1,533 research outputs found
Cooperative effects in one-dimensional random atomic gases: Absence of single atom limit
We study superradiance in a one-dimensional geometry, where N>>1 atoms are
randomly distributed along a line. We present an analytic calculation of the
photon escape rates based on the diagonalization of the N x N coupling matrix
Uij = cos xij, where xij is the dimensionless random distance between any two
atoms. We show that unlike a three-dimensional geometry, for a one- dimensional
atomic gas the single-atom limit is never reached and the photon is always
localized within the atomic ensemble. This localization originates from
long-range cooperative effects and not from disorder as expected on the basis
of the theory of Anderson localization.Comment: 5 pages, 3 figure
Effect of superradiance on transport of diffusing photons in cold atomic gases
We show that in atomic gases cooperative effects like superradiance and
subradiance lead to a potential between two atoms that decays like . In
the case of superradiance, this potential is attractive for close enough atoms
and can be interpreted as a coherent mesoscopic effect. The contribution of
superradiant pairs to multiple scattering properties of a dilute gas, such as
photon elastic mean free path and group velocity, is significantly different
from that of independent atoms. We discuss the conditions under which these
effects may be observed and compare our results to recent experiments on photon
transport in cold atomic gases.Comment: 4 pages and 1 figur
Superradiance and multiple scattering of photons in atomic gases
We study the influence of cooperative effects such as superradiance and
subradiance, on the scattering properties of dilute atomic gases. We show that
cooperative effects lead to an effective potential between pairs of atoms that
decays like . In the case of superradiance, this potential is attractive
for close enough atoms and can be interpreted as a coherent mesoscopic effect.
We consider a model of multiple scattering of a photon among superradiant pairs
and calculate the elastic mean free path and the group velocity. We study first
the case of a scalar wave which allows to obtain and to understand basic
features of cooperative effects and multiple scattering. We then turn to the
general problem of a vector wave. In both cases, we obtain qualitatively
similar results and derive, for the case of a scalar wave, analytic expressions
of the elastic mean free path and of the group velocity for an arbitrary (near
resonance) detuning.Comment: 12 pages, 7 figure
Intensity correlations and mesoscopic fluctuations of diffusing photons in cold atoms
We study the angular correlation function of speckle patterns that result
from multiple scattering of photons by cold atomic clouds. We show that this
correlation function becomes larger than the value given by Rayleigh law for
classical scatterers. These large intensity fluctuations constitute a new
mesoscopic interference effect specific to atom-photon interactions, that could
not be observed in other systems such as weakly disordered metals. We provide a
complete description of this behavior and expressions that allow for a
quantitative comparison with experiments.Comment: 4 pages, 2 figure
E pluribus unum: Using group model building with many interdependent organizations to create integrated health care networks.
Cooperative effects and disorder: A scaling analysis of the spectrum of the effective atomic Hamiltonian
We study numerically the spectrum of the non-Hermitian effective Hamiltonian
that describes the dipolar interaction of a gas of atoms with the
radiation field. We analyze the interplay between cooperative effects and
disorder for both scalar and vectorial radiation fields. We show that for dense
gases, the resonance width distribution follows, both in the scalar and
vectorial cases, a power law that originates
from cooperative effects between more than two atoms. This power law is
different from the behavior, which has been
considered as a signature of Anderson localization of light in random systems.
We show that in dilute clouds, the center of the energy distribution is
described by Wigner's semicircle law in the scalar and vectorial cases. For
dense gases, this law is replaced in the vectorial case by the Laplace
distribution. Finally, we show that in the scalar case the degree of resonance
overlap increases as a power law of the system size for dilute gases, but
decays exponentially with the system size for dense clouds.Comment: 11 pages, 12 figure
Universality of vertex corrections to the electrical conductivity in models with elastically scattered electrons
We study quantum coherence of elastically scattered lattice fermions. We
calculate vertex corrections to the electrical conductivity of electrons
scattered either on thermally equilibrated or statically distributed random
impurities. We demonstrate that the sign of the vertex corrections to the Drude
conductivity is in both cases negative. Quantum coherence due to elastic
back-scatterings always leads to diminution of diffusion.Comment: ReVTEX, 9 pages, 8 EPS figure
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