review progress on the spintronics proposal for quantum computing where the quantum bits (qubits) are implemented with electron spins. We calculate the exchange interaction of coupled quantum dots and present experiments, where the exchange coupling is measured via transport. Then, experiments on single spins on dots are described, where long spin relaxation times, on the order of a millisecond, are observed. We consider spin-orbit interaction as sources of spin decoherence and find theoretically that also long decoherence times are expected. Further, we describe the concept of spin filtering using quantum dots and show data of successful experiments. We also show an implementation of a read out scheme for spin qubits and define how qubits can be measured with high precision. Then, we propose new experiments, where the spin decoherence time and the Rabi oscillations of single electrons can be measured via charge transport through quantum dots. Finally, all these achievements have promising applications both in conventional and quantum information processing. PACS: 03.67.Lx, 03.67.Mn, 73.23.Hk, 85.35.Be     

Diagnosability of repairable faults

The diagnosis problem for discrete event systems consists in deciding whether some fault event occurred or not in the system, given partial observations on the run of that system. Diagnosability checks whether a correct diagnosis can be issued in bounded time after a fault, for all faulty runs of that system. This problem appeared two decades ago and numerous facets of it have been explored, mostly for permanent faults. It is known for example that diagnosability of a system can be checked in polynomial time, while the construction of a diagnoser is exponential. The present paper examines the case of transient faults, that can appear and be repaired. Diagnosability in this setting means that the occurrence of a fault should always be detected in bounded time, but also before the fault is repaired, in order to prepare for the detection of the next fault or to take corrective measures while they are needed. Checking this notion of diagnosability is proved to be PSPACE-complete. It is also shown that faults can be reliably counted provided the system is diagnosable for faults and for repairs.     

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Book reviews

     

Some aspects of crack propagation under monotonic and cyclic load

In the present paper a summarizing survey is given on crack propagation in AlZnMgCuO·5 under both cyclic and monotonic loads. Details of the investigation are already published elsewhere. By systematic arrangement of the results it can be shown that micromechanisms and macroscopic crack propagation exhibit remarkable similarities under both loading conditions.     

Study of Interatomic Potentials in ZnS—Crystal-GRID Experiments Versus Ab Initio Calculations

Crystal-GRID measurements have been performed with ZnS single crystals. For the first time, an asymmetric Crystal-GRID line shape could be observed. The preliminary data evaluation indicates that the reported lifetime of the 3221 keV level in ³³S is too short. A value of about 60 fs has been found. Due to this “long” lifetime the line shape is much less structured than calculated with the reported lifetime.     

Historical perspective and current trends in emission microscopy, mirror electron microscopy and low-energy electron microscopy. An introduction to the proceedings of the Second International Symposium and Workshop on Emission microscopy and Related Techniques

Emission microscopes and related instruments comprise a specialized class of electron microscopes that have in common an acceleration field in combination with the first stage of imaging (i.e., an immersion objective lens, also called a cathode lens or emission lens). These imaging techniques include photoelectron emission microscopy (PEEM or PEM), electron emission induced by heat, ions, or neutral particles, mirror electron microscopy (MEM), and low-energy electron microscopy (LEEM), among others. In these instruments the specimen is placed on a flat cathode or is the cathode itself. The low-energy electrons that are emitted, reflected, or backscattered from the specimen are first accelerated and then imaged by means of an electron lens system resembling that of a transmission electron microscope. The image is formed in a parallel mode in all of the above instruments, in contrast to the image in scanning electron microscopes, where the information is collected sequentially by scanning the specimen. A brief history and introduction to emission microscopy, MEM, and LEEM is presented as a background for the Proceedings of the Second International Symposium and Workshop on this subject, held in Seattle, Washington, August 16-17, 1990. Current trends in this field gleaned from the presentations at that meeting are discussed.     

Discrete-time analysis of linear and nonlinear systems using analogcircuit simulators

A method to teach discrete-time systems analysis concepts and skills to engineering students is presented and discussed. The method presented in this investigation uses commercially available analog-digital circuit simulators and a delay element and sampler constructed from analog components. The delay element and sampler used with standard, continuous-time amplifiers and summers, allow the user to simulate discrete-time systems in a manner identical to that used for continuous-time systems. Construction of the discrete-time delay element and sampler using the PSpice circuit simulator is presented as is their use in a variety of well-known linear discrete-time systems including problems from economics, population dynamics and digital signal processing. The well-known nonlinear quadratic model of chaos is also simulated in this investigation. The results of the linear and nonlinear discrete-time system analysis are compared to the analytical results and show that the delay element and sampler are stable and accurate components useful in modeling a wide variety of linear and nonlinear discrete-time systems