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Journal of the Optical Society of America B

Laser cooling and trapping of atoms

H. Metcalf, P. van der Straten

Published May 1, 2003• 201 citations
Publisher

Abstract

A review is presented of some of the principal techniques of laser cooling and trapping that have been developed during the past 20 years. Its approach is primarily experimental, but its quantitative descriptions are consistent in notation with most of the theoretical literature. It begins with a simplified introduction to optical forces on atoms, including both cooling and trapping. Then its three main sections discuss its three selected features, (1) quantization of atomic motion, (2) effects of the multilevel structure of atoms, and (3) the effects of polychromatic light. Each of these features is an expansion in a different direction from the simplest model of a classical, two-level atom moving in a monochromatic laser field.

DOI
10.1364/josab.20.000887

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A review is presented of some of the principal techniques of laser cooling and trapping that have been developed during the past 20 years. Its approach is primarily experimental, but its quantitative descriptions are consistent in notation with most of the theoretical literature. It begins with a simplified introduction to optical forces on atoms, including both cooling and trapping. Then its three main sections discuss its three selected features, (1) quantization of atomic motion, (2) effects of the multilevel structure of atoms, and (3) the effects of polychromatic light. Each of these features is an expansion in a different direction from the simplest model of a classical, two-level atom moving in a monochromatic laser field.
The usual form of electromagnetic forces is given by but for neutral atoms q = 0. The next order of force is the dipole term, but this also vanishes because neutral atoms have no inherent dipole moment. However, a dipole moment can be induced by a field, and this is most efficient if the field is alternating near the atomic resonance frequency. Since these frequencies are typically in the optical range, dipole moments are efficiently induced by shining nearly resonant light on the atoms. Laser cooling and trapping rely on the interaction between laser light and atoms to exert a controllable force on the atoms, and many sophisticated schemes have been developed using the special properties of the interaction. This chapter begins with the simplest schemes for exerting optical forces on atoms, namely a single-frequency light field interacting with a two-level atom. Although this is the simplest possible scheme, it is pedagogically valuable because it shows many of the features that will be encountered further on. The ultimate temperature using such a scheme is limited, but lower temperatures can be achieved with multilevel atoms. This topic is addressed later in the chapter. Two kinds of optical forces When an atom absorbs nearly resonant light, it makes a transition to the excited state, and its subsequent return to the ground state can be either by spontaneous or by stimulated emission. The nature of the optical force that arises from these two different processes is quite different, and will be described separately [1]. The spontaneous emission case is different from the familiar quantum mechanical calculations using state vectors to describe the state of the system, because spontaneous emission causes the state of the system to evolve from a pure state into a mixed state (see Exercise 6.9). Spontaneous emission is an essential ingredient for the dissipative nature of the optical forces in monochromatic light, and the density matrix is needed to describe it (see Chap. 6). When an atom absorbs light, its energy excites the atom and its angular momentum changes the electron's orbit. Both of these are internal atomic properties. However, the linear momentum of the light, can change only the external, translational motion of the atom.
We consider the mechanical forces exerted on atoms by light as a tool to make the atom more concrete as a particle. It can be pushed, trapped and observed in its own scattered light. Recent theoretical and experimental progress in the field of cooling and trapping of atoms demonstrates their palpable reality. The theory of the mechanical manifestations of light is reviewed, and its application to laser cooling is discussed. Traps for atoms are considered. The feasibility of pure light traps is discussed, and the use of other types of traps in laser applications is reviewed. We present briefly the main experiments proving the possibility of laser cooling and trapping. A brief discussion of applications and future experimental possibilities is given.
Summary form only given. A brief history of the laser cooling and trapping of atoms will be presented, covering some of the cooling and trapping techniques that have been developed over the past 15 years. The cooling and trapping technology is already being applied in numerous areas of science and engineering. Applications to be discussed include atomic clocks, atom interferometers for inertial sensors and for the measurement of the fine structure constant, as well as studies in polymer dynamics and protein motion.
This book provides a thorough introduction to the interaction of atoms and atomic ions with optical and magnetic fields. Particular emphasis is placed on the wealth of important multilevel effects, where atomic vapours exhibit anisotropic behaviour. As well as covering the classic two-level atom approach to light-atom interactions, a general multi-level formalism is also described in detail, and used to discuss optical pumping, two-dimensional spectroscopy and nonlinear optical dynamics. The final chapter deals with the mechanical effects of light, including the cooling and trapping of atoms. With full theoretical and experimental coverage, and over 250 illustrations, the book will be of great interest to graduate students of laser spectroscopy, quantum electronics and quantum optics, and to researchers in these fields.
Introduction and review of quantum mechanics: Review of Quantum Mechanics.- The Density Matrix.- Force on Two-Level Atoms.- Multi-Level Atoms.- General Properties Concerning Laser Cooling. Cooling and Trapping: Deceleration of an Atomic Beam.- Optical Molasses.- Cooling Below the Doppler Limit.- The Dipole Force.- Magentic Trapping of Neutral Atoms.- Optical Traps for Neutral Atoms.- Evaporative Cooling. Applications: Newtonian Atom Optics.- Ultra-Cold Collisions.- deBrooglie-Wave Optics.- Optical Lattices.- Bose -Einstein Condensation.- Dark States.