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

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