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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Nonlinear dynamical effects with a cold two-level atom in an optical lattice

Victor Yu. Argonov, Sergey V. Prants

Published Jan 24, 2006• 0 citations
Publisher

Abstract

We treat semiclassical nonlinear dynamics of a two-level atom in the strong standing-wave field created in a high-quality cavity and in a free space with the help of two counterpropagating laser beams. First of all, we consider the dynamics of the strongly coupled atom-field system in an ideal cavity without any losses and discuss briefly nonlinear dynamical effects of random walking of an atom in a fully deterministic standing wave and Hamiltonian dynamical fractals. Then we study how these effects change in a free space when we take into account relaxation and external pumping of the standing wave. We discuss briefly new dissipative effects of synchronization between electronic and mechanical degrees of freedom of atoms, limit cycles of different periods and their bifurcations, and strange atomic attractors. Finally, we treat spontaneous relaxation as a random events in the equations of motion and study what happens with atomic fractality and limit cycles. It is shown that a self-similar structure of the scattering function (at least, on a few first levels) is conserved in the presence of spontaneous jumps. As to limit cycles, we could not resolve limit cycles of different period, but the fact of synchronization of internal and external atomic degrees of freedom is strictly established in the system with spontaneous jumps.

DOI
10.1117/12.675008

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We treat semiclassical nonlinear dynamics of a two-level atom in the strong standing-wave field created in a high-quality cavity and in a free space with the help of two counterpropagating laser beams. First of all, we consider the dynamics of the strongly coupled atom-field system in an ideal cavity without any losses and discuss briefly nonlinear dynamical effects of random walking of an atom in a fully deterministic standing wave and Hamiltonian dynamical fractals. Then we study how these effects change in a free space when we take into account relaxation and external pumping of the standing wave. We discuss briefly new dissipative effects of synchronization between electronic and mechanical degrees of freedom of atoms, limit cycles of different periods and their bifurcations, and strange atomic attractors. Finally, we treat spontaneous relaxation as a random events in the equations of motion and study what happens with atomic fractality and limit cycles. It is shown that a self-similar structure of the scattering function (at least, on a few first levels) is conserved in the presence of spontaneous jumps. As to limit cycles, we could not resolve limit cycles of different period, but the fact of synchronization of internal and external atomic degrees of freedom is strictly established in the system with spontaneous jumps.
We study the coupled translational and internal dynamics of a two-level atom in a spatially periodic optical potential created by a single-mode field in a high-finesse Fabry-Perot cavity. The strongly-coupled atom-cavity system with atomic center-of-mass motion included in modeled by semiclassical nonlinear Heisenberg equations of motion with two degrees of freedom. At exact resonance between the internal atomic transition and the cavity eigen frequency, the internal and translational degrees of freedom are separated from each other resulting in periodically modulated Rabi oscillations and regular translational motion. Near the atom-field resonance, the coupled dynamics is found to be irregular and even chaotic in the sense of external sensitivity to initial conditions. It is manifested as a chaotic motion of an atom in an absolutely regular spatially periodic potential in the absence of any random fluctuations. This effect may be explained as a result of the interaction of nonlinear resonances in the atom-field nonlinear pendulum whose low- frequency translational motion is modulated by the high- frequency Rabi oscillations. The irregular motion is studied in detail in numerical experiments using the arsenal of methods of the dynamical systems theory. The results obtained are compared with recent experiments with ultra cold single atoms interacting with single photons in the strong-coupling regime. We estimate the ranges of the system's control parameters, the detuning, the recoil frequency, and the mean number of intracavity photons, in which the interaction of nonlinear resonances may lead, under realistic conditions, to local instability of the center-of-mass motion and some manifestations of dynamical chaos, the effect which is of interest in studying the quantum-classical correspondence and quantum chaos in atomic optics.
We consider dissipative dynamics of atoms in a strong standing laser wave and find a nonlinear dynamical effect of synchronization between center-of-mass motion and internal Rabi oscillations. The synchronization manifests itself in the phase space as limit cycles, which may have different periods and riddled basins of attraction. The effect can be detected in fluorescence spectra of atoms as equidistant sideband frequencies with the space between adjacent peaks to be inversely proportional to the value of the period of the respective limit cycle. With increasing intensity of the laser field, we numerically observe cascades of bifurcations that eventually end up in settling a strange chaotic attractor. A broadband noise is shown to destroy a fine structure of the bifurcation scenario, but prominent features of period-1 and period-3 limit cycles survive under a weak noise. The character of the atomic motion is analyzed with the help of the friction force whose zeroes are attractor or repellor points in the velocity space. We find ranges of the laser parameters where the atomic motion resembles a random but deterministic walking of atoms erratically jumping between different wells of the optical potential. Such a random walking is shown to be fractal.
Abstract Coherent dynamics of cold atoms in a 2D optical lattice with interfering laser beams is studied with account for internal and external degrees of freedom of an atom. A system of differential equations for coupled degrees of freedom obtained in the semiclassical approximation has regular and chaotic solutions depending on the atomic-field detuning from resonance. The Hamilton chaos is manifested in the form of chaotic Rabi oscillations and random walks of cold atoms in the lattice for relatively small resonance detunings. It is shown that the deterministic chaos appears as a result of jumps in the value of the electric dipole moment of an atom approaching the nodes of a 2D standing wave. This in turn causes a pseudorandom behavior of momenta of atoms and, as a consequence, their random walks in the absolutely rigid 2D optical lattice without any external modulation of its parameters. It is shown in numerical experiments with 106 atoms that their distributions over the lattice for different resonance detunings differ significantly. This fact can be used for detecting the effect of random walks of cold atoms in a real experiment by the absorption image method.
We report a clear evidence of atomic fractals in the nonlinear motion of a two-level atom in a standing-wave microcavity. Fractal-like structures, typical for chaotic scattering, are numerically found in the dependencies of outgoing positions and momenta of scattered atoms on their ingoing values and in the dependence of exit times of cold atoms on their initial momenta in the generic semiclassical models of cavity QED (1) with atoms in a far-detuned amplitude (phase)-modulated standing wave and (2) with coupled atomic external and internal degrees of freedom. Tiny interplay between all the degrees of freedom in the second model is responsible for trapping atoms even in a very short microcavity. It may lead simultaneously, at least, to two kinds of atomic fractals, a countable fractal (a set of initial momenta generating separatrix-like atomic trajectories) and a seemingly uncountable fractal with a set of momenta generating infinite walkings of atoms inside the cavity.
The nonlinear dynamical effects of synchronization and bifurcations of Rabi oscillations and oscillations of the center of mass of an atom moving in the field of a standing light wave are theoretically and numerically examined. After completion of a transient process, synchronized oscillations, stable with respect to small noise, are established both upon ballistic motion of the atom and upon its oscillations in the optical potential well. The bifurcations of limit cycle generation with different periods (tripling of the period is most pronounced) and the passage to a chaotic strange attractor through an intermittency are obtained.
The motion of a two-level atom with an internal degree of freedom that interacts with the field of a single-mode standing light wave in a high- Q cavity is treated as a scattering problem. At detunings between atomic and field resonances δ, atomic recoil frequencies α, initial atomic momenta ρ 0 , and numbers of excitations N at which the semiclassical equations have chaotic solutions (in the sense of their exponential sensitivity to small changes in the initial conditions), atomic scattering is fractal with strongly pronounced self-similarity of the dependence of time T at which the atom leaves the cavity at its initial momentum; the fractal dimension of this curve is 1.84. In the chaotic regime, there are two infinite sets of atomic initial momenta for which T =∞ (in the idealized case of the absence of any loss). They correspond to separatrix-like trajectories along which the atom asymptotically approaches certain configuration space points and to the trajectories of infinitely long chaotic wandering of atoms in the cavity. These trajectories make up countable and uncountable fractals, respectively. Correlations between Rabi atomic oscillations and atomic motion can lead to Doppler-Rabi resonances, that is, deep oscillations of the internal energy of the atom, for large detunings δ that satisfy the condition |αρ 0 |≈|δ|.