279 research outputs found
Dynamic screening in solar and stellar nuclear reactions
In the hot, dense plasma of solar and stellar interiors, the Coulomb
interaction is screened by the surrounding plasma. Although the standard
Salpeter approximation for static screening is widely accepted and used in
stellar modeling, the question of dynamic screening has been revisited. In
particular, Shaviv and Shaviv apply the techniques of molecular dynamics to the
conditions in the solar core in order to numerically determine the dynamic
screening effect. By directly calculating the motion of ions and electrons due
to Coulomb interactions, they compute the effect of screening without the
mean-field assumption inherent in the Salpeter approximation. Here we reproduce
their numerical analysis of the screening energy in the plasma of the solar
core and conclude that the effects of dynamic screening are relevant and should
be included in the treatment of the plasma, especially in the computation of
stellar nuclear reaction rates.Comment: Astrophysics and Space Science, Special Issue Solar & Stellar
Modelling Corrected sign error. Now consistent with final published versio
Modification of radiation pressure due to cooperative scattering of light
Cooperative spontaneous emission of a single photon from a cloud of N atoms
modifies substantially the radiation pressure exerted by a far-detuned laser
beam exciting the atoms. On one hand, the force induced by photon absorption
depends on the collective decay rate of the excited atomic state. On the other
hand, directional spontaneous emission counteracts the recoil induced by the
absorption. We derive an analytical expression for the radiation pressure in
steady-state. For a smooth extended atomic distribution we show that the
radiation pressure depends on the atom number via cooperative scattering and
that, for certain atom numbers, it can be suppressed or enhanced.Comment: 8 pages, 2 Figure
Life after charge noise: recent results with transmon qubits
We review the main theoretical and experimental results for the transmon, a
superconducting charge qubit derived from the Cooper pair box. The increased
ratio of the Josephson to charging energy results in an exponential suppression
of the transmon's sensitivity to 1/f charge noise. This has been observed
experimentally and yields homogeneous broadening, negligible pure dephasing,
and long coherence times of up to 3 microseconds. Anharmonicity of the energy
spectrum is required for qubit operation, and has been proven to be sufficient
in transmon devices. Transmons have been implemented in a wide array of
experiments, demonstrating consistent and reproducible results in very good
agreement with theory.Comment: 6 pages, 4 figures. Review article, accepted for publication in
Quantum Inf. Pro
External Electromagnetic Fields of a Slowly Rotating Magnetized Star with Gravitomagnetic Charge
We study Maxwell equations in the external background spacetime of a slowly
rotating magnetized NUT star and find analytical solutions for the exterior
electric fields after separating the equations of electric field into angular
and radial parts in the lowest order approximation. The star is considered
isolated and in vacuum, with dipolar magnetic field aligned with the axis of
rotation. The contribution to the external electric field of star from the NUT
charge is considered in detail.Comment: 6 pages, 2 figures, accepted for publication in Astrophysics and
Space Scienc
Electromagnetic Casimir densities for a wedge with a coaxial cylindrical shell
Vacuum expectation values of the field square and the energy-momentum tensor
for the electromagnetic field are investigated for the geometry of a wedge with
a coaxal cylindrical boundary. All boundaries are assumed to be perfectly
conducting and both regions inside and outside the shell are considered. By
using the generalized Abel-Plana formula, the vacuum expectation values are
presented in the form of the sum of two terms. The first one corresponds to the
geometry of the wedge without the cylindrical shell and the second term is
induced by the presence of the shell. The vacuum energy density induced by the
shell is negative for the interior region and is positive for the exterior
region. The asymptotic behavior of the vacuum expectation values are
investigated in various limiting cases. It is shown that the vacuum forces
acting on the wedge sides due to the presence of the cylindrical boundary are
always attractive.Comment: 21 pages, 7 figure
On the energy-momentum tensor for a scalar field on manifolds with boundaries
We argue that already at classical level the energy-momentum tensor for a
scalar field on manifolds with boundaries in addition to the bulk part contains
a contribution located on the boundary. Using the standard variational
procedure for the action with the boundary term, the expression for the surface
energy-momentum tensor is derived for arbitrary bulk and boundary geometries.
Integral conservation laws are investigated. The corresponding conserved
charges are constructed and their relation to the proper densities is
discussed. Further we study the vacuum expectation value of the energy-momentum
tensor in the corresponding quantum field theory. It is shown that the surface
term in the energy-momentum tensor is essential to obtain the equality between
the vacuum energy, evaluated as the sum of the zero-point energies for each
normal mode of frequency, and the energy derived by the integration of the
corresponding vacuum energy density. As an application, by using the zeta
function technique, we evaluate the surface energy for a quantum scalar field
confined inside a spherical shell.Comment: 25 pages, 2 figures, section and appendix on the surface energy for a
spherical shell are added, references added, accepted for publication in
Phys. Rev.
Stress-energy tensor for a quantised bulk scalar field in the Randall-Sundrum brane model
We calculate the vacuum expectation value of the stress-energy tensor for a
quantised bulk scalar field in the Randall-Sundrum model, and discuss the
consequences of its local behaviour for the self-consistency of the model. We
find that, in general, the stress-energy tensor diverges in the vicinity of the
branes. Our main conclusion is that the stress-energy tensor is sufficiently
complicated that it has implications for the effective potential, or radion
stabilisation, methods that have so far been used.Comment: 16 pages, 3 figures. Minor changes made and references added. To
appear in Phys. Rev.
Wightman function and scalar Casimir densities for a wedge with two cylindrical boundaries
Wightman function, the vacuum expectation values of the field square and the
energy-momentum tensor are investigated for a massive scalar field with general
curvature coupling parameter inside a wedge with two coaxial cylindrical
boundaries. It is assumed that the field obeys Dirichlet boundary condition on
bounding surfaces. The application of a variant of the generalized Abel-Plana
formula enables to extract from the expectation values the contribution
corresponding to the geometry of a wedge with a single shell and to present the
interference part in terms of exponentially convergent integrals. The local
properties of the vacuum are investigated in various asymptotic regions of the
parameters. The vacuum forces acting on the boundaries are presented as the sum
of self-action and interaction terms. It is shown that the interaction forces
between the separate parts of the boundary are always attractive. The
generalization to the case of a scalar field with Neumann boundary condition is
discussed.Comment: 19 pages, 3 figure
Casimir Effect on the Worldline
We develop a method to compute the Casimir effect for arbitrary geometries.
The method is based on the string-inspired worldline approach to quantum field
theory and its numerical realization with Monte-Carlo techniques. Concentrating
on Casimir forces between rigid bodies induced by a fluctuating scalar field,
we test our method with the parallel-plate configuration. For the
experimentally relevant sphere-plate configuration, we study curvature effects
quantitatively and perform a comparison with the ``proximity force
approximation'', which is the standard approximation technique. Sizable
curvature effects are found for a distance-to-curvature-radius ratio of a/R >~
0.02. Our method is embedded in renormalizable quantum field theory with a
controlled treatment of the UV divergencies. As a technical by-product, we
develop various efficient algorithms for generating closed-loop ensembles with
Gaussian distribution.Comment: 27 pages, 10 figures, Sect. 2.1 more self-contained, improved data
for Fig. 6, minor corrections, new Refs, version to be published in JHE
Thermal Casimir effect in ideal metal rectangular boxes
The thermal Casimir effect in ideal metal rectangular boxes is considered
using the method of zeta functional regularization. The renormalization
procedure is suggested which provides the finite expression for the Casimir
free energy in any restricted quantization volume. This expression satisfies
the classical limit at high temperature and leads to zero thermal Casimir force
for systems with infinite characteristic dimensions. In the case of two
parallel ideal metal planes the results, as derived previously using thermal
quantum field theory in Matsubara formulation and other methods, are reproduced
starting from the obtained expression. It is shown that for rectangular boxes
the temperature-dependent contribution to the electromagnetic Casimir force can
be both positive and negative depending on side lengths. The numerical
computations of the scalar and electromagnetic Casimir free energy and force
are performed for cubesComment: 10 pages, 4 figures, to appear in Europ. Phys. J.
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