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Physical Review A

Anomalous diffusion and Lévy walks in optical lattices

Stephan Marksteiner, K. Ellinger, P. Zoller

Published May 1, 1996• 198 citations
Publisher

Abstract

We study theoretically the spatial diffusion (transport) of two-level atoms in one- and two-dimensional optical molasses derived from counterpropagating laser beams. We use both quantum Monte Carlo and semiclassical methods to study the microscopic characteristics of the atomic motion and their effect on the macroscopic behavior of the spatial distribution. We find that there exists a certain critical depth of the optical potential below which the atomic trajectories show L\'evy flights in space that last on a definite time scale (L\'evy walks). This behavior leads to a transition from Gaussian spatial diffusion to anomalous diffusion while crossing this critical potential depth. We show that only atoms with very high momentum are responsible for these L\'evy walks. This observation allows us to predict the critical parameters via a semiclassical Fokker-Planck equation approach. \textcopyright{} 1996 The American Physical Society.

DOI
10.1103/physreva.53.3409

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We study theoretically the spatial diffusion (transport) of two-level atoms in one- and two-dimensional optical molasses derived from counterpropagating laser beams. We use both quantum Monte Carlo and semiclassical methods to study the microscopic characteristics of the atomic motion and their effect on the macroscopic behavior of the spatial distribution. We find that there exists a certain critical depth of the optical potential below which the atomic trajectories show L'evy flights in space that last on a definite time scale (L'evy walks). This behavior leads to a transition from Gaussian spatial diffusion to anomalous diffusion while crossing this critical potential depth. We show that only atoms with very high momentum are responsible for these L'evy walks. This observation allows us to predict the critical parameters via a semiclassical Fokker-Planck equation approach. \textcopyright{} 1996 The American Physical Society.
Recently, anomalous superdiffusion of ultracold 87Rb atoms in an optical lattice has been observed along with a fat-tailed, Lévy type, spatial distribution. The anomalous exponents were found to depend on the depth of the optical potential. We find, within the framework of the semiclassical theory of Sisyphus cooling, three distinct phases of the dynamics as the optical potential depth is lowered: normal diffusion; Lévy diffusion; and x∼t(3/2) scaling, the latter related to Obukhov's model (1959) of turbulence. The process can be formulated as a Lévy walk, with strong correlations between the length and duration of the excursions. We derive a fractional diffusion equation describing the atomic cloud, and the corresponding anomalous diffusion coefficient.
We point out a connection between anomalous transport in an optical lattice and Tsallis' generalized statistics. Specifically, we show that the momentum equation for the semiclassical Wigner function which describes atomic motion in the optical potential, belongs to a class of transport equations recently studied by Borland [Phys. Lett. A 245, 67 (1998)]. The important property of these ordinary linear Fokker-Planck equations is that their stationary solutions are exactly given by Tsallis distributions. An analytical expression of the Tsallis index q in terms of the microscopic parameters of the quantum-optical problem is given and the spatial coherence of the atomic wave packets is discussed.
We carry out quantum and semiclassical numerical simulations of the dynamics, particularly diffusion, of atoms in an optical lattice. We consider specifically the situation of a $J=1/2\ensuremath{\leftrightarrow}J=3/2$ atomic transition, whose experimental counterpart can be realized in Mg+{\mathrm{Mg}}^{+}Mg+ experiments. The semiclassical results, which cover a larger range of parameters than would be practical for fully quantum calculations, confirm and extend the work of Marksteiner et al. [Phys. Rev. A 53, 3409 (1996)], characterize the transition from normal to anomalous diffusion and the atomic trajectories which give rise to it, and provide a physical explanation for the difference between these two regimes. The full quantum calculations reveal uniquely quantum features of individual atomic trajectories, and in addition, predict significantly lower diffusion, by a factor of 1.2 to 2, than their semiclassical counterparts.
We study dissipative transport of spontaneously emitting atoms in a one-dimensional (1D) standing-wave laser field in the regimes where the underlying deterministic Hamiltonian dynamics is regular and chaotic. A Monte Carlo stochastic wave-function method is applied to simulate semiclassically the atomic dynamics with coupled internal and translational degrees of freedom. It is shown in numerical experiments and confirmed analytically that chaotic atomic transport can take the form either of ballistic motion or a random walking with specific statistical properties. The character of spatial and momentum diffusion in the ballistic atomic transport is shown to change abruptly in the atom-laser detuning regime where the Hamiltonian dynamics is irregular in the sense of dynamical chaos. We find a clear correlation between the behavior of the momentum diffusion coefficient and Hamiltonian chaos probability which is a manifestation of chaoticity of the fundamental atom-light interaction in the diffusivelike dissipative atomic transport. We propose to measure a linear extent of atomic clouds in a 1D optical lattice and predict that, beginning with those values of the mean cloud's momentum for which the probability of Hamiltonian chaos is close to 1, the linear extent of the corresponding clouds should increase sharply. A sensitive dependence of statistical characteristics of dissipative transport on the values of the detuning allows us to manipulate the atomic transport by changing the laser frequency.
We experimentally study anomalous diffusion of ultracold atoms in a one dimensional polarization optical lattice. The atomic spatial distribution is recorded at different times and its dynamics and shape are analyzed. We find that the width of the cloud exhibits a power-law time dependence with an exponent that depends on the lattice depth. Moreover, the distribution exhibits fractional self-similarity with the same characteristic exponent. The self-similar shape of the distribution is found to be well fitted by a Lévy distribution, but with a characteristic exponent that differs from the temporal one. Numerical simulations suggest that this is due to long trapping times in the lattice and correlations between the atom's velocity and flight duration.
It is well established that classical chaotic motion is suppressed by quantum mechanics. The paradigm theoretical system has been the ?-kicked rotor, where the suppression manifests itself as dynamical localization. This effect is saturation of momentum growth after the quantum break time, with a resulting exponentially localized distribution. Classical analyses of this system have also identified regimes of anomalous diffusion, characterized by “Levy-type” flights in phase space [1]. This behavior is considerably more complex than simple diffusion and has attracted much interest in recent years. The quantum system was predicted to exhibit an increase in localization length near regions of anomalous diffusion [2], and recent theoretical work predicts very large growth of energy in this case, even for systems that are far from the semiclassical limit [3].