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Search Results: 1 - 10 of 4929 matches for " Wolfgang Alt "
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An objective lens for efficient fluorescence detection of single atoms
Wolfgang Alt
Physics , 2001, DOI: 10.1078/0030-4026-00133
Abstract: We present the design of a diffraction limited, long working distance monochromatic objective lens for efficient light collection. Consisting of four spherical lenses, it has a numerical aperture of 0.29, an effective focal length of 36 mm and a working distance of 36.5 mm. This inexpensive system allows us to detect 8*10^4 fluorescence photons per second from a single cesium atom stored in a magneto-optical trap.
Coupling of cytoplasm and adhesion dynamics determines cell polarization and locomotion
Wolfgang Alt,Martin Bock,Christoph M?hl
Quantitative Biology , 2009,
Abstract: Observations of single epidermal cells on flat adhesive substrates have revealed two distinct morphological and functional states, namely a non-migrating symmetric unpolarized state and a migrating asymmetric polarized state. These states are characterized by different spatial distributions and dynamics of important biochemical cell components: F-actin and myosin-II form the contractile part of the cytoskeleton, and integrin receptors in the plasma membrane connect F-actin filaments to the substratum. In this way, focal adhesion complexes are assembled, which determine cytoskeletal force transduction and subsequent cell locomotion. So far, physical models have reduced this phenomenon either to gradients in regulatory control molecules or to different mechanics of the actin filament system in different regions of the cell. Here we offer an alternative and self-organizational model incorporating polymerization, pushing and sliding of filaments, as well as formation of adhesion sites and their force dependent kinetics. All these phenomena can be combined into a non-linearly coupled system of hyperbolic, parabolic and elliptic differential equations. Aim of this article is to show how relatively simple relations for the small-scale mechanics and kinetics of participating molecules may reproduce the emergent behavior of polarization and migration on the large-scale cell level.
Egocentric Path Integration Models and their Application to Desert Arthropods
Tobias Merkle,Martin Rost,Wolfgang Alt
Quantitative Biology , 2005,
Abstract: Path integration enables desert arthropods to find back to their nest on the shortest track from any position. To perform path integration successfully, speeds and turning angles along the preceding outbound path have to be measured continuously and combined to determine an internal {\em global vector} leading back home at any time. A number of experiments have given an idea how arthropods might use allothetic or idiothetic signals to perceive their orientation and moving speed. We systematically review the four possible model descriptions of mathematically precise path integration, whereby we favour and elaborate the hitherto not used variant of egocentric cartesian coordinates. Its simple and intuitive structure is demonstrated in comparison to the other models. Measuring two speeds, the forward moving speed and the angular turning rate, and implementing them into a linear system of differential equations provides the necessary information during outbound route, reorientation process and return path. In addition, we propose several possible types of systematic errors that can cause deviations from the correct homeward course. Deviations have been observed for several species of desert arthropods in different experiments, but their origin is still under debate. Using our egocentric path integration model we propose simple error indices depending on path geometry that will allow future experiments to rule out or corroborate certain error types.
Optical control of the refractive index of a single atom
Tobias Kampschulte,Wolfgang Alt,Stefan Brakhane,Martin Eckstein,René Reimann,Artur Widera,Dieter Meschede
Physics , 2010, DOI: 10.1103/PhysRevLett.105.153603
Abstract: We experimentally demonstrate the elementary case of electromagnetically induced transparency (EIT) with a single atom inside an optical cavity probed by a weak field. We observe the modification of the dispersive and absorptive properties of the atom by changing the frequency of a control light field. Moreover, a strong cooling effect has been observed at two-photon resonance, increasing the storage time of our atoms twenty-fold to about 16 seconds. Our result points towards all-optical switching with single photons.
Generalized Voronoi Tessellation as a Model of Two-dimensional Cell Tissue Dynamics
Martin Bock,Amit Kumar Tyagi,Jan-Ulrich Kreft,Wolfgang Alt
Physics , 2009,
Abstract: Voronoi tessellations have been used to model the geometric arrangement of cells in morphogenetic or cancerous tissues, however so far only with flat hypersurfaces as cell-cell contact borders. In order to reproduce the experimentally observed piecewise spherical boundary shapes, we develop a consistent theoretical framework of multiplicatively weighted distance functions, defining generalized finite Voronoi neighborhoods around cell bodies of varying radius, which serve as heterogeneous generators of the resulting model tissue. The interactions between cells are represented by adhesive and repelling force densities on the cell contact borders. In addition, protrusive locomotion forces are implemented along the cell boundaries at the tissue margin, and stochastic perturbations allow for non-deterministic motility effects. Simulations of the emerging system of stochastic differential equations for position and velocity of cell centers show the feasibility of this Voronoi method generating realistic cell shapes. In the limiting case of a single cell pair in brief contact, the dynamical nonlinear Ornstein-Uhlenbeck process is analytically investigated. In general, topologically distinct tissue conformations are observed, exhibiting stability on different time scales, and tissue coherence is quantified by suitable characteristics. Finally, an argument is derived pointing to a tradeoff in natural tissues between cell size heterogeneity and the extension of cellular lamellae.
Microwave control of atomic motional states in a spin-dependent optical lattice
Noomen Belmechri,Leonid F?rster,Wolfgang Alt,Artur Widera,Dieter Meschede,Andrea Alberti
Physics , 2013, DOI: 10.1088/0953-4075/46/10/104006
Abstract: Spin-dependent optical potentials allow us to use microwave radiation to manipulate the motional state of trapped neutral atoms (F\"orster et al. 2009 Phys. Rev. Lett. 103, 233001). Here, we discuss this method in greater detail, comparing it to the widely-employed Raman sideband coupling method. We provide a simplified model for sideband cooling in a spin-dependent potential, and we discuss it in terms of the generalized Lamb-Dicke parameter. Using a master equation formalism, we present a quantitative analysis of the cooling performance for our experiment, which can be generalized to other experimental settings. We additionally use microwave sideband transitions to engineer motional Fock states and coherent states, and we devise a technique for measuring the population distribution of the prepared states.
Single atoms in a standing-wave dipole trap
Wolfgang Alt,Dominik Schrader,Stefan Kuhr,Martin Mueller,Victor Gomer,Dieter Meschede
Physics , 2002, DOI: 10.1103/PhysRevA.67.033403
Abstract: We trap a single cesium atom in a standing-wave optical dipole trap. Special experimental procedures, designed to work with single atoms, are used to measure the oscillation frequency and the atomic energy distribution in the dipole trap. These methods rely on unambiguously detecting presence or loss of the atom using its resonance fluorescence in the magneto-optical trap.
An optical conveyor belt for single neutral atoms
Dominik Schrader,Stefan Kuhr,Wolfgang Alt,Martin Mueller,Victor Gomer,Dieter Meschede
Physics , 2001, DOI: 10.1007/s003400100722
Abstract: Using optical dipole forces we have realized controlled transport of a single or any desired small number of neutral atoms over a distance of a centimeter with sub-micrometer precision. A standing wave dipole trap is loaded with a prescribed number of cesium atoms from a magneto-optical trap. Mutual detuning of the counter-propagating laser beams moves the interference pattern, allowing us to accelerate and stop the atoms at preselected points along the standing wave. The transportation efficiency is close to 100%. This optical "single-atom conveyor belt" represents a versatile tool for future experiments requiring deterministic delivery of a prescribed number of atoms on demand.
Super-resolution microscopy of single atoms in optical lattices
Andrea Alberti,Carsten Robens,Wolfgang Alt,Stefan Brakhane,Micha? Karski,René Reimann,Artur Widera,Dieter Meschede
Physics , 2015,
Abstract: We report on image processing techniques and experimental procedures to determine the lattice-site positions of single atoms in an optical lattice with high reliability, even for limited acquisition time or optical resolution. Determining the positions of atoms beyond the diffraction limit relies on parametric deconvolution in close analogy to methods employed in super-resolution microscopy. We develop a deconvolution method that makes effective use of the prior knowledge of the optical transfer function, noise properties, and discreteness of the optical lattice. We show that accurate knowledge of the image formation process enables a dramatic improvement on the localization reliability. This is especially relevant for closely packed ensembles of atoms where the separation between particles cannot be directly optically resolved. Furthermore, we demonstrate experimental methods to precisely reconstruct the point spread function with sub-pixel resolution from fluorescence images of single atoms, and we give a mathematical foundation thereof. We also discuss discretized image sampling in pixel detectors and provide a quantitative model of noise sources in electron multiplying CCD cameras.
Ultra-low birefringence dodecagonal vacuum glass cell
Stefan Brakhane,Wolfgang Alt,Dieter Meschede,Geol Moon,Carsten Robens,Andrea Alberti
Physics , 2015,
Abstract: We eport on an ultra-low birefringence dodecagonal glass cell for ultra-high vacuum applications. The epoxy-bonded trapezoidal windows of the cell are made of SF57 glass, which exhibits a very low stress-induced birefringence. We characterize the birefringence $\Delta n$ of each window with the cell under vacuum conditions, obtaining values around $\num{e-8}$. After baking the cell at $\SI{150}{\degreeCelsius}$, we reach a pressure below $\SI{e-10}{\milli \bar}$. In addition, each window is antireflection coated on both sides, which is highly desirable for quantum optics experiments and precision measurements.
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