137 research outputs found
Van der Waals interaction and spontaneous decay of an excited atom in a superlens-type geometry
Within the framework of macroscopic quantum electrodynamics, the resonant van
der Waals potential experienced by an excited two-level atom near a planar
magneto-electric two-layer system consisting of a slab of left-handed material
and a perfect mirror is studied. It is shown that disregarding of material
absorption leads to unphysical results, with divergent values for the potential
away from the surface. Under appropriate conditions, the setup is found to
feature a barrier near the surface which can be employed to levitate particles
or used as a trapping or cooling mechanism. Finally, the problem of spontaneous
decay [J. K\"{a}stel and M. Fleischhauer, Phys. Rev. A \textbf{68}, 011804(R)
(2005)] is revisited. Disregarding of absorption is shown to drastically
falsify the dependence on the atomic position of the decay rate.Comment: 10 Pages, 6 figure
Atomic entanglement near a realistic microsphere
We study a scheme for entangling two-level atoms located close to the surface
of a dielectric microsphere. The effect is based on medium-assisted spontaneous
decay, rigorously taking into account dispersive and absorptive properties of
the microsphere. We show that even in the weak-coupling regime, where the
Markov approximation applies, entanglement up to 0.35 ebits between two atoms
can be created. However, larger entanglement and violation of Bell's inequality
can only be achieved in the strong-coupling regime.Comment: 16 pages, 4 figures, Late
Resonant Energy Exchange between Atoms in Dispersing and Absorbing Surroundings
Within the framework of quantization of the macroscopic electromagnetic
field, a master equation describing both the resonant dipole-dipole interaction
(RDDI) and the resonant atom-field interaction (RAFI) in the presence of
dispersing and absorbing macroscopic bodies is derived, with the relevant
couplings being expressed in terms of the surroundings-assisted Green tensor.
It is shown that under certain conditions the RDDI can be regarded as being
governed by an effective Hamiltonian. The theory, which applies to both weak
and strong atom-field coupling, is used to study the resonant energy exchange
between two (two-level) atoms sharing initially a single excitation. In
particular, it is shown that in the regime of weak atom-field coupling there is
a time window, where the energy transfer follows a transfer-rate law of the
type obtained by ordinary second-order perturbation theory. Finally, the
spectrum of the light emitted during the energy transfer is studied and the
line splittings are discussed.Comment: 9 pages, 5 figs, Proceedings of ICQO'2002, Raubichi, to appear in
Optics and Spectroscop
Joint Load Balancing and Interference Management for Small-Cell Heterogeneous Networks with Limited Backhaul Capacity
Atomic multipole relaxation rates near surfaces
The spontaneous relaxation rates for an atom in free space and close to an
absorbing surface are calculated to various orders of the electromagnetic
multipole expansion. The spontaneous decay rates for dipole, quadrupole and
octupole transitions are calculated in terms of their respective primitive
electric multipole moments and the magnetic relaxation rate is calculated for
the dipole and quadrupole transitions in terms of their respective primitive
magnetic multipole moments. The theory of electromagnetic field quantization in
magnetoelectric materials is used to derive general expressions for the decay
rates in terms of the dyadic Green function. We focus on the decay rates in
free space and near an infinite half space. For the decay of atoms near to an
absorbing dielectric surface we find a hierarchy of scaling laws depending on
the atom-surface distance z.Comment: Updated to journal version. 16 page
Decreased Streptococcus pneumoniae susceptibility to oral antibiotics among children in rural Vietnam: a community study
Spontaneous decay in the presence of dispersing and absorbing bodies: general theory and application to a spherical cavity
A formalism for studying spontaneous decay of an excited two-level atom in
the presence of dispersing and absorbing dielectric bodies is developed. An
integral equation, which is suitable for numerical solution, is derived for the
atomic upper-state-probability amplitude. The emission pattern and the power
spectrum of the emitted light are expressed in terms of the Green tensor of the
dielectric-matter formation including absorption and dispersion. The theory is
applied to the spontaneous decay of an excited atom at the center of a
three-layered spherical cavity, with the cavity wall being modeled by a
band-gap dielectric of Lorentz type. Both weak coupling and strong coupling are
studied, the latter with special emphasis on the cases where the atomic
transition is (i) in the normal-dispersion zone near the medium resonance and
(ii) in the anomalous-dispersion zone associated with the band gap. In a
single-resonance approximation, conditions of the appearance of Rabi
oscillations and closed solutions to the evolution of the atomic state
population are derived, which are in good agreement with the exact numerical
results.Comment: 12 pages, 6 figures, typos fixed, 1 figure adde
Casimir-Polder forces: A non-perturbative approach
Within the frame of macroscopic QED in linear, causal media, we study the
radiation force of Casimir-Polder type acting on an atom which is positioned
near dispersing and absorbing magnetodielectric bodies and initially prepared
in an arbitrary electronic state. It is shown that minimal and multipolar
coupling lead to essentially the same lowest-order perturbative result for the
force acting on an atom in an energy eigenstate. To go beyond perturbation
theory, the calculations are based on the exact center-of-mass equation of
motion. For a nondriven atom in the weak-coupling regime, the force as a
function of time is a superposition of force components that are related to the
electronic density-matrix elements at a chosen time. Even the force component
associated with the ground state is not derivable from a potential in the
ususal way, because of the position dependence of the atomic polarizability.
Further, when the atom is initially prepared in a coherent superposition of
energy eigenstates, then temporally oscillating force components are observed,
which are due to the interaction of the atom with both electric and magnetic
fields.Comment: 23 pages, 3 figures, additional misprints correcte
Body-assisted van der Waals interaction between two atoms
Using fourth-order perturbation theory, a general formula for the van der
Waals potential of two neutral, unpolarized, ground-state atoms in the presence
of an arbitrary arrangement of dispersing and absorbing magnetodielectric
bodies is derived. The theory is applied to two atoms in bulk material and in
front of a planar multilayer system, with special emphasis on the cases of a
perfectly reflecting plate and a semi-infinite half space. It is demonstrated
that the enhancement and reduction of the two-atom interaction due to the
presence of a perfectly reflecting plate can be understood, at least in the
nonretarded limit, by using the method of image charges. For the semi-infinite
half space, both analytical and numerical results are presented.Comment: 17 pages, 9 figure
Disentangling the circumnuclear environs of Centaurus A: II. On the nature of the broad absorption line
We report on atomic gas (HI) and molecular gas (as traced by CO(2-1))
redshifted absorption features toward the nuclear regions of the closest
powerful radio galaxy, Centaurus A (NGC 5128). Our HI observations using the
Very Long Baseline Array allow us to discern with unprecedented sub-parsec
resolution HI absorption profiles toward different positions along the 21 cm
continuum jet emission in the inner 0."3 (or 5.4 pc). In addition, our CO(2-1)
data obtained with the Submillimeter Array probe the bulk of the absorbing
molecular gas with little contamination by emission, not possible with previous
CO single-dish observations. We shed light with these data on the physical
properties of the gas in the line of sight, emphasizing the still open debate
about the nature of the gas that produces the broad absorption line (~55 km/s).
First, the broad H I line is more prominent toward the central and brightest 21
cm continuum component than toward a region along the jet at a distance ~ 20
mas (or 0.4 pc) further from it. This suggests that the broad absorption line
arises from gas located close to the nucleus, rather than from diffuse and more
distant gas. Second, the different velocity components detected in the CO(2-1)
absorption spectrum match well other molecular lines, such as those of
HCO+(1-0), except the broad absorption line that is detected in HCO+(1-0) (and
most likely related to that of the H I). Dissociation of molecular hydrogen due
to the AGN seems to be efficient at distances <= 10 pc, which might contribute
to the depth of the broad H I and molecular lines.Comment: 17 pages, 9 figures, accepted for publication in Ap
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