532 research outputs found
Approximated center-of-mass motion for systems of interacting particles with space- and velocity-dependent friction and anharmonic potential
We study the center-of-mass motion in systems of trapped interacting
particles with space- and velocity-dependent friction and anharmonic traps. Our
approach, based on a dynamical ansatz assuming a fixed density profile, allows
us to obtain information at once for a wide range of binary interactions and
interaction strengths, at linear and nonlinear levels. Our findings are first
tested on different simple models by comparison with direct numerical
simulations. Then, we apply the method to characterize the motion of the center
of mass of a magneto-optical trap and its dependence on the number of trapped
atoms. Our predictions are compared with experiments performed on a large Rb85
magneto-optical trap.Comment: 9 pages, 8 figure
Decay dynamics in the coupled-dipole model
Cooperative scattering in cold atoms has gained renewed interest, in
particular in the context of single-photon superradiance, with the recent
experimental observation of super-and subradiance in dilute atomic clouds.
Numerical simulations to support experimental signatures of cooperative
scattering are often limited by the number of dipoles which can be treated,
well below the number of atoms in the experiments. In this paper, we provide
systematic numerical studies aimed at matching the regime of dilute atomic
clouds. We use a scalar coupled-dipole model in the low excitation limit and an
exclusion volume to avoid density-related effects. Scaling laws for super-and
subradiance are obtained and the limits of numerical studies are pointed out.
We also illustrate the cooperative nature of light scattering by considering an
incident laser field, where half of the beam has a phase shift. The
enhanced subradiance obtained under such condition provides an additional
signature of the role of coherence in the detected signal
Subradiance in a Large Cloud of Cold Atoms
Since Dicke's seminal paper on coherence in spontaneous radiation by atomic
ensembles, superradiance has been extensively studied. Subradiance, on the
contrary, has remained elusive, mainly because subradiant states are weakly
coupled to the environment and are very sensitive to nonradiative decoherence
processes.Here we report the experimental observation of subradiance in an
extended and dilute cold-atom sample containing a large number of particles. We
use a far detuned laser to avoid multiple scattering and observe the temporal
decay after a sudden switch-off of the laser beam. After the fast decay of most
of the fluorescence, we detect a very slow decay, with time constants as long
as 100 times the natural lifetime of the excited state of individual atoms.
This subradiant time constant scales linearly with the cooperativity parameter,
corresponding to the on-resonance optical depth of the sample, and is
independent of the laser detuning, as expected from a coupled-dipole model
Cooperativity in light scattering by cold atoms
A cloud of cold N two-level atoms driven by a resonant laser beam shows
cooperative effects both in the scattered radiation field and in the radiation
pressure force acting on the cloud center-of-mass. The induced dipoles
synchronize and the scattered light presents superradiant and/or subradiant
features. We present a quantum description of the process in terms of a master
equation for the atomic density matrix in the scalar, Born-Markov
approximations, reduced to the single-excitation limit. From a perturbative
approach for weak incident field, we derive from the master equation the
effective Hamiltonian, valid in the linear regime. We discuss the validity of
the driven timed Dicke ansatz and of a partial wave expansion for different
optical thicknesses and we give analytical expressions for the scattered
intensity and the radiation pressure force on the center of mass. We also
derive an expression for collective suppression of the atomic excitation and
the scattered light by these correlated dipoles.Comment: 15 pages, 8 figure
Intensity fluctuations signature of 3D Anderson localization of light
Apart from the difficulty of producing highly scattering samples, a major
challenge in the observation of Anderson localization of 3D light is
identifying an unambiguous signature of the phase transition in experimentally
feasible situations. In this letter we establish a clear correspondence between
the collapse of the conductance, the increase in intensity fluctuations at the
localization transition and the scaling analysis results based on the Thouless
number, thus connecting the macroscopic and microscopic approaches of
localization. Furthermore, the transition thus inferred is fully compatible
both with the results based on the eigenvalue analysis of the microscopic
description and with the effective-medium Ioffe-Regel criterion
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