719 research outputs found
Oscillations of the purity in the repeated-measurement-based generation of quantum states
Repeated observations of a quantum system interacting with another one can
drive the latter toward a particular quantum state, irrespectively of its
initial condition, because of an {\em effective non-unitary evolution}. If the
target state is a pure one, the degree of purity of the system approaches
unity, even when the initial condition of the system is a mixed state. In this
paper we study the behavior of the purity from the initial value to the final
one, that is unity. Depending on the parameters, after a finite number of
measurements, the purity exhibits oscillations, that brings about a lower
purity than that of the initial state, which is a point to be taken care of in
concrete applications.Comment: 5 pages, 3 figure
Coupled structural, thermal, phase-change and electromagnetic analysis for superconductors, volume 1
This research program has dealt with the theoretical development and computer implementation of reliable and efficient methods for the analysis of coupled mechanical problems that involve the interaction of mechanical, thermal, phase-change and electromagnetic subproblems. The focus application has been the modeling of superconductivity and associated quantum-state phase-change phenomena. In support of this objective the work has addressed the following issues: (1) development of variational principles for finite elements; (2) finite element modeling of the electromagnetic problem; (3) coupling of thermal and mechanical effects; and (4) computer implementation and solution of the superconductivity transition problem. The research was carried out over the period September 1988 through March 1993. The main accomplishments have been: (1) the development of the theory of parametrized and gauged variational principles; (2) the application of those principled to the construction of electromagnetic, thermal and mechanical finite elements; and (3) the coupling of electromagnetic finite elements with thermal and superconducting effects; and (4) the first detailed finite element simulations of bulk superconductors, in particular the Meissner effect and the nature of the normal conducting boundary layer. The grant has fully supported the thesis work of one doctoral student (James Schuler, who started on January 1989 and completed on January 1993), and partly supported another thesis (Carmelo Militello, who started graduate work on January 1988 completing on August 1991). Twenty-three publications have acknowledged full or part support from this grant, with 16 having appeared in archival journals and 3 in edited books or proceedings
Coupled structural, thermal, phase-change and electromagnetic analysis for superconductors, volume 2
Two families of parametrized mixed variational principles for linear electromagnetodynamics are constructed. The first family is applicable when the current density distribution is known a priori. Its six independent fields are magnetic intensity and flux density, magnetic potential, electric intensity and flux density and electric potential. Through appropriate specialization of parameters the first principle reduces to more conventional principles proposed in the literature. The second family is appropriate when the current density distribution and a conjugate Lagrange multiplier field are adjoined, giving a total of eight independently varied fields. In this case it is shown that a conventional variational principle exists only in the time-independent (static) case. Several static functionals with reduced number of varied fields are presented. The application of one of these principles to construct finite elements with current prediction capabilities is illustrated with a numerical example
Population trapping due to cavity losses
In population trapping the occupation of a decaying quantum level keeps a
constant non-zero value. We show that an atom-cavity system interacting with an
environment characterized by a non-flat spectrum, in the non-Markovian limit,
exhibits such a behavior, effectively realizing the preservation of
nonclassical states against dissipation. Our results allow to understand the
role of cavity losses in hybrid solid state systems and pave the way to the
proper description of leakage in the recently developed cavity quantum
electrodynamic systems.Comment: 4 pages, 3 figures, version accepted for publication on Phys. Rev.
Influence of dissipation on the extraction of quantum states via repeated measurements
A quantum system put in interaction with another one that is repeatedly
measured is subject to a non-unitary dynamics, through which it is possible to
extract subspaces. This key idea has been exploited to propose schemes aimed at
the generation of pure quantum states (purification). All such schemes have so
far been considered in the ideal situations of isolated systems. In this paper,
we analyze the influence of non-negligible interactions with environment during
the extraction process, with the scope of investigating the possibility of
purifying the state of a system in spite of the sources of dissipation. A
general framework is presented and a paradigmatic example consisting of two
interacting spins immersed in a bosonic bath is studied. The effectiveness of
the purification scheme is discussed in terms of purity for different values of
the relevant parameters and in connection with the bath temperature.Comment: 10 pages, 3 figure
Polyadenylation regulates the stability of Trypanosoma brucei mitochondrial RNAs
Polyadenylation of RNAs plays a critical role in modulating rates of RNA turnover and ultimately in controlling gene expression in all systems examined to date. In mitochondria, the precise mechanisms by which RNAs are degraded, including the role of polyadenylation, are not well understood. Our previous in organello pulse-chase experiments suggest that poly(A) tails stimulate degradation of mRNAs in the mitochondria of the protozoan parasite Trypanosoma brucei (Militello, K. T., and Read, L. K. (2000) Mol. Cell. Biol. 21, 731-742). In this report, we developed an in vitro assay to directly examine the effects of specific 3′-sequences on RNA degradation. We found that a salt-extracted mitochondrial membrane fraction preferentially degraded polyadenylated mitochondrially and non-mitochondrially encoded RNAs over their non-adenylated counterparts. A poly(A) tail as short as 5 nucleotides was sufficient to stimulate rapid degradation, although an in vivo tail length of 20 adenosines supported the most rapid decay. A poly(U) extension did not promote rapid RNA degradation, and RNA turnover was slowed by the addition of uridine residues to the poly(A) tail. To stimulate degradation, the poly(A) element must be located at the 3′ terminus of the RNA. Finally, we demonstrate that degradation of polyadenylated RNAs occurs in the 3′ to 5′ direction through the action of a hydrolytic exonuclease. These experiments demonstrate that the poly(A) tail can act as a cis-acting element to facilitate degradation of T. brucei mitochondrial mRNAs
Tripartite thermal correlations in an inhomogeneous spin-star system
We exploit the tripartite negativity to study the thermal correlations in a
tripartite system, that is the three outer spins interacting with the central
one in a spin-star system. We analyze the dependence of such correlations on
the homogeneity of the interactions, starting from the case where central-outer
spin interactions are identical and then focusing on the case where the three
coupling constants are different. We single out some important differences
between the negativity and the concurrence.Comment: 8 pages, 9 figure
Quantum Zeno subspaces
The quantum Zeno effect is recast in terms of an adiabatic theorem when the
measurement is described as the dynamical coupling to another quantum system
that plays the role of apparatus. A few significant examples are proposed and
their practical relevance discussed. We also focus on decoherence-free
subspaces.Comment: 5 pages, 1 figure, to be published in Phys. Rev. Let
Transcriptional and post-transcriptional in organello labeling of Trypanosoma brucei mitochondrial RNA.
Abstract In organello labelling of Trypanosoma brucei mitochondrial (mt) RNA was characterised with respect to nucleotide requirements and drug sensitivity. Mitochondrial transcriptional activity is maximal in the presence of all ribonucleosidetriphosphate NTPs, and can be inhibited by UTP depletion. Mitochondrial transcription can also be partially inhibited by actinomycin D (actD) or ethidium bromide (EtBr). Post-transcriptional UTP incorporation is insensitive to actinomycin D or ethidium bromide. Proteins were identi®ed that interact with transcriptional and post-transcriptionally labelled RNAs, and con®rm the in vitro RNA-binding properties discovered for a number of T. brucei mt proteins. These experiments reveal new strategies for studying mt transcription and processing in T. brucei mitochondria. 7 2000 Published by Elsevier Science Ltd on behalf of the Australian Society for Parasitology Inc. All rights reserved. In organello T. brucei mt transcription was initially characterised by Harris et al. Transcriptional activity was maximal in the presence of all four ribonucleosidetriphosphates (NTPs) and this activity was designated 100
Coupled Structural, Thermal, Phase-change and Electromagnetic Analysis for Superconductors, Volume 2
Described are the theoretical development and computer implementation of reliable and efficient methods for the analysis of coupled mechanical problems that involve the interaction of mechanical, thermal, phase-change and electromag subproblems. The focus application has been the modeling of superconductivity and associated quantum-state phase change phenomena. In support of this objective the work has addressed the following issues: (1) development of variational principles for finite elements, (2) finite element modeling of the electromagnetic problem, (3) coupling of thermel and mechanical effects, and (4) computer implementation and solution of the superconductivity transition problem. The main accomplishments have been: (1) the development of the theory of parametrized and gauged variational principles, (2) the application of those principled to the construction of electromagnetic, thermal and mechanical finite elements, and (3) the coupling of electromagnetic finite elements with thermal and superconducting effects, and (4) the first detailed finite element simulations of bulk superconductors, in particular the Meissner effect and the nature of the normal conducting boundary layer. The theoretical development is described in two volumes. Volume 1 describes mostly formulation specific problems. Volume 2 describes generalization of those formulations
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