2,637 research outputs found
Orbits of quantum states and geometry of Bloch vectors for -level systems
Physical constraints such as positivity endow the set of quantum states with
a rich geometry if the system dimension is greater than two. To shed some light
on the complicated structure of the set of quantum states, we consider a
stratification with strata given by unitary orbit manifolds, which can be
identified with flag manifolds. The results are applied to study the geometry
of the coherence vector for n-level quantum systems. It is shown that the
unitary orbits can be naturally identified with spheres in R^{n^2-1} only for
n=2. In higher dimensions the coherence vector only defines a non-surjective
embedding into a closed ball. A detailed analysis of the three-level case is
presented. Finally, a refined stratification in terms of symplectic orbits is
considered.Comment: 15 pages LaTeX, 3 figures, reformatted, slightly modified version,
corrected eq.(3), to appear in J. Physics
Degrees of controllability for quantum systems and applications to atomic systems
Precise definitions for different degrees of controllability for quantum
systems are given, and necessary and sufficient conditions are discussed. The
results are applied to determine the degree of controllability for various
atomic systems with degenerate energy levels and transition frequencies.Comment: 20 pages, IoP LaTeX, revised and expanded versio
Metagenomic sequencing unravels gene fragments with phylogenetic signatures of O2-tolerant NiFe membrane-bound hydrogenases in lacustrine sediment
Many promising hydrogen technologies utilising hydrogenase enzymes have been slowed by the fact that most hydrogenases are extremely sensitive to O2. Within the group 1 membrane-bound NiFe hydrogenase, naturally occurring tolerant enzymes do exist, and O2 tolerance has been largely attributed to changes in iron–sulphur clusters coordinated by different numbers of cysteine residues in the enzyme’s small subunit. Indeed, previous work has provided a robust phylogenetic signature of O2 tolerance [1], which when combined with new sequencing technologies makes bio prospecting in nature a far more viable endeavour. However, making sense of such a vast diversity is still challenging and could be simplified if known species with O2-tolerant enzymes were annotated with information on metabolism and natural environments. Here, we utilised a bioinformatics approach to compare O2-tolerant and sensitive membrane-bound NiFe hydrogenases from 177 bacterial species with fully sequenced genomes for differences in their taxonomy, O2 requirements, and natural environment. Following this, we interrogated a metagenome from lacustrine surface sediment for novel hydrogenases via high-throughput shotgun DNA sequencing using the Illumina™ MiSeq platform. We found 44 new NiFe group 1 membrane-bound hydrogenase sequence fragments, five of which segregated with the tolerant group on the phylogenetic tree of the enzyme’s small subunit, and four with the large subunit, indicating de novo O2-tolerant protein sequences that could help engineer more efficient hydrogenases
ZZE-Configuration of chromophore Ăź-153 in C-phycocyanin from Mastigocladus laminosus
The photochemistry of C-phycocyanin has been studied after denaturation in the dark. It shows
an irreversible reaction which has characteristics of a Ζ,Ζ,Ε- to Z,Z,Z-isomerization of dihydrobilins.
Its amplitude depends on the reaction conditions, with a maximum corresponding to 15%
conversion of one of the three PC chromophores. This chromophore is suggested to be Ăź-153, for
which recent X-ray data T. Schirmer, W. Bode, and R. Huber, J. Mol. Biol., submitted, show
ring D being highly twisted out of the plane of the other rings. During unfolding, there is thus a
probability of falling into the photochemically labile Z,Z,^-configuration
Beyond the pale?: the implications of the RSLG Report for non-CURL modern university libraries: Perspectives on the support libraries group: Final report
We have shown that the cluster-mass reconstruction method
which combines strong and weak gravitational lensing data, developed
in the first paper in the series, successfully reconstructs the
mass distribution of a simulated cluster. In this paper we apply the method to the
ground-based high-quality multi-colour data of RX J1347.5-114
Efficient Algorithms for Optimal Control of Quantum Dynamics: The "Krotov'' Method unencumbered
Efficient algorithms for the discovery of optimal control designs for
coherent control of quantum processes are of fundamental importance. One
important class of algorithms are sequential update algorithms generally
attributed to Krotov. Although widely and often successfully used, the
associated theory is often involved and leaves many crucial questions
unanswered, from the monotonicity and convergence of the algorithm to
discretization effects, leading to the introduction of ad-hoc penalty terms and
suboptimal update schemes detrimental to the performance of the algorithm. We
present a general framework for sequential update algorithms including specific
prescriptions for efficient update rules with inexpensive dynamic search length
control, taking into account discretization effects and eliminating the need
for ad-hoc penalty terms. The latter, while necessary to regularize the problem
in the limit of infinite time resolution, i.e., the continuum limit, are shown
to be undesirable and unnecessary in the practically relevant case of finite
time resolution. Numerical examples show that the ideas underlying many of
these results extend even beyond what can be rigorously proved.Comment: 19 pages, many figure
GaBoDS: The Garching-Bonn Deep Survey -- II. Confirmation of EIS cluster candidates by weak gravitational lensing
We report the first confirmation of colour-selected galaxy cluster candidates
by means of weak gravitational lensing. Significant lensing signals were
identified in the course of the shear-selection programme of dark matter haloes
in the Garching-Bonn Deep Survey, which currently covers 20 square degrees of
deep, high-quality imaging data on the southern sky. The detection was made in
a field that was previously covered by the ESO Imaging Survey (EIS) in 1997. A
highly significant shear-selected mass-concentration perfectly coincides with
the richest EIS cluster candidate at z~0.2, thus confirming its cluster nature.
Several other shear patterns in the field can also be identified with cluster
candidates, one of which could possibly be part of a filament at z~0.45.Comment: 4 pages, 4 figures, submitted to A&A Letter
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