Improved noise-adding radiometer for microwave receivers

Use of input switch and noise reference standard is avoided by using noise-adding technique. Excess noise from solid state noise-diode is coupled into receiver through directional coupler and square-wave modulated at low rate. High sensitivity receivers for radioastronomy applications are utilized with greater confidence in stability of radiometer

Front Propagation in the Pearling Instability of Tubular Vesicles

Recently Bar-Ziv and Moses discovered a dynamical shape transformation induced in cylindrical lipid bilayer vesicles by the action of laser tweezers. We develop a hydrodynamic theory of fluid bilayers in interaction with the surrounding water and argue that the effect of the laser is to induce a sudden tension in the membrane. We refine our previous analysis to account for the fact that the shape transformation is not uniform but propagates outward from the laser trap. Applying the marginal stability criterion to this situation gives us an improved prediction for the selected initial wavelength and a new prediction for the propagation velocity, both in rough agreement with the experimental values. For example, a tubule of initial radius 0.7\micron\ has a predicted initial sinusoidal perturbation in its diameter with wavelength 5.5\micron, as observed. The perturbation propagates as a front with the qualitatively correct front velocity a bit less than 100\micron/sec. In particular we show why this velocity is initially constant, as observed, and so much smaller than the natural scale set by the tension. We also predict that the front velocity should increase linearly with laser power. Finally we introduce an approximate hydrodynamic model applicable to the fully nonlinear regime. This model exhibits propagating fronts as well as fully-developed ``pearled" vesicles similar to those seen in the experiments.Comment: 42 pages, 6 eps figures included with text in uuencoded file, ps file available from ftp://dept.physics.upenn.edu/pub/Nelson/pearl_propagation.ps submitted to Journal de Physiqu

Reversibility of Red blood Cell deformation

The ability of cells to undergo reversible shape changes is often crucial to their survival. For Red Blood Cells (RBCs), irreversible alteration of the cell shape and flexibility often causes anemia. Here we show theoretically that RBCs may react irreversibly to mechanical perturbations because of tensile stress in their cytoskeleton. The transient polymerization of protein fibers inside the cell seen in sickle cell anemia or a transient external force can trigger the formation of a cytoskeleton-free membrane protrusion of micrometer dimensions. The complex relaxation kinetics of the cell shape is shown to be responsible for selecting the final state once the perturbation is removed, thereby controlling the reversibility of the deformation. In some case, tubular protrusion are expected to relax via a peculiar "pearling instability".Comment: 4 pages, 3 figure

Wave localization in binary isotopically disordered one-dimensional harmonic chains with impurities having arbitrary cross section and concentration

The localization length for isotopically disordered harmonic one-dimensional chains is calculated for arbitrary impurity concentration and scattering cross section. The localization length depends on the scattering cross section of a single scatterer, which is calculated for a discrete chain having a wavelength dependent pulse propagation speed. For binary isotopically disordered systems composed of many scatterers, the localization length decreases with increasing impurity concentration, reaching a mimimum before diverging toward infinity as the impurity concentration approaches a value of one. The concentration dependence of the localization length over the entire impurity concentration range is approximated accurately by the sum of the behavior at each limiting concentration. Simultaneous measurements of Lyapunov exponent statistics indicate practical limits for the minimum system length and the number of scatterers to achieve representative ensemble averages. Results are discussed in the context of future investigations of the time-dependent behavior of disordered anharmonic chains.Comment: 8 pages, 10 figures, submitted to PR

ShapeFit and ShapeKick for Robust, Scalable Structure from Motion

We introduce a new method for location recovery from pair-wise directions that leverages an efficient convex program that comes with exact recovery guarantees, even in the presence of adversarial outliers. When pairwise directions represent scaled relative positions between pairs of views (estimated for instance with epipolar geometry) our method can be used for location recovery, that is the determination of relative pose up to a single unknown scale. For this task, our method yields performance comparable to the state-of-the-art with an order of magnitude speed-up. Our proposed numerical framework is flexible in that it accommodates other approaches to location recovery and can be used to speed up other methods. These properties are demonstrated by extensively testing against state-of-the-art methods for location recovery on 13 large, irregular collections of images of real scenes in addition to simulated data with ground truth

Dark states of dressed Bose-Einstein condensates

We combine the ideas of dressed Bose-Einstein condensates, where an intracavity optical field allows one to design coupled, multicomponent condensates, and of dark states of quantum systems, to generate a full quantum entanglement between two matter waves and two optical waves. While the matter waves are macroscopically populated, the two optical modes share a single photon. As such, this system offers a way to influence the behaviour of a macroscopic quantum system via a microscopic ``knob''.Comment: 6 pages, no figur

Iterative Approach to Gravitational Lensing Theory

We develop an iterative approach to gravitational lensing theory based on approximate solutions of the null geodesic equations. The approach can be employed in any space-time which is ``close'' to a space-time in which the null geodesic equations can be completely integrated, such as Minkowski space-time, Robertson-Walker cosmologies, or Schwarzschild-Kerr geometries. To illustrate the method, we construct the iterative gravitational lens equations and time of arrival equation for a single Schwarzschild lens. This example motivates a discussion of the relationship between the iterative approach, the standard thin lens formulation, and an exact formulation of gravitational lensing.Comment: 27 pages, 2 figures, submitted to Phys.Rev.D, minor revisions, new reference

Binary Bose-Einstein Condensate Mixtures in Weakly and Strongly Segregated Phases

We perform a mean-field study of the binary Bose-Einstein condensate mixtures as a function of the mutual repulsive interaction strength. In the phase segregated regime, we find that there are two distinct phases: the weakly segregated phase characterized by a `penetration depth' and the strongly segregated phase characterized by a healing length. In the weakly segregated phase the symmetry of the shape of each condensate will not take that of the trap because of the finite surface tension, but its total density profile still does. In the strongly segregated phase even the total density profile takes a different symmetry from that of the trap because of the mutual exclusion of the condensates. The lower critical condensate-atom number to observe the complete phase segregation is discussed. A comparison to recent experimental data suggests that the weakly segregated phase has been observed.Comment: minor change