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Cooling and Trapping Neutral Atoms (1985)

S. Brandt

Published Apr 22, 1992• 1 citation
Publisher

Abstract

Abstract Atomic beams, pioneered by Stern (Episodes 30 and 52), opened the way to study non-interacting atoms which, essentially, have only one component in velocity, namely, that in beam direction. Ions could even be confined in a trap(Episode 80) with small values of all three velocity components. We shall see now how that was achieved also for neutral atoms. A measure of quality for a gas of trapped atoms is its temperature, i.e., the average kinetic energy of the atoms expressed in Kelvin. By carefully exploiting the inner structure of atoms it became possible, with the help of laser light and magnetic fields, to cool and trap small samples of highly dilute atomic gases, reaching temperatures as low as 180 nK (180 × 10‒9 K).

DOI
10.1093/oso/9780199544691.003.098

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Abstract Atomic beams, pioneered by Stern (Episodes 30 and 52), opened the way to study non-interacting atoms which, essentially, have only one component in velocity, namely, that in beam direction. Ions could even be confined in a trap(Episode 80) with small values of all three velocity components. We shall see now how that was achieved also for neutral atoms. A measure of quality for a gas of trapped atoms is its temperature, i.e., the average kinetic energy of the atoms expressed in Kelvin. By carefully exploiting the inner structure of atoms it became possible, with the help of laser light and magnetic fields, to cool and trap small samples of highly dilute atomic gases, reaching temperatures as low as 180 nK (180 × 10‒9 K).
Magnetic trapping of laser-cooled neutral atoms has been demonstrated at NBS and is progressing in other laboratories. Confinement of any object requires exchanging kinetic for potential energy, and for neutral atoms, this necessarily derives from shifts of internal energy levels. This is implemented through the force experienced by the atomic magnetic moment in a carefully designed, highly inhomogeneous field. Since magnetic fields of convenient strength can shift atomic energy levels by only a few gigahertz (temperature ≌ 0.1 K), neutral atom traps are very shallow and must be loaded with very cold atoms. We present some motivations for using magnetic traps and then discuss the constraints and optimal configurations of various arrangements. For example, no trap can be isotropic. Next we discuss both the classical and quantum mechanical motions of trapped atoms. These motions are important for at least two reasons: First, several schemes under study for further cooling depend on knowing the position and velocity of the atoms, as well as the vector field at each point along the orbit. Second, magnetic traps depend on the atomic moment remaining aligned with the field as the atom orbits in the trap, and this precludes rapid motion through a low-field region.
Abstract : We have trapped large numbers of neutral atoms, and cooled them to millikelvin temperatures. Our next objective is to cool them to microkelvin temperatures. Dense samples of atoms cooled to microkelvin temperatures promise to open up new and exciting areas of physics. The lack of interaction of the low velocity atoms due to their reduced thermal motion, together with the possibility of indefinitely long interaction times, make samples of trapped atoms ideal for high resolution spectroscopy and for use as atomic frequency standards. Our main current effort in neutral atom trapping is cooling of trapped atoms to low temperatures. While this remains a difficult and elusive goal (to date, micro-kelvin temperatures have only been achieved with untrapped atoms), the rewards for supercooling trapped atoms appear to be high. The long confinement times, together with the reduced thermal motion of cold atoms, could result in a new era of ultra-high resolution spectroscopy and precise frequency standards. Potentially more exciting is the possibility of combining the high densities achievable in traps and the long deBroglie wavelength of ultra-cold atoms to observe novel quantum collective phenomena.
In order to confine any object, it is necessary to exchange kinetic for potential energy in the trapping field, and in neutral atom traps, the potential energy must be stored as internal atomic energy. Thus practical traps for ground-state neutral atoms are necessarily very shallow compared with thermal energy because the energy level shifts that result from conveniently sized fields are typically considerably smaller than k B T , even for T = 1 K. Neutral atom trapping therefore depends on substantial cooling of a thermal atomic sample, and is often connected with the cooling process. In most practical cases, atoms are loaded from magneto-optical traps, where they have been efficiently accumulated and cooled to mK temperatures (see Sec. 20.3), or from optical molasses, where they have been optically cooled to μK temperatures (see Sec. 19.4). The small depth of typical neutral atom traps dictates stringent vacuum requirements, because an atom cannot remain trapped after a collision with a thermal energy background gas molecule. Since these atoms are vulnerable targets for thermal energy background gas, the mean free time between collisions must exceed the desired trapping time. The cross-section for destructive collisions is quite large because even a gentle collision (i.e. large impact parameter) can impart enough energy to eject an atom from a trap. At pressure P sufficiently low to be of practical interest, the trapping time is ~(10 -8 / P ) s, where P is in torr. This chapter begins with a discussion of ordinary, single-center traps, but also considers arrays of traps. Such “optical lattices” are playing an increasingly important role in research in atomic physics, so discussion of their properties is an appropriate part of the education of a student of the subject. Dipole force optical traps Single-beam optical traps for two-level atoms The simplest imaginable optical trap consists of a single, strongly focused Gaussian laser beam (see Fig. 20.1) [214, 215] whose intensity at the focus varies transversely with r as where is the beam waist size. Such a trap has a well-studied and important macroscopic classical analog in a phenomenon called optical tweezers (see App. 1.C) [216–218].
We present a simple review of the basic physical processes allowing one to control, with laser beams, the velocity and the position of neutral atoms. The control of the velocity corresponds to a cooling of atoms, that is, a reduction of the atomic velocity spread around a given value. The control of the position means a trapping of atoms in real space. The best present performances will be given, in terms of the lowest temperatures and the highest densities. The corresponding highest quantum degeneracy will also be estimated. It is imposed by fundamental limits, which will be described briefly. We also give the general trends in this field of research and outline the new directions which look promising for observing quantum statistical effects in laser cooled atomic samples, but which are for the moment restricted by unsolved problems. Introducing the Simple Schemes The radiative forces acting on atoms in a light field can be split into two parts, a reactive one and a dissipative one. The dissipative force (radiation pressure), which basically involves scattering processes, is velocity dependent. We will see that this dependence leads to the Doppler cooling scheme and to the concept of optical molasses, and we will give the corresponding minimal achievable temperature. The dissipative force can be made position dependent, through a gradient of the magnetic field, so that the atoms are also trapped in the so-called magneto-optical trap.