The pointer basis and the feedback stabilization of quantum systems

The dynamics for an open quantum system can be `unravelled' in infinitely many ways, depending on how the environment is monitored, yielding different sorts of conditioned states, evolving stochastically. In the case of ideal monitoring these states are pure, and the set of states for a given monitoring forms a basis (which is overcomplete in general) for the system. It has been argued elsewhere [D. Atkins et al., Europhys. Lett. 69, 163 (2005)] that the `pointer basis' as introduced by Zurek and Paz [Phys. Rev. Lett 70, 1187(1993)], should be identified with the unravelling-induced basis which decoheres most slowly. Here we show the applicability of this concept of pointer basis to the problem of state stabilization for quantum systems. In particular we prove that for linear Gaussian quantum systems, if the feedback control is assumed to be strong compared to the decoherence of the pointer basis, then the system can be stabilized in one of the pointer basis states with a fidelity close to one (the infidelity varies inversely with the control strength). Moreover, if the aim of the feedback is to maximize the fidelity of the unconditioned system state with a pure state that is one of its conditioned states, then the optimal unravelling for stabilizing the system in this way is that which induces the pointer basis for the conditioned states. We illustrate these results with a model system: quantum Brownian motion. We show that even if the feedback control strength is comparable to the decoherence, the optimal unravelling still induces a basis very close to the pointer basis. However if the feedback control is weak compared to the decoherence, this is not the case

Half-Skyrmions and Spike-Vortex Solutions of Two-Component Nonlinear Schrodinger Systems

Recently, skyrmions with integer topological charges have been observed numerically but have not yet been shown rigorously on two-component systems of nonlinear Schrodinger equations (NLSE) describing a binary mixture of Bose-Einstein condensates. Besides, half-skyrmions characterized by half-integer topological charges can also be found in the nonlinear sigma model which is a model of the Bose-Einstein condensate of the Schwinger bosons. Here we prove rigorously the existence of half-skyrmions which may come from a new type of soliton solutions called spike-vortex solutions of two-component systems of NLSE on the entire plane. These spike-vortex solutions having spikes in one component and a vortex in the other component may form half-skyrmions. By Liapunov-Schmidt reduction process, we may find spike-vortex solutions of two-component systems of NLSE.Comment: to appear in J.Math.Phy

Isolation of Yeast DNA Replication Mutants in Permeabilized Cells

A random population of temperature-sensitive mutants was screened by assaying for defects in DNA synthesis in a permeabilized yeast DNA replication system. Twenty mutants defective in in vitro DNA synthesis have been isolated. In this paper we describe eight of these mutants. Seven of them fall into three complementation groups--cdc2, cdc8, and cdc16--involved in the control of the cell-division cycle. Because synthesis in vitro represents propagation of replication forks active in vivo at the time of permeabilization, our finding that cdc2 and cdc16 mutants can incorporate dTMP into DNA in such permeabilized cells at 23 degrees C but not at 37 degrees C supports the conclusion that these two mutations directly affect DNA synthesis at replication forks. Such an involvement was previously suggested by in vivo analysis for CDC2 but was less clear for CDC16. Finally, the usefulness of our screening procedure is demonstrated by the isolation of replication mutants in previously undescribed complementation groups. One strain shows a serious defect in in vivo DNA synthesis but normal RNA synthesis

Taxonomy Of Thermopsis (Fabaceae) In North-America

Comprehensive reevaluation of both herbarium specimens and field observations of the North American Thermopsis leads to our recognition of 10 species. Three species occur in the southern Appalachians: T. villosa, T. mollis, and T. fraxinifolia. The Rocky Mountains and intermountain regions are populated by the relatively variable and widespread species T. divaricarpa, T. montana, and T. rhombifolia. The Pacific coastal mountains of California are dominated by the variable T. californica (with three infraspecific taxa), which gives way to T. gracilis in northern California and western Oregon. Thermopsis macrophylla and T. robusta have restricted ranges in California.Integrative Biolog

Immigrant Entrepreneurs Creating Jobs and Strengthening the U.S. Economy in Growing Industries

The focus of this report evolved from a 2010 conference at Babson College on "Immigrant Entrepreneurship in Massachusetts" sponsored by The Immigrant Learning Center, Inc. (ILC) from which two key ideas emerged. One is that there is an "immigrant entrepreneurship ecology" that includes immigrant neighborhood storefront businesses; immigrant high-tech and health science entrepreneurs; immigrant non-tech growth businesses; and immigrant transnational businesses. A second idea was that these growing, non-tech industries (including transportation, food and building services) have not attracted much attention. Interestingly, these sectors can be crucial to the expansion of the green economy. Within this context, The ILC decided to look at these three sectors in Massachusetts as well as in New York and Pennsylvania.Moreover, the report dramatically illustrates how immigrant entrepreneurs look for niches in underserved markets. For example, vans and other alternatives to mass transit serve unmet transportation needs in urban areas. Food intended to be a "taste of home" for compatriots in local restaurants and grocery stores becomes popular and influences the eating habits of other Americans. Workers who enter industries like landscaping or cleaning because they don't require much English gain experience and see opportunities to start their own companies. Businesses like these add value to American life by expanding the economy rather than taking away from native businesses