In the 1970s and 1980s technological developments, notably the development of lasers but also advances in ultra-high vacuum techniques and electronics, produced a revolution in atomic physics comparable with that caused by the development of spectroscopy in the 1870s. Single atoms can be observed and experimented on and many new and strange phenomena investigated. The atom will lose the excess energy by spontaneous emission. True trapping can be achieved if atoms which have magnetic dipole moments are used. The Penning trap obviously only works on charged particles. However a different technique enables individual neutral atoms to be trapped and experimented on. This technique is known as 'laser cooling'. If a three-dimensional array of lasers is set up. tuned to the appropriate frequency, atoms injected into the centre of the combined field move more and more sluggishly.
In the 1970s and 1980s technological developments, notably the development of lasers but also advances in ultra-high vacuum techniques and electronics, produced a revolution in atomic physics comparable with that caused by the development of spectroscopy in the 1870s. Single atoms can be observed and experimented on and many new and strange phenomena investigated. The atom will lose the excess energy by spontaneous emission. True trapping can be achieved if atoms which have magnetic dipole moments are used. The Penning trap obviously only works on charged particles. However a different technique enables individual neutral atoms to be trapped and experimented on. This technique is known as 'laser cooling'. If a three-dimensional array of lasers is set up. tuned to the appropriate frequency, atoms injected into the centre of the combined field move more and more sluggishly.
The trapping of charged elementary particles and ions by various electromagnetic field configurations is a well established art. Last month (page 17) we told of a lone electron that was confined for almost a year in the vacuum of a Penning trap, yielding a superbly delicate observation of lowenergy relativistic hysteresis. But the trapping of electrically neutral particles—atoms in particular—is quite another matter. In the absence of the simple Lorentz force acting on a charged particle, one has to reach for a much weaker, second-order handle—the atom's magnetic or induced electric dipole moment. A variety of ingenious magnetic and laser traps have been proposed for atoms, but they all suffer a severe limitation: The weak dipole interaction with the trapping field yields only very shallow trapping potential wells, generally incapable of holding an atom moving much faster than a man can run—about 10 meters/second.
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
Cooling is indispensable for trapping and observing single free atoms.In atomic physics, this can be achieved efficiently by laser cooling.Conventional methods to laser cool atoms are based on repeated optical pumping cycles followed by spontaneous emission of a photon from the atom.Such pumping schemes can only be applied to certain atomic species, which have a closed optical transition.Drawbacks caused by photons spontaneously emitted from the atom can be avoided in the strongly coupled atom-cavity system in which energy and entropy can be removed by photons lost from the cavity.This thesis reports on the first observation of this cooling mechanism which does not rely on spontaneous emission.In the experiment, a single atom is captured in an intracavity dipole trap.Illuminating the system with a weak, slightly blue-detuned light beam extends the storage time of the atom in the trap.The observed cooling force is at least five times larger than the force achieved by free-space cooling methods at comparable excitation of a two-level atom.Utilising cavity cooling, single atoms are prepared strongly-coupled to the mode of a high-finesse cavity.This allows to observe two well-resolved normal-mode peaks in both the cavity transmission as well as in the trap lifetime.The experiment is in agreement with a Monte Carlo simulation, demonstrating the localisation of the atom to within a tenth of a wavelength.The ability to individually excite the normal modes opens new possibilities to use this system for applications in quantum information science, including the realisation of quantum-logic gates.In the future cavity cooling might be applied to systems which cannot be cooled by conventional methods as for instance molecules or an atom carrying a quantum bit.
Abstract That the laser cooling and trapping of neutral atoms are interrelated has already been said in the introduction to Chapter 5. We shall, therefore, now consider various techniques whereby cold atoms can be trapped. All the known techniques for trapping laser-cooled neutral atoms can be classified according to a few basic methods. These basic methods are optical trapping using the forces of the electric dipole interaction between atoms and laser fields, magnetic trapping based on the use of the forces of the magnetic dipole interaction, mixed magneto-optical trapping using simultaneous interaction between atoms and magnetic and laser fields, and mixed gravitooptical and gravito-magnetic trapping. Traps for neutral atoms can have a wide variety of geometries and dimensions: (a) macroscopic traps with a size of a »λ, (b) microscopic traps with a size of a ≃λ/2, (c) various combinations of macroscopic and microscopic traps (one-, two-, and three-dimensional) in the case of optical lattices, and finally (d) nanoscopic traps with a size of a «λ/2.
Abstract : The program is to slow, trap and cool neutral atoms. Laser light is used to slow and cool atoms and magnetic forces are used to trap them. Ultimately we hope to be able to trap atoms for days and cool them to temperatures on the order of .000001 K in addition to the interesting scientific and technical challenges involved in trapping and cooling atoms, we hope to open several new lines of investigation with these cooled atoms. Cold trapped atoms constitute a good system for studying collective phenomena such as Bose condensation, low energy - Na and Na - Na, and coherent optical effects (the deBrogile wavelength can exceed the optical wavelength). They also offer opportunities for performing ultra-high resolution spectroscopy and are extremely promising candidates for a new generation of frequency standards.
Introduction This chapter describes remarkable new experiments that have been made possible through recent advances in laser cooling of atoms. Cooling of atoms in a gas by laser light is in itself surprising and this technique is important in a range of applications. In particular the experiments described here take atoms that have been laser cooled and then use other methods to reach even lower temperatures, at which wonderful and fascinating quantum effects occur. As well as revealing new phenomena, these experiments explore the deep analogies between the roles played by light waves in lasers and matter waves. These techniques are widely accessible and enable skilled researchers in laboratories all over the world to explore exciting new physics. The key physics and techniques of laser cooling are described in depth in Chapter 6. What is “temperature”? We use the words “cold” and “hot” every day; for example to refer to the temperature of the air outside. But what do “cold” and “hot” mean for the air’s molecules? Broadly speaking, we can say that atoms in a hot gas move faster than those in a cold gas and that cooling, by taking energy out of the gas, slows the atoms down. In this description of the gas as a collection of atoms behaving like billiard balls, or little hard spheres, the lowest possible temperature occurs when the atoms stop moving but, as we shall see, this picture does not give an accurate description at very low energies, for which quantum effects become important. In this chapter we will look at the fascinating properties of atomic gases cooled to temperatures within one millionth of a degree Kelvin from absolute zero. We shall use the term “ultra-cold” to refer to such extremely low temperatures that have only recently been achieved in experiments on atomic gases. The techniques used to cool liquids and solids have a longer history and have made available a different set of possibilities (see Chapter 8). Note that most of this chapter refers to gases in which the individual particles are atoms, but the general statements about temperature scales apply equally well to gases of molecules, e.g. air, whose major constituents, oxygen and nitrogen, exist as diatomic molecules.