Macroscopic limit of a solvable dynamical model

The interaction between an ultrarelativistic particle and a linear array made up of $N$ two-level systems (^^ ^^ AgBr" molecules) is studied by making use of a modified version of the Coleman-Hepp Hamiltonian. Energy-exchange processes between the particle and the molecules are properly taken into account, and the evolution of the total system is calculated exactly both when the array is initially in the ground state and in a thermal state. In the macroscopic limit ($N \rightarrow \infty$), the system remains solvable and leads to interesting connections with the Jaynes-Cummings model, that describes the interaction of a particle with a maser. The visibility of the interference pattern produced by the two branch waves of the particle is computed, and the conditions under which the spin array in the $N \rightarrow \infty$ limit behaves as a ^^ ^^ detector" are investigated. The behavior of the visibility yields good insights into the issue of quantum measurements: It is found that, in the thermodynamical limit, a superselection-rule space appears in the description of the (macroscopic) apparatus. In general, an initial thermal state of the ^^ ^^ detector" provokes a more substantial loss of quantum coherence than an initial ground state. It is argued that a system decoheres more as the temperature of the detector increases. The problem of ^^ ^^ imperfect measurements" is also shortly discussed.Comment: 30 pages, report BA-TH/93-13

Exponential behavior of a quantum system in a macroscopic medium

An exponential behavior at all times is derived for a solvable dynamical model in the weak-coupling, macroscopic limit. Some implications for the quantum measurement problem are discussed, in particular in connection with dissipation.Comment: 8 pages, report BA-TH/94-17

EXTENSION OF THE WIGNER-ARAKI-YANASE THEOREM IN OZAWA FORMULATION

We present new conditions leading to the Wigner-Araki-Yanase theorem in a formulation recently proposed by Ozawa. We show that normalizable initial states and the noise operator of a detector play an important role in enabling an exact measuring process in the Heisenberg picture. Three examples clarify these ideas

Raman spectroscopy as a tool to study TiC formation during controlled ball milling

Titanium and carbon elemental powder mixtures with compositions of Ti100–xCx (x=50,40,30) were milled under a helium atmosphere using a magnetoball mill. For Ti50C50 and Ti60C40 powder mixtures, the combined results of external mill temperature monitoring and x-ray diffraction (XRD) analysis indicated that, after a specific incubation period, titanium carbide (TiC) was formed rapidly via a highly exothermic mechanically induced reaction. However, contrary to the current understanding of mechanically induced self-propagating reactions, Raman spectroscopy clearly showed the formation of nonstoichiometric TiC in Ti50C50 and Ti60C40 powders prior to the sudden exothermic event occurring inside the mill. This result has not been reported in previous studies that used only XRD analysis to characterize the as-milled powders. It is now thought that a significant component of the heat generated after the incubation period may be due to a combination of rapid grain growth and/or recrystallization of the preexisting TiC, rather than the direct formation of TiC. When milling Ti70C30, the reaction to form TiC proceeded gradually as milling progressed

The role of roof rats (Rattus rattus) in the spread of Salmonella Enteritidis and S. Infantis contamination in layer farms in eastern Japan

The prevalence of Salmonella in four layer farms in eastern Japan was investigated between 2004 and 2006 to determine the role of roof rats (Rattus rattus) in the epizootology of Salmonella enterica subsp. enterica serovar Enteritidis (S. Enteritidis). Persistent S. Enteritidis and S. Infantis contamination of the environment and pooled egg samples were detected in three out of four layer farms. A total of 113 (13·3%) and 158 (18·6%) out of 851 rats examined were positive for S. Enteritidis and S. Infantis, respectively. By pulsed-field gel electrophoresis, only one indistinguishable pulsed-field pattern was yielded by S. Enteritidis strains from rats, eggs and environmental samples from each of the two contaminated layer farms. Although, a variety of pulsed-field patterns were generated by S. Enteritidis isolates from rats, eggs, and the environment of the other contaminated farms, there are, however, some S. Enteritidis strains that are closely related clones. These results suggest that roof rats are carriers of S. Enteritidis and S. Infantis and that persistent S. Enteritidis and S. Infantis infections in a rat population may play an important role in the spread and maintenance of these pathogens inside the layer premises