3,770 research outputs found
A comparison of glacier melt on debris-covered glaciers in the northern and southern Caucasus
The glacier coverage in the Caucasus Mountains underwent considerable changes during the last decades. In some regions, the observed reduction in glacier area is comparable to those in the European Alps and the extent of supra-glacial debris increased on many glaciers. Only a few glaciers in the Caucasus are monitored on a regular basis, while for most areas no continuous field measurements are available. In this study, regional differences of the conditions for glacier melt with a special focus on debris covered glacier tongues in the well-studied Adyl-su basin on the northern slope of the Caucasus Mountains (Russia) is compared with the Zopkhito basin which has similar characteristics but is located on the southern slope in Georgia. The paper focuses on the effect of supra-glacial debris cover on glacier summer melt. There are systematic differences in the distribution and increase of the debris cover on the glaciers of the two basins. In the Adyl-su basin an extensive debris cover on the glacier tongues is common, however, only those glacier tongues that are positioned at the lowest elevations in the Zopkhito basin show a considerable extent of supra-glacial debris. The observed increase in debris cover is considerably stronger in the north. Field experiments show that thermal resistance of the debris cover in both basins is somewhat higher than in other glaciated regions of the world, but there is also a significant difference between the two regions. A simple ablation model accounting for the effect of debris cover on ice melt shows that melt rates are considerably higher in the northern basin despite a wider debris distribution. This difference between the two regions can be attributed to different meteorological conditions which are characterised by more frequent cloud cover and precipitation in the south. Furthermore ablation is strongly influenced by the occurrence of supra-glacial debris cover in both basins, reducing the total amount of melt on the studied glaciers by about 25 %. This effect mitigates glacier retreat in the lower sectors of the ablation zones considerably. The sensitivity to moderate changes in the debris cover, however, is rather small which implies only gradual changes of the melt regime due to debris cover dynamics during the near future
Casimir torque between corrugated metallic plates
We consider two parallel corrugated plates and show that a Casimir torque
arises when the corrugation directions are not aligned. We follow the
scattering approach and calculate the Casimir energy up to second order in the
corrugation amplitudes, taking into account nonspecular reflections,
polarization mixing and the finite conductivity of the metals. We compare our
results with the proximity force approximation, which overestimates the torque
by a factor 2 when taking the conditions that optimize the effect. We argue
that the Casimir torque could be measured for separation distances as large as
1 Comment: 7 pages, 3 figures, contribution to QFEXT07 proceeding
Transverse-mode coupling in a Kerr medium
We analyze nonlinear transverse mode coupling in a Kerr medium placed in an
optical cavity and its influence on bistability and different kinds of quantum
noise reduction. Even for an input beam that is perfectly matched to a cavity
mode, the nonlinear coupling produces an excess noise in the fluctuations of
the output beam. Intensity squeezing seems to be particularly robust with
respect to mode coupling, while quadrature squeezing is more sensitive.
However, it is possible to find a mode the quadrature squeezing of which is not
affected by the coupling.Comment: 11 pages, 6 figures, LaTe
Lateral Casimir-Polder force with corrugated surfaces
We derive the lateral Casimir-Polder force on a ground state atom on top of a
corrugated surface, up to first order in the corrugation amplitude. Our
calculation is based on the scattering approach, which takes into account
nonspecular reflections and polarization mixing for electromagnetic quantum
fluctuations impinging on real materials. We compare our first order exact
result with two commonly used approximation methods. We show that the proximity
force approximation (large corrugation wavelengths) overestimates the lateral
force, while the pairwise summation approach underestimates it due to the
non-additivity of dispersion forces. We argue that a frequency shift
measurement for the dipolar lateral oscillations of cold atoms could provide a
striking demonstration of nontrivial geometrical effects on the quantum vacuum.Comment: 12 pages, 6 figures, contribution to QFEXT07 proceeding
Long range gravity tests and the Pioneer anomaly
Experimental tests of gravity performed in the solar system show a good
agreement with general relativity. The latter is however challenged by the
Pioneer anomaly which might be pointing at some modification of gravity law at
ranges of the order of the size of the solar system. As this question could be
related to the puzzles of ``dark matter'' or ``dark energy'', it is important
to test it with care. There exist metric extensions of general relativity which
preserve the well verified equivalence principle while possibly changing the
metric solution in the solar system. Such extensions have the capability to
preserve compatibility with existing gravity tests while opening free space for
the Pioneer anomaly. They constitute arguments for new mission designs and new
space technologies as well as for having a new look at data of already
performed experiments.Comment: 8 page
Generation of three-dimensional prototype models based on cone beam computed tomography
Purpose: The purpose of this study was to generate three-dimensional models based on digital volumetric data that can be used in basic and advanced education. Methods: Four sets of digital volumetric data were established by cone beam computed tomography (CBCT) (Accuitomo, J. Morita, Kyoto, Japan). Datasets were exported as Dicom formats and imported into Mimics and Magic software programs to separate the different tissues such as nerve, tooth and bone. These data were transferred to a Polyjet 3D Printing machine (Eden 330, Object, Israel) to generate the models. Results: Three-dimensional prototype models of certain limited anatomical structures as acquired volumetrically were fabricated. Conclusions: Generating three-dimensional models based on CBCT datasets is possible. Automated routine fabrication of these models, with the given infrastructure, is too time-consuming and therefore too expensiv
Quantum vacuum properties of the intersubband cavity polariton field
We present a quantum description of a planar microcavity photon mode strongly
coupled to a semiconductor intersubband transition in presence of a
two-dimensional electron gas. We show that, in this kind of system, the vacuum
Rabi frequency can be a significant fraction of the intersubband
transition frequency . This regime of ultra-strong light-matter
coupling is enhanced for long wavelength transitions, because for a given
doping density, effective mass and number of quantum wells, the ratio
increases as the square root of the intersubband
emission wavelength. We characterize the quantum properties of the ground state
(a two-mode squeezed vacuum), which can be tuned {\it in-situ} by changing the
value of , e.g., through an electrostatic gate. We finally point out
how the tunability of the polariton quantum vacuum can be exploited to generate
correlated photon pairs out of the vacuum via quantum electrodynamics phenomena
reminiscent of the dynamic Casimir effect.Comment: Final version accepted in PR
A spectroscopy approach to measure the gravitational mass of antihydrogen
We study a method to induce resonant transitions between antihydrogen
() quantum states above a material surface in the gravitational field
of the Earth. The method consists of applying a gradient of magnetic field,
which is temporally oscillating with the frequency equal to a frequency of
transition between gravitational states of antihydrogen. A corresponding
resonant change in the spatial density of antihydrogen atoms could be measured
as a function of the frequency of applied field. We estimate an accuracy of
measuring antihydrogen gravitational states spacing and show how a value of the
gravitational mass of the atom could be deduced from such a
measurement. We also demonstrate that a method of induced transitions could be
combined with a free-fall-time measurement in order to further improve the
precision
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