31,662 research outputs found
Trans-spectral orbital angular momentum transfer via four-wave mixing in Rb vapor
We report the transfer of phase structure and, in particular, of orbital angular momentum from near-infrared pump light to blue light generated in a four-wave-mixing process in Rb-85 vapor. The intensity and phase profile of the two pump lasers at 780 and 776 nm, shaped by a spatial light modulator, influences the phase and intensity profile of light at 420 nm, which is generated in a subsequent coherent cascade. In particular, we observe that the phase profile associated with orbital angular momentum is transferred entirely from the pump light to the blue. Pumping with more complicated light profiles results in the excitation of spatial modes in the blue that depend strongly on phase matching, thus demonstrating the parametric nature of the mode transfer. These results have implications on the inscription and storage of phase information in atomic gases
Study of certain launching techniques using long orbiting tethers
A study of the basic equations governing orbital transfers using long orbiting tethers is presented. A very simple approximation to the general transfer equation is derived for the case of short tethers and low eccentricity orbits. Numerical examples are calculated for the case of injection into a circular orbit from a platform in eccentric orbit and injection into eccentric orbit from a platform in circular orbit. For the case of long tethers, a method is derived for reducing tether mass and increasing payload mass by tapering the tether to maintain constant stress per unit of tether cross section. Formulas are presented for calculating the equilibrium orbital parameters taking into account the mass of the platform, tether, and payload
A weakly random Universe?
The cosmic microwave background (CMB) radiation is characterized by
well-established scales, the 2.7 K temperature of the Planckian spectrum and
the amplitude of the temperature anisotropy. These features were
instrumental in indicating the hot and equilibrium phases of the early history
of the Universe and its large scale isotropy, respectively. We now reveal one
more intrinsic scale in CMB properties. We introduce a method developed
originally by Kolmogorov, that quantifies a degree of randomness (chaos) in a
set of numbers, such as measurements of the CMB temperature in some region.
Considering CMB as a composition of random and regular signals, we solve the
inverse problem of recovering of their mutual fractions from the temperature
sky maps. Deriving the empirical Kolmogorov's function in the Wilkinson
Microwave Anisotropy Probe's maps, we obtain the fraction of the random signal
to be about 20 per cent, i.e. the cosmological sky is a weakly random one. The
paper is dedicated to the memory of Vladimir Arnold (1937-2010).Comment: 4 pages, 3 figs, A & A (Lett) in press; to match the published
versio
Adaptive reflection and focusing of Bose-Einstein condensates
We report adjustable magnetic `bouncing' and focusing of a dilute Rb
Bose gas. Both the condensate production and manipulation are realised using a
particularly straight-forward apparatus. The bouncing region is comprised of
approximately concentric ellipsoidal magnetic equipotentials with a centre that
can be adjusted vertically. We extend, and discuss the limitations of, simple
Thomas-Fermi and Monte-Carlo theoretical models for the bouncing, which at
present find close agreement with the condensate's evolution. Very strong
focusing has been inferred and the observation of atomic matter-wave
diffraction should be possible. Prospects look bright for applications in
matter-wave atom-optics, due to the very smooth nature of the mirror
Degree of randomness: numerical experiments for astrophysical signals
Astrophysical and cosmological signals such as the cosmic microwave
background radiation, as observed, typically contain contributions of different
components, and their statistical properties can be used to distinguish one
from the other. A method developed originally by Kolmogorov is involved for the
study of astrophysical signals of randomness of various degrees. Numerical
performed experiments based on the universality of Kolmogorov distribution and
using a single scaling of the ratio of stochastic to regular components, reveal
basic features in the behavior of generated signals also in terms of a critical
value for that ratio, thus enable the application of this technique for various
observational datasetsComment: 6 pages, 9 figures; Europhys.Letters; to match the published versio
A simple extended-cavity diode laser
Operating a laser diode in an extended cavity which provides frequency-selective feedback is a very effective method of reducing the laser's linewidth and improving its tunability. We have developed an extremely simple laser of this type, built from inexpensive commercial components with only a few minor modifications, A 780 nm laser built to this design has an output power of 80 mW, a Linewidth of 350 kHz, and it has been continuously locked to a Doppler-free rubidium transition for several days
On the accuracy of conservation of adiabatic invariants in slow-fast systems
Let the adiabatic invariant of action variable in slow-fast Hamiltonian
system with two degrees of freedom have two limiting values along the
trajectories as time tends to infinity. The difference of two limits is
exponentially small in analytic systems. An iso-energetic reduction and
canonical transformations are applied to transform the slow-fast systems to
form of systems depending on slowly varying parameters in a complexified phase
space. On the basis of this method an estimate for the accuracy of conservation
of adiabatic invariant is given for such systems.Comment: 27 pages, 14 figure
Pionic Color Transparency
We use a semi-classical approximation to investigate the effects of color
transparency on pion electroproduction reactions. The resulting reduced nuclear
interactions produce significant, but not dominating, differences with the
results of conventional distorted-wave, Glauber-type treatments at kinematics
accessible to Jefferson Laboratory. Nuclear effects that could mimic the
influence of color transparency are also discussed.Comment: 10 pages, 6 figure
Development of aircraft lavatory compartments with improved fire resistance characteristics, phase 1: Fire containment test of a wide body aircraft lavatory module
A test was conducted to evaluate the fire containment characteristics of a Boeing 747 lavatory module. Results showed that the fire was contained within the lavatory during the 30-minute test period with the door closed. The resistance of the lavatory wall and ceiling panels and general lavatory construction to burn-through under the test conditions was demonstrated
Global Theory of Quantum Boundary Conditions and Topology Change
We analyze the global theory of boundary conditions for a constrained quantum
system with classical configuration space a compact Riemannian manifold
with regular boundary . The space \CM of self-adjoint
extensions of the covariant Laplacian on is shown to have interesting
geometrical and topological properties which are related to the different
topological closures of . In this sense, the change of topology of is
connected with the non-trivial structure of \CM. The space \CM itself can
be identified with the unitary group \CU(L^2(\Gamma,\C^N)) of the Hilbert
space of boundary data L^2(\Gamma,\C^N). A particularly interesting family of
boundary conditions, identified as the set of unitary operators which are
singular under the Cayley transform, \CC_-\cap \CC_+ (the Cayley manifold),
turns out to play a relevant role in topology change phenomena. The singularity
of the Cayley transform implies that some energy levels, usually associated
with edge states, acquire an infinity energy when by an adiabatic change the
boundary condition reaches the Cayley submanifold \CC_-. In this sense
topological transitions require an infinite amount of quantum energy to occur,
although the description of the topological transition in the space \CM is
smooth. This fact has relevant implications in string theory for possible
scenarios with joint descriptions of open and closed strings. In the particular
case of elliptic self--adjoint boundary conditions, the space \CC_- can be
identified with a Lagrangian submanifold of the infinite dimensional
Grassmannian. The corresponding Cayley manifold \CC_- is dual of the Maslov
class of \CM.Comment: 29 pages, 2 figures, harvma
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