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Laser Cooling and Trapping

Published Jan 1, 2008• 2 citations

Abstract

Laser cooling and trapping of neutral atoms is a rapidly expanding area of physics research that has seen dramatic new developments over the last decade. These include the ability to cool atoms down to unprecedented kinetic temperatures (as low as one micro Kelvin) and to bold samples of a gas isolated in the middle of a vacuum system for many seconds. This unique new level of control of atomic motion is

DOI: 10.1142/9789812813787_others02

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Low temperature physics without a cryostat: laser cooling and trapping of atoms

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Of the many application of lasers, the one with possibly the most dramatic impact on fundamental physics is laser cooling and trapping: the Nobel Prize for physics was awarded for laser cooling in 1997 [1]. The staggering progress achieved by laser cooling is reflected by considering a thought experiment that begins ‘take an atom. . . ’. Such an experiment was impossible to realize in practice because room temperature atoms move at supersonic speeds and, as atoms are neutral, there was no easy way to grab hold of them. Laser cooling allows one to obtain samples of atoms essentially at rest, with temperatures in the microKelvin region being routine. At such low temperatures, it becomes easy to trap or move atoms with modest electromagnetic forces [2, 3] and a whole new class of experiments previously confined to the realm of fantasy becomes possible. For example, one can throw atoms upwards and use precision spectroscopy to measure time or gravity with unprecedented accuracy. One can place atoms at a desired location on a surface creating a novel lithographic technique or place an atom just above the surface to create a qubit in a quantum information processor. One can take many atoms and cool them further by evaporation to form a Bose-Einstein condensate [4] or degenerate Fermi gas. This last development has propelled atomic and optical physics right into the heart of condensed matter physics and resulted in the award of the Nobel Prize in 2001.

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The past few years have seen remarkable developments in the ability to control atomic motion by optical techniques. Among the key tools that have been developed for this purpose are the techniques of laser cooling, the significance of which was recognized by the award of the 1997 Nobel Prize in Physics to Steven Chu, Claude Cohen-Tannoudji, and William Phillips. These techniques have made it possible to cool atoms to temperatures lower than have been produced in any other physical system - around 10(-8) degrees Kelvin above absolute zero - and to confine those in traps for extended periods of observation.

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Significant advances have been made in the ability to control the motion of neutral atoms. Cooling and trapping atoms present new possibilities for studies of ultracold atoms and atomic interactions. The techniques of laser cooling and deceleration of atomic beams, magnetic and laser trapping of neutral atoms, and a number of recent advances in the use of radiative forces to manipulate atoms are reviewed.

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Charles S. Adams, Ifan G. Hughes

Of the many applications of lasers, the one with possibly the most dramatic impact on fundamental physics is laser cooling and trapping: the Nobel Prize for physics was awarded for laser cooling in 1997. Laser cooling allows one to obtain samples of atoms essentially at rest, with temperatures in the micro-Kelvin region being routine. At such low temperatures, it becomes easy to trap or move atoms with modest electromagnetic forces and a whole new class of experiments previously confined to the realm of fantasy becomes possible. The high brightness of masers and lasers makes it possible to access a regime of atomic excitation for which Einstein’s rate equations are inadequate. Three-dimensional optical molasses using counter-propagating laser beams along three mutually orthogonal directions was first observed in 1985 by Chu et al. The Sisyphus mechanism works well for atoms that are already cold and has a smaller capture velocity than conventional Doppler cooling.

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Slowing down atoms using laser cooling, together with techniques like magnetic trapping and evaporative cooling, has led to numerous exciting results over the last decade [1], with the realization of Bose-Einstein condensation as the most spectacular achievement [2]. Laser-cooling schemes are applicable to atoms since closed multilevel systems can be realized and many consecutive absorptionemission cycles can be induced. This leads to a significant momentum transfer from the photons of the laser to the atom, resulting in a change of the velocity of the atom. There currently is a great interest in producing cold molecules for the study of cold molecule-molecule collisions, molecular quantum collective effects, Doppler-free spectroscopy, frequency standards, as well as many other applications already realized for atoms [3]. Slowing down of neutral molecules, a prerequisite for being able to trap them, has proven to be considerably more difficult to be achieved, however. The above-mentioned laser-cooling scheme cannot be applied to slow down molecules as no simple closed-level systems can be found; the unavoidable off-resonant radiative decay of molecules from the excited state to other vibrational levels in the electronic ground state hampers efficient momentum transfer [4]. Once molecules are present at sufficiently low kinetic energies, below a fraction of a cm21, they can be trapped in various ways. Trapping of neutral molecules in inhomogeneous magnetic [3] and electric fields [5] and trapping based on the use of evanescent fields close to the dielectricvacuum interface have been proposed [6]. It has also been suggested that extremely high laser powers might be used to induce a dipole moment in polarizable molecules, leading to trapping of these molecules in the laser beam [7]. Other methods to produce slow molecules have been put forward and have recently been successfully applied to produce the first samples of trapped molecules. By injecting molecules in a cold buffer gas, sufficiently low kinetic energies of the injected molecules can be obtained after thermalization that they can subsequently be trapped in an inhomogeneous magnetic field. This has recently been demonstrated for CaH injected in 3He buffer gas at temperatures of around 0.3 K [8]. Associative collisions between ultracold trapped atoms [9] have proven to be

Jan 1, 1999134 citations