1,594 research outputs found
Fighting Decoherence by Feedback-controlled Dissipation
Repeated closed-loop control operations acting as piecewise-constant
Liouville superoperators conditioned on the outcomes of regularly performed
measurements may effectively be described by a fixed-point iteration for the
density matrix. Even when all Liouville superoperators point to the completely
mixed state, feedback of the measurement result may lead to a pure state, which
can be interpreted as selective dampening of undesired states. Using a
microscopic model, we exemplify this for a single qubit, which can be purified
in an arbitrary single-qubit state by tuning the measurement direction and two
qubits that may be purified towards a Bell state by applying a special
continuous two-local measurement. The method does not require precise knowledge
of decoherence channels and works for large reservoir temperatures provided
measurement, processing, and control can be implemented in a continuous
fashion.Comment: to appear in PR
Schweizer Lokalparteien im Wandel : erste Ergebnisse einer Befragung der Präsidentinnen und Präsidenten der Schweizer Lokalparteien 2002/2003
Schweizer Lokalparteien im Wandel : erste Ergebnisse einer Befragung der Präsidentinnen und Präsidenten der Schweizer Lokalparteien 2002/2003
Single electron transistor strongly coupled to vibrations: Counting Statistics and Fluctuation Theorem
Using a simple quantum master equation approach, we calculate the Full
Counting Statistics of a single electron transistor strongly coupled to
vibrations. The Full Counting Statistics contains both the statistics of
integrated particle and energy currents associated to the transferred electrons
and phonons. A universal as well as an effective fluctuation theorem are
derived for the general case where the various reservoir temperatures and
chemical potentials are different. The first relates to the entropy production
generated in the junction while the second reveals internal information of the
system. The model recovers Franck-Condon blockade and potential applications to
non-invasive molecular spectroscopy are discussed.Comment: extended discussion, to appear in NJ
Structural attributes contributing to locomotor performance in the ostrich
As the fastest long-endurance runner, the bipedal ostrich (Struthio camelus) was selected as a prime model organism to investigate the physical attributes underlying this advanced locomotor performance. A specific integrative approach combining morphological, morphometric, kinematic and pedobarographic methods was developed. The comparative morphometric analysis of the hind limbs of all ratite species revealed that leg segment ratios in the ostrich are the most specialised for efficient locomotion, especially when taking into consideration its unique supra-jointed toe posture. In addition, the crural muscle mass is more concentrated towards the hip joint in the ostrich than in its ratite relatives. According to the Law of the Pendulum, this concentration of mass towards the pivot point – in concert with the relatively longest and lightest distal leg elements – represents a mechanical optimisation of limb swinging capacities. While musculature clearly drives limb movement, the passive guidance and constraint of motion range by ligamentous structures combined with joint surface contours allows a high level of energy output efficiency during all stages of locomotion and ensures articular stability during slow locomotion as well as high-speed performance. So far, the influence of these passive effects in locomotion has been largely ignored. In order to quantify the guiding effect of these anatomical structures, kinematic data of adult ostriches during walking and running were collected. Subsequently, these data were compared with results from manual manipulation experiments performed with the limbs of anatomical specimens – both fully intact and with muscles removed – leaving only the ligament system intact. This investigation revealed that the range of motion among leg segments was nearly identical in all sample groups, especially in regard to maximum extension values. This indicates that ostrich hind limb dynamics are managed to a significant degree by passive elements that ensure a controlled swing-plane with minimal deviation from an optimal attitude. Further dissections allowed some of these features to be described in detail, with an emphasis on functional-morphological examination of the intertarsal joint. The intertarsal joint contains a significant locking mechanism, briefly mentioned in historical documents, but described and functionally analysed herein for the first time. The functional examination qualified the interplay of three collateral ligaments, the tendinous M. fibularis brevis and specific joint surface protrusions as the basis for this effect which remains absent in smaller ground-dwelling bird species. A proximate quantification, based on comparative morphological and kinematic data, revealed function of Struthio's passively locked intertarsal joint as a potent stabiliser in the supporting limb during the ground-contact phase of locomotion. During stance phase, it is crucial that the supporting limb is stabilised internally and in relation to the substrate. As yet, no study exists concerning use and loading of the actual ground contact elements. The toes must absorb body mass, guarantee stable grip and provide energetic push off. Obvious specialisations of the ostrich's phalangeal complex include toe reduction (leaving only 3rd and 4th toe), claw reduction (only at 3rd toe) and a permanently elevated metatarsophalangeal joint. Using a relatively new methodology to examine in vivo toe function, pedobarography was employed on specifically trained ostriches to allow extensive collection of Centre of Pressure (CoP) and load distribution (LD) data. In contrast to a relatively predictable CoP trajectory at all speeds, conspicuous LD differences were observed between slow and fast trials. Load was distributed rather inconsistently during walking, while a typical tripod-like toe-print occurred in all running trials to presumably deliver additional stability during the comparatively short stance phase. Significant grip is provided by the highly directed impact of the 3rd toe claw-tip, suggesting its important function as a positional anchor during running. Pedobarographic analysis further showed the importance of the 4th toe as an outrigger to maintain balance, rendering a future reduction highly unlikely. In conclusion, the application of interdisciplinary methodologies allowed comprehensive data collection and integration of the model organism within its ecological context. The data gained from this thesis increases the current knowledge about ostrich locomotion by identifying distinct structural attributes as essential elements for extreme cursorial performance. The present data may alter existing models for calculation of the metabolic cost of terrestrial locomotion and aid in the reconstruction of theropod locomotion, as these branch sciences often overlook the important role of ligaments and passively-coupled motion cycles in reducing the cost of locomotion
Sequence differences between histones of procyclic Trypanosoma brucei brucei and higher eukaryotes
Four histones, a, b, c, d from procyclic Trypanosoma brucei brucei, which show similarities with the amino acid composition of the core histones H3, H2A, H2B and H4, were isolated and cleaved with Endoproteinase Glu-C. The fragments were separated by FPLC reversed phase chromatography and a subset of the fragments (a5, a9, b6, c8, d3, d9, d11) was subjected to sequence analysis. A 54-71% identity was found in the sequences of the fragment c8 and the C-terminal half of H2B and of three fragments of protein d covering the N-terminal half as well as the C-terminal region of H4. The amino acid sequence of the fragment a9 showed a 57 and 54% identity with H3 sequences of Saccharomyces cerevisiae and Xenopus laevis. Neither the a5 nor the b6 sequence could be aligned with histone sequences of other eukaryotes. The significant differences of 21-48% between the T. b. brucei, histone sequences and those of calf thymus histones, which are more pronounced than the differences of Tetrahymena pyriformis and the higher eukaryote, resulted partially from replacements of amino acids with different properties and indicate specific patterns of histone-histone and/or histone-DNA contact sites in the nucleosome of T. b. brucei. These differences, together with the lack of a functional histone H1, may be sufficient to explain the lack of a salt-dependent formation of the nucleosome filament into the 30 nm fibre, which reflects alternative methods of organizing and processing the genetic information in the nucleus of the protozoan parasite and which may be of chemotherapeutic significanc
Systematic Perturbation Theory for Dynamical Coarse-Graining
We demonstrate how the dynamical coarse-graining approach can be
systematically extended to higher orders in the coupling between system and
reservoir. Up to second order in the coupling constant we explicitly show that
dynamical coarse-graining unconditionally preserves positivity of the density
matrix -- even for bath density matrices that are not in equilibrium and also
for time-dependent system Hamiltonians. By construction, the approach correctly
captures the short-time dynamics, i.e., it is suitable to analyze non-Markovian
effects. We compare the dynamics with the exact solution for highly
non-Markovian systems and find a remarkable quality of the coarse-graining
approach. The extension to higher orders is straightforward but rather tedious.
The approach is especially useful for bath correlation functions of simple
structure and for small system dimensions.Comment: 17 pages, 5 figures, version accepted for publication in PR
Hydrodynamical Survey of First Overtone Cepheids
A hydrodynamical survey of the pulsational properties of first overtone
Galactic Cepheids is presented. The goal of this study is to reproduce their
observed light- and radial velocity curves. The comparison between the models
and the observations is made in a quantitative manner on the level of the
Fourier coefficients. Purely radiative models fail to reproduce the observed
features, but convective models give good agreement.
It is found that the sharp features in the Fourier coefficients are indeed
caused by the P1/P4 = 2 resonance, despite the very large damping of the 4th
overtone. For the adopted mass-luminosity relation the resonance center lies
near a period of 4.2d +/- 0.2 as indicated by the observed radial velocity
data, rather than near 3.2d as the light-curves suggest.Comment: ApJ, 12 pages, (slightly) revise
HE 0437-5439 -- an unbound hyper-velocity main-sequence B-type star
We report the discovery of a 16th magnitude star, HE0437-5439, with a
heliocentric radial velocity of +723+-3km/s. A quantitative spectral analysis
of high-resolution optical spectra obtained with the VLT and the UVES
spectrograph shows that HE0437-5439 is a main sequence B-type star with
Teff=20350K, log g=3.77, solar within a factor of a few helium abundance and
metal content, rotating at v sin i=54km/s. Using appropriate evolutionary
tracks we derive a mass of 8 Msun and a corresponding distance of 61 kpc. Its
galactic rest frame velocity is at least 563km/s, almost twice the local
Galactic escape velocity, indicating that the star is unbound to the Galaxy.
Numerical kinematical experiments are carried out to constrain its place of
birth. It has been suggested that such hyper-velocity stars can be formed by
the tidal disruption of a binary through interaction with the super-massive
black hole at the Galactic center (GC). HE0437-5439 needs about 100Myrs to
travel from the GC to its presentposition, much longer than its main sequence
lifetime of 25Myrs. This can only be reconciled if HE0437-5439 is a blue
straggler star. In this case, the predicted proper motion is so small that it
can only be measured by future space missions. Since the star is much closer to
the Large Magellanic Cloud (LMC, 18kpc) than to the GC, it can reach its
position from the center of the LMC. The proper motion predicted in this case
is about 2mas/y (relative to the LMC), large enough to be measurable with
conventional techniques from the ground. The LMC origin could also be tested by
a high-precision abundance analysis.Comment: 13 pages, 4 figures. Astrophysical Journal Letters, accepte
Alteration in elemental and functional composition of heated peat humic acids
The article analyzes the effect of thermal modification of different-type peat on the alteration of elemental and functional composition of peat humic acids. Based on the data of IR-spectra and readings of electron paramagnetic resonance, structural alterations are identified. It is shown that the impact of peat characteristics on humic acids is preserved after thermal modification. It is revealed that the strongest alteration of humic acid composition and properties caused by peat heating are typical to humic acid samples extracted from the peat with low decomposition degree
- …