Hamiltonian Chaos with a Cold Atom in an Optical Lattice

Abstract

We consider a basic model of the lossless interaction between a moving 2-level atom and a standing-wave single-mode laser field. Classical treatment of the translational atomic motion provides the semiclassical Hamilton-Schrodinger equations of motion which are a 5-dimensional nonlinear dynamical system with two integrals of motion. The atomic dynamics can be regular or chaotic (in the sense of exponential sensitivity to small variations in initial conditions and/or the system’s control parameters) in dependence on values of the control parameters, the atom-field detuning and recoil frequency. We develop a semiclassical theory of the chaotic atomic transport in terms of a random walk of the atomic electric dipole moment u which is one of the components of a Bloch vector. Based on a jump-like behavior of this variable for atoms crossing nodes of the standing laser wave, we construct a stochastic map that specifies the center-of-mass motion. We find the relations between the detuning, recoil frequency and the atomic energy, under which atoms may move in a rigid optical lattice in a chaotic way. We obtain the analytical conditions under which deterministic atomic transport has fractal properties and explain a hierarchical structure of the dynamical fractals. Quantum treatment of the atomic motion in a standing wave is studied in the dressed state picture where the atom moves in two optical potentials simultaneously. If the values of the detuning and a characteristic atomic frequency are of the same order, than there is a probability of nonadiabatic transitions of the atom upon crossing nodes of the standing wave. At the same condition exactly, we observe sudden changes (jumps) in the atomic dipole moment u when the atom crosses the nodes. Those jumps are accompanied by splitting of atomic wave packets at the nodes. Such a proliferation of wave packets at the nodes of a standing wave is a manifestation of classical atomic chaotic transport. In particular, the effect of simultaneous trapping of an atom in a well of one of the optical potential and its flight in the other potential is a quantum analogue of a chaotic classical walking of an atom. At large values of the detuning, the quantum evolution is shown to be adiabatic in accordance with a regular character of the classical atomic motion.