Optimized Entanglement Purification

We investigate novel protocols for entanglement purification of qubit Bell pairs. Employing genetic algorithms for the design of the purification circuit, we obtain shorter circuits achieving higher success rates and better final fidelities than what is currently available in the literature. We provide a software tool for analytical and numerical study of the generated purification circuits, under customizable error models. These new purification protocols pave the way to practical implementations of modular quantum computers and quantum repeaters. Our approach is particularly attentive to the effects of finite resources and imperfect local operations - phenomena neglected in the usual asymptotic approach to the problem. The choice of the building blocks permitted in the construction of the circuits is based on a thorough enumeration of the local Clifford operations that act as permutations on the basis of Bell states

Interatomic potentials for ionic systems with density functional accuracy based on charge densities obtained by a neural network

Based on an analysis of the short range chemical environment of each atom in a system, standard machine learning based approaches to the construction of interatomic potentials aim at determining directly the central quantity which is the total energy. This prevents for instance an accurate description of the energetics of systems where long range charge transfer is important as well as of ionized systems. We propose therefore not to target directly with machine learning methods the total energy but an intermediate physical quantity namely the charge density, which then in turn allows to determine the total energy. By allowing the electronic charge to distribute itself in an optimal way over the system, we can describe not only neutral but also ionized systems with unprecedented accuracy. We demonstrate the power of our approach for both neutral and ionized NaCl clusters where charge redistribution plays a decisive role for the energetics. We are able to obtain chemical accuracy, i.e. errors of less than a milli Hartree per atom compared to the reference density functional results. The introduction of physically motivated quantities which are determined by the short range atomic environment via a neural network leads also to an increased stability of the machine learning process and transferability of the potential.Comment: 4 figure

Mathematics Is Physics

In this essay, I argue that mathematics is a natural science---just like physics, chemistry, or biology---and that this can explain the alleged "unreasonable" effectiveness of mathematics in the physical sciences. The main challenge for this view is to explain how mathematical theories can become increasingly abstract and develop their own internal structure, whilst still maintaining an appropriate empirical tether that can explain their later use in physics. In order to address this, I offer a theory of mathematical theory-building based on the idea that human knowledge has the structure of a scale-free network and that abstract mathematical theories arise from a repeated process of replacing strong analogies with new hubs in this network. This allows mathematics to be seen as the study of regularities, within regularities, within ..., within regularities of the natural world. Since mathematical theories are derived from the natural world, albeit at a much higher level of abstraction than most other scientific theories, it should come as no surprise that they so often show up in physics. This version of the essay contains an addendum responding to Slyvia Wenmackers' essay and comments that were made on the FQXi website.Comment: 15 pages, LaTeX. Second prize winner in 2015 FQXi Essay Contest (see http://fqxi.org/community/forum/topic/2364

Optimization of flux-surface density variation in stellarator plasmas with respect to the transport of collisional impurities

Avoiding impurity accumulation is a requirement for steady-state stellarator operation. The accumulation of impurities can be heavily affected by variations in their density on the flux-surface. Using recently derived semi-analytic expressions for the transport of a collisional impurity species with high-$Z$ and flux-surface density-variation in the presence of a low-collisionality bulk ion species, we numerically optimize the impurity density-variation on the flux-surface to minimize the radial peaking factor of the impurities. These optimized density-variations can reduce the core impurity density by $0.75^Z$ (with $Z$ the impurity charge number) in the Large Helical Device case considered here, and by $0.89^Z$ in a Wendelstein 7-X standard configuration case. On the other hand, when the same procedure is used to find density-variations that maximize the peaking factor, it is notably increased compared to the case with no density-variation. This highlights the potential importance of measuring and controlling these variations in experiments.Comment: 19 figures, 17 pages. Accepted into Nuclear Fusio

Formation of interstellar silicate dust via nanocluster aggregation : Insights from quantum chemistry simulations

The issue of formation of dust grains in the interstellar medium is still a matter of debate. One of the most developed proposals suggests that atomic and heteromolecular seeds bind together to initiate a nucleation process leading to the formation of nanostructures resembling very small grain components. In the case of silicates, nucleated systems can result in molecular nanoclusters with diameters ≤ 2 nm. A reasonable path to further increase the size of these proto-silicate nanoclusters is by mutual aggregation. The present work deals with modeling this proto-silicate nanocluster aggregation process by means of quantum chemical density functional theory calculations. We simulate nanocluster aggregation by progressively reducing the size of a periodic array of initially well-separated nanoclusters. The resulting aggregation leads to a set of silicate bulk structures with gradually increasing density which we analyze with respect to structure, energetics and spectroscopic properties. Our results indicate that aggregation is a highly energetically favorable process, in which the infrared spectra of the finally formed amorphous silicates match well with astronomical observations

The new "p-n junction": Plasmonics enables photonic access to the nanoworld

Since the development of the light microscope in the 16th century, optical device size and performance have been limited by diffraction. Optoelectronic devices of today are much bigger than the smallest electronic devices for this reason. Achieving control of light-material interactions for photonic device applications at the nanoscale requires structures that guide electromagnetic energy with subwavelength-scale mode confinement. By converting the optical mode into nonradiating surface plasmons, electromagnetic energy can be guided in structures with lateral dimensions of less than 10% of the free-space wavelength. A variety of methods-including electron-beam lithography and self-assembly-have been used to construct both particle and planar plasmon waveguides. Recent experimental studies have confirmed the strongly coupled collective plasmonic modes of metallic nanostructures. In plasmon waveguides consisting of closely spaced silver rods, electromagnetic energy transport over distances of 0.5 mu m has been observed. Moreover, numerical simulations suggest the possibility of multi-centimeter plasmon propagation in thin metallic stripes. Thus, there appears to be no fundamental scaling limit to the size and density of photonic devices, and ongoing work is aimed at identifying important device performance criteria in the subwavelength size regime. Ultimately, it may be possible to design an entire class of subwavelength-scale optoelectronic components (waveguides, sources, detectors, modulators) that could form the building blocks of an optical device technology-a technology scalable to molecular dimensions, with potential imaging, spectroscopy, and interconnection applications in computing, communications, and chemical/biological detection

Proteomics-on-a-chip for Biomarker discovery

In proteomics research still two-dimensional gel electrophoresis (2D-GE) is currently used for biomarker discovery. We applied free flow electrophoresis (FFE) separation technology combined with biomolecular interaction sensing using Surface Plasmon Resonance (SPR) imaging in an integrated proteomics-on-a-chip device as a proof of concept for biomarker discovery

Review On Laser Lightcraft Research At DLR Stuttgart

A review on 15 years research on remote laser propulsion with a parabolic thruster at DLR is presented. Mission scenarios were analyzed for nanosatellite launch to Low Earth Orbit (LEO) using a ground-based high energy laser as energy supply significantly optimizing the mass-to-payload-ratio. Experimental work was carried out using a home-made, electron-beam sustained CO2 high energy laser in the 10 kW class with around 10 μs pulse length. The parabolic thruster was compared with the Lightcraft Technology Demonstrator in air-breathing mode as well as with Polyoxymethylene (POM) as an ablative propellant with respect to laser pulse energy and beam profile taking into account for standardization issues of ballistic pendula. Experiments showed good performance of pure air-breathing mode without propellant down to 200 mbar ambient pressure allowing for a drastic propellant reduction for the initial flight phase during dense atmosphere. The commonly used hydrodynamic point explosion model with a strong shock wave was analyzed with respect to the optimization of the impulse coupling coefficient in geometric scaling by the adaptation of nozzle diameter and length to the range of the applied laser pulse energy. The usage of ablative propellants like POM, inevitable in the vacuum of space, yields enhancement of impulse coupling under atmospheric conditions which can partly be attributed to combustion. Various polymer-metal composites were developed and analyzed in order to achieve a higher specific impulse, but failed due to material inhomogeneity. Started up with wire-guided flights, using only air as propellant in a laser-induced breakdown, the detonation reproducibility by means of an ignition pin on the axis of symmetry of the thruster was proven in the free flight experiments yielding an altitude up to 8 m, limited by the laboratory ceiling. Nevertheless, flight dynamic analysis of a tilted pin as steering gear and hovering experiments near ground level revealed crucial coupling of lateral and angular motion and the demand of spin-stabilization for a beam-riding flight. A review of related publications, in cooperation with US AFRL, University of Stuttgart and Nagoya University, is given as a compendium