391 research outputs found
An experimental study to discriminate between the validity of diffraction theories for off-Bragg replay
We show that experiments clearly verify the assumptions made by the
first-order two-wave coupling theory for one dimensional lossless unslanted
planar volume holographic gratings using the beta-value method rather than
Kogelnik's K-vector closure method. Apart from the fact that the diffraction
process is elastic, a much more striking difference between the theories
becomes apparent particularly in the direction of the diffracted beam in
off-Bragg replay. We therefore monitored the direction of the diffracted beam
as a function of the off-Bragg replay angle in two distinct cases: [a] the
diffracted beam lies in the plane of incidence and [b] the sample surface
normal, the grating vector and the incoming beam do not form a plane which
calls for the vectorial theory and results in conical scattering.Comment: Corrected Eqs. (3) & (6); 14 pages, 8 figure
New Aspects of Geometric Phases in Experiments with polarized Neutrons
Geometric phase phenomena in single neutrons have been observed in
polarimeter and interferometer experiments. Interacting with static and time
dependent magnetic fields, the state vectors acquire a geometric phase tied to
the evolution within spin subspace. In a polarimeter experiment the
non-additivity of quantum phases for mixed spin input states is observed. In a
Si perfect-crystal interferometer experiment appearance of geometric phases,
induced by interaction with an oscillating magnetic field, is verified. The
total system is characterized by an entangled state, consisting of neutron and
radiation fields, governed by a Jaynes-Cummings Hamiltonian. In addition, the
influence of the geometric phase on a Bell measurement, expressed by the
Clauser-Horne-Shimony-Holt (CHSH) inequality, is studied. It is demonstrated
that the effect of geometric phase can be balanced by an appropriate change of
Bell angles.Comment: 17 pages, 9 figure
Observation of nonadditive mixed state phases with polarized neutrons
In a neutron polarimetry experiment the mixed state relative phases between
spin eigenstates are determined from the maxima and minima of measured
intensity oscillations. We consider evolutions leading to purely geometric,
purely dynamical and combined phases. It is experimentally demonstrated that
the sum of the individually determined geometric and dynamical phases is not
equal to the associated total phase which is obtained from a single
measurement, unless the system is in a pure state.Comment: RevTex, 4 pages, 4 figures, accepted by PR
Mirrors for slow neutrons from holographic nanoparticle-polymer free-standing film-gratings
We report on successful tests of holographically arranged grating-structures
in nanoparticle-polymer composites in the form of 100 microns thin
free-standing films, i.e. without sample containers or covers that could cause
unwanted absorption/incoherent scattering of very-cold neutrons. Despite their
large diameter of 2 cm, the flexible materials are of high optical quality and
yield mirror-like reflectivity of about 90% for neutrons of 4.1 nm wavelength
Measurements of rainfall rate, drop size distribution, and variability at middle and higher latitudes: application to the combined DPR-GMI algorithm
The Global Precipitation Measurement mission is a major U.S.–Japan joint mission to understand the physics of the Earth’s global precipitation as a key component of its weather, climate, and hydrological systems. The core satellite carries a dual-precipitation radar and an advanced microwave imager which provide measurements to retrieve the drop size distribution (DSD) and rain rates using a Combined Radar-Radiometer Algorithm (CORRA). Our objective is to validate key assumptions and parameterizations in CORRA and enable improved estimation of precipitation products, especially in the middle-to-higher latitudes in both hemispheres. The DSD parameters and statistical relationships between DSD parameters and radar measurements are a central part of the rainfall retrieval algorithm, which is complicated by regimes where DSD measurements are abysmally sparse (over the open ocean). In view of this, we have assembled optical disdrometer datasets gathered by research vessels, ground stations, and aircrafts to simulate radar observables and validate the scattering lookup tables used in CORRA. The joint use of all DSD datasets spans a large range of drop concentrations and characteristic drop diameters. The scaling normalization of DSDs defines an intercept parameter NW, which normalizes the concentrations, and a scaling diameter Dm, which compresses or stretches the diameter coordinate axis. A major finding of this study is that a single relationship between NW and Dm, on average, unifies all datasets included, from stratocumulus to heavier rainfall regimes. A comparison with the NW–Dm relation used as a constraint in versions 6 and 7 of CORRA highlights the scope for improvement of rainfall retrievals for small drops (Dm lt; 1 mm) and large drops (Dm gt; 2 mm). The normalized specific attenuation–reflectivity relationships used in the combined algorithm are also found to match well the equivalent relationships derived using DSDs from the three datasets, suggesting that the currently assumed lookup tables are not a major source of uncertainty in the combined algorithm rainfall estimates
Experimental demonstration of the stability of Berry's phase for a spin-1/2 particle
The geometric phase has been proposed as a candidate for noise resilient
coherent manipulation of fragile quantum systems. Since it is determined only
by the path of the quantum state, the presence of noise fluctuations affects
the geometric phase in a different way than the dynamical phase. We have
experimentally tested the robustness of Berry's geometric phase for spin-1/2
particles in a cyclically varying magnetic field. Using trapped polarized
ultra-cold neutrons it is demonstrated that the geometric phase contributions
to dephasing due to adiabatic field fluctuations vanish for long evolution
times.Comment: 4 pages, 4 figure
Primary prevention of overweight and obesity in adolescents: An overview of systematic reviews
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