3,764 research outputs found
Optical bistability in sideband output modes induced by squeezed vacuum
We consider two-level atoms in a ring cavity interacting with a broadband
squeezed vacuum centered at frequency and an input monochromatic
driving field at frequency . We show that, besides the central mode
(at \o), many other {\em sideband modes} are produced at the output, with
frequencies shifted from by multiples of .
Here we analyze the optical bistability of the two nearest sideband modes, one
red-shifted and the other blue-shifted.Comment: Replaced with final published versio
Particle Creation by a Moving Boundary with Robin Boundary Condition
We consider a massless scalar field in 1+1 dimensions satisfying a Robin
boundary condition (BC) at a non-relativistic moving boundary. We derive a
Bogoliubov transformation between input and output bosonic field operators,
which allows us to calculate the spectral distribution of created particles.
The cases of Dirichlet and Neumann BC may be obtained from our result as
limiting cases. These two limits yield the same spectrum, which turns out to be
an upper bound for the spectra derived for Robin BC. We show that the particle
emission effect can be considerably reduced (with respect to the
Dirichlet/Neumann case) by selecting a particular value for the oscillation
frequency of the boundary position
Inertial forces in the Casimir effect with two moving plates
We combine linear response theory and dimensional regularization in order to
derive the dynamical Casimir force in the low frequency regime. We consider two
parallel plates moving along the normal direction in dimensional space. We
assume the free-space values for the mass of each plate to be known, and obtain
finite, separation-dependent mass corrections resulting from the combined
effect of the two plates. The global mass correction is proportional to the
static Casimir energy, in agreement with Einstein's law of equivalence between
mass and energy for stressed rigid bodies.Comment: 9 pages, 1 figure; title and abstract changed; to appear in Physical
Review
Energy density and pressure of long wavelength gravitational waves
Inflation leads us to expect a spectrum of gravitational waves (tensor
perturbations) extending to wavelengths much bigger than the present observable
horizon. Although these gravity waves are not directly observable, the energy
density that they contribute grows in importance during the radiation- and
dust-dominated ages of the universe. We show that the back reaction of tensor
perturbations during matter domination is limited from above, since
gravitational waves of wavelength have a share of the total energy
density during matter domination that is at most
equal to the share of the total energy density that they had when the mode
exited the Hubble radius during inflation. This work is to
be contrasted to that of Sahni, who analyzed the energy density of gravity
waves only insofar as their wavelengths are smaller than . Such a
cut-off in the spectral energy of gravity waves leads to the breakdown of
energy conservation, and we show that this anomaly is eliminated simply by
taking into account the energy density and pressure of long wavelength
gravitational waves as well as short wavelength ones.Comment: Updated one reference; 17 pages, no figure
Casimir forces between arbitrary compact objects: Scalar and electromagnetic field
We develop an exact method for computing the Casimir energy between arbitrary
compact objects, both with boundary conditions for a scalar field and
dielectrics or perfect conductors for the electromagnetic field. The energy is
obtained as an interaction between multipoles, generated by quantum source or
current fluctuations. The objects' shape and composition enter only through
their scattering matrices. The result is exact when all multipoles are
included, and converges rapidly. A low frequency expansion yields the energy as
a series in the ratio of the objects' size to their separation. As examples, we
obtain this series for two spheres with Robin boundary conditions for a scalar
field and dielectric spheres for the electromagnetic field. The full
interaction at all separations is obtained for spheres with Robin boundary
conditions and for perfectly conducting spheres.Comment: 24 pages, 3 figures, contribution to QFEXT07 proceeding
Disorder in quantum vacuum: Casimir-induced localization of matter waves
Disordered geometrical boundaries such as rough surfaces induce important
modifications to the mode spectrum of the electromagnetic quantum vacuum. In
analogy to Anderson localization of waves induced by a random potential, here
we show that the Casimir-Polder interaction between a cold atomic sample and a
rough surface also produces localization phenomena. These effects, that
represent a macroscopic manifestation of disorder in quantum vacuum, should be
observable with Bose-Einstein condensates expanding in proximity of rough
surfaces
On the Nature of the Cosmological Constant Problem
General relativity postulates the Minkowski space-time to be the standard
flat geometry against which we compare all curved space-times and the
gravitational ground state where particles, quantum fields and their vacuum
states are primarily conceived. On the other hand, experimental evidences show
that there exists a non-zero cosmological constant, which implies in a deSitter
space-time, not compatible with the assumed Minkowski structure. Such
inconsistency is shown to be a consequence of the lack of a application
independent curvature standard in Riemann's geometry, leading eventually to the
cosmological constant problem in general relativity.
We show how the curvature standard in Riemann's geometry can be fixed by
Nash's theorem on locally embedded Riemannian geometries, which imply in the
existence of extra dimensions. The resulting gravitational theory is more
general than general relativity, similar to brane-world gravity, but where the
propagation of the gravitational field along the extra dimensions is a
mathematical necessity, rather than being a a postulate. After a brief
introduction to Nash's theorem, we show that the vacuum energy density must
remain confined to four-dimensional space-times, but the cosmological constant
resulting from the contracted Bianchi identity is a gravitational contribution
which propagates in the extra dimensions. Therefore, the comparison between the
vacuum energy and the cosmological constant in general relativity ceases to be.
Instead, the geometrical fix provided by Nash's theorem suggests that the
vacuum energy density contributes to the perturbations of the gravitational
field.Comment: LaTex, 5 pages no figutres. Correction on author lis
Desempenho produtivo de caprinos mestiços da raça Anglo-Nubiana, no Acre.
O experimento foi conduzido na Fazenda Experimental do Centro de Pesquisa Agroflorestal do Acre - CPAF-Acre, localizada no município de Rio Branco. Foram utilizados inicialmente 20 matrizes e dois reprodutores mestiços da raça An-glo-nubiana, criados em regime semi-intensivo de exploração, tendo como base alimentar pastagens de colonião (Panicum maximum) e brizantão (Brachiaria brizan-tha). O sistema de acasalamento adotado foi o da monta natural a campo, onde o macho permanecia com as fêmeas durante todo o ano. Foram obtidos os índices de prolificidade (1,26%) natalidade (125,2 %); fertilidade ao parto (87,5%); intervalo entre partos (266,2 dias); proporção sexual (50%); mortalidade de 0-6 meses (14,63%); peso ao nascer para machos (2,81 i<g), para fêmeas (2,71 kg) e fêmeas com três partos em dois anos (7,9%). Ocorreram parições em quase todos os meses do ano (exceto setembro), com uma maior concentração no período de junho a agosto (413%). As cabras que tiveram parto duplo apresentaram um peso ao parto maior (P< 0,01), em relação àquelas com panos simples. Conclui-se que o desempenho reprodutivo do rebanho estudado foi bom, podendo, no entanto, atingir índices mais elevados, desde que se introduzam, no sistema de criação, algumas práticas de manejo reprodutivo e melhores condições nutriclonais.bitstream/item/161362/1/975.pd
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
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