2,357 research outputs found
Topological Protection and Quantum Noiseless Subsystems
Encoding and manipulation of quantum information by means of topological
degrees of freedom provides a promising way to achieve natural fault-tolerance
that is built-in at the physical level. We show that this topological approach
to quantum information processing is a particular instance of the notion of
computation in a noiseless quantum subsystem. The latter then provide the most
general conceptual framework for stabilizing quantum information and for
preserving quantum coherence in topological and geometric systems.Comment: 4 Pages LaTeX. Published versio
In vivo analysis of staphylococcus aureus-infected mice reveals differential temporal and spatial expression patterns of fhuD2
Staphylococcus aureus is an opportunistic human pathogen and a major cause of invasive infections such as bacteremia, endocarditis, pneumonia and wound infections. FhuD2 is a staphylococcal lipoprotein involved in the uptake of iron-hydroxymate and is under the control of the iron uptake regulator Fur. The protein is part of an investigational multi-component vaccine formulation that has shown protective efficacy in several murine models of infection. Even though fhuD2 expression was shown to be upregulated in murine kidneys infected with S. aureus, it is unknown whether the bacterium undergoes increased iron deprivation during prolonged infection. Furthermore, different infection niches of S. aureus might provide different environments and iron availability resulting in different fhuD2 expression pattern within different host organs. To address these questions, we characterized the in vitro expression of the fhuD2 gene and confirmed Fur-dependent iron-regulation of its expression. We further investigated its expression in mice infected with a bioluminescent reporter strain of S. aureus expressing the luciferase operon under the control of the fhuD2 promoter. The emission of bioluminescence in different organs was followed over a seven-day time course, as well as quantitative real-time PCR analysis of the RNA transcribed from the endogenous fhuD2 gene. Using this approach, we could show that fhuD2 expression was induced during infection in all organs analyzed and that differences in expression were observed in the temporal expression profiles, and between infected organs. Our data suggest that S. aureus undergoes increased iron deprivation during progression of infection in diverse host organs and accordingly induces dedicated iron acquisition mechanisms. Since FhuD2 plays a central role in providing the pathogen with the required iron, further knowledge of the patterns of fhuD2 expression in vivo during infection is instrumental in better defining the role of this antigen in S. aureus pathogenesis and as a vaccine antigen
Photosystem II: Thermodynamics and Kinetics of Electron Transport from QA- to QB(QB- ) and Deleterious Effects of Copper(II)
Studies on thermodynamics and kinetics of electron transfer from QA- to QB(QB-) were performed by monitoring laser flash induced changes of the relative fluorescence emission as a function of temperature (220 K < T < 310 K) in isolated thylakoids and PS II membrane fragments. In addition, effects of bivalent metal ions on PS II were investigated by measuring conventional fluorescence induction curves, oxygen evolution, manganese content and atrazine binding mostly in PS II membrane fragments. It was found: a) the normalized level of the fluorescence remaining 10 s after the actinic flash (Ft/F0) steeply increases at temperatures below -10 to - 20 °C, b) the fast phase of the transient fluorescence change becomes markedly retarded with decreasing temperatures, c) among different cations (Cu2+, Zn2+, Cd2+, Ni2+, Co2+) only Cu2+ exhibits marked effects in the concentration range below 100 μᴍ and d) Cu2+ decreases the normalized variable fluorescence, inhibits oxygen evolution and diminishes the affinity to atrazine binding without affecting the number of binding sites. The content of about four manganeses per functionally competent oxygen evolving complex is not changed by [Cu2+] < 70 μᴍ. Based on these findings it is concluded: i) a temperature dependent equilibrium between an inactive (I) and active (A) state of QA- reoxidation by QB(QB- ) is characterized by standard enthalpies ΔH° of 95 kJ mol-1 and 60 kJ mol-1 and standard entropies ΔS° of 370 kJ K-1 mol-1 and 240 kJ K-1 mol-1 in isolated thylakoids and PS II membrane fragments, respectively, ii) the activation energies of QA- reoxidation by plastoquinone bound to the QB site are about 30 kJ mol-1 (thylakoids) and 40 kJ mol-1 (PS II membrane fragments) in 220 K < T < 300 K, and iii) Cu2+ causes at least a two-fold effect on PS II by modifying the atrazine binding affinity at lower concentrations ( ~ 5 μᴍ) and interference with the redox active tyrosine Yz at slightly higher concentration ( ~ 10 μᴍ) leading to blockage of oxygen evolution
Vacuum Structures in Hamiltonian Light-Front Dynamics
Hamiltonian light-front dynamics of quantum fields may provide a useful
approach to systematic non-perturbative approximations to quantum field
theories. We investigate inequivalent Hilbert-space representations of the
light-front field algebra in which the stability group of the light-front is
implemented by unitary transformations. The Hilbert space representation of
states is generated by the operator algebra from the vacuum state. There is a
large class of vacuum states besides the Fock vacuum which meet all the
invariance requirements. The light-front Hamiltonian must annihilate the vacuum
and have a positive spectrum. We exhibit relations of the Hamiltonian to the
nontrivial vacuum structure.Comment: 16 pages, report \# ANL-PHY-7524-TH-93, (Latex
Taylor-Lagrange renormalization scheme, Pauli-Villars subtraction, and light-front dynamics
We show how the recently proposed Taylor-Lagrange renormalization scheme can
lead to extensions of singular distributions which are reminiscent of the
Pauli-Villars subtraction. However, at variance with the Pauli-Villars
regularization scheme, no infinite mass limit is performed in this scheme. As
an illustration of this mechanism, we consider the calculation of the
self-energy in second order perturbation theory in the Yukawa model, within the
covariant formulation of light-front dynamics. We show in particular how
rotational invariance is preserved in this scheme.Comment: 9 pages, 1 figure To be published in Physical Review
Dark Matter and Dark Energy via Non-Perturbative (Flavour) Vacua
A non-perturbative field theoretical approach to flavour physics
(Blasone-Vitiello formalism) has been shown to imply a highly non-trivial
vacuum state. In a previous work, we implemented the approach on a simple
supersymmetric model (free Wess-Zumino), with flavour mixing, which was
regarded as a model for free neutrinos and sneutrinos. The resulting effective
vacuum (called "flavour vacuum") was found to be characterized by a strong SUSY
breaking. In this paper we explore the phenomenology of the model and we argue
that the flavour vacuum is a consistent source for both Dark Energy (thanks to
the bosonic sector of the model) and Dark Matter (via the fermionic one). Quite
remarkably, besides the parameters connected with neutrino physics, in this
model no other parameters have been introduced, possibly leading to a
predictive theory of Dark Energy/Matter. Despite its oversimplification, such a
toy model already seems capable to shed some light on the observed energy
hierarchy between neutrino physics, Dark Energy and Dark Matter. Furthermore,
we move a step forth in the construction of a more realistic theory, by
presenting a novel approach for calculating relevant quantities and hence
extending some results to interactive theories, in a completely
non-perturbative way.Comment: 14 pages, 2 figure
Estimating Kinetic Parameters for the Spontaneous Polymerization of Glycidol at Elevated Temperatures
The ring-opening polymerization of glycidol at elevated temperatures is investigated. To
improve the synthesis of dendritic polyether polyols, experiments are carried out without
initiator to identify the influence of thermal side reactions. This results in a step-growth
polymerization caused by the spontaneous combination
of monomers. Kinetic parameters of the
side reactions are estimated by fitting simulated
number- and weight-average molecular weights
to the experimental values measured at different
reaction times during the polymerization. The
reactions are conducted at three different temperatures
of 90, 105, and 120 8C. It is shown that
thermal side reactions lead to high dispersities of
the final product and are highly sensitive to the
reactor operating temperature
Semicausal operations are semilocalizable
We prove a conjecture by DiVincenzo, which in the terminology of Preskill et
al. [quant-ph/0102043] states that ``semicausal operations are
semilocalizable''. That is, we show that any operation on the combined system
of Alice and Bob, which does not allow Bob to send messages to Alice, can be
represented as an operation by Alice, transmitting a quantum particle to Bob,
and a local operation by Bob. The proof is based on the uniqueness of the
Stinespring representation for a completely positive map. We sketch some of the
problems in transferring these concepts to the context of relativistic quantum
field theory.Comment: 4 pages, 1 figure, revte
A Way Out of the Quantum Trap
We review Event Enhanced Quantum Theory (EEQT). In Section 1 we address the
question "Is Quantum Theory the Last Word". In particular we respond to some of
recent challenging staments of H.P. Stapp. We also discuss a possible future of
the quantum paradigm - see also Section 5. In Section 2 we give a short sketch
of EEQT. Examples are given in Section 3. Section 3.3 discusses a completely
new phenomenon - chaos and fractal-like phenomena caused by a simultaneous
"measurement" of several non-commuting observables (we include picture of
Barnsley's IFS on unit sphere of a Hilbert space). In Section 4 we answer
"Frequently Asked Questions" concerning EEQT.Comment: Replacement. Corrected affiliation. Latex, one .jpg figure. To appear
in Proc. Conf. Relativistic Quantum Measurements, Napoli 1998, Ed. F.
Petruccion
Evolution in Quantum Causal Histories
We provide a precise definition and analysis of quantum causal histories
(QCH). A QCH consists of a discrete, locally finite, causal pre-spacetime with
matrix algebras encoding the quantum structure at each event. The evolution of
quantum states and observables is described by completely positive maps between
the algebras at causally related events. We show that this local description of
evolution is sufficient and that unitary evolution can be recovered wherever it
should actually be expected. This formalism may describe a quantum cosmology
without an assumption of global hyperbolicity; it is thus more general than the
Wheeler-DeWitt approach. The structure of a QCH is also closely related to
quantum information theory and algebraic quantum field theory on a causal set.Comment: 20 pages. 8 figures. (v3: minor corrections, additional references
[2,3]) to appear in CQ
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