ELECTROMAGNETIC TRAPPING OF COLD ATOMS

ELECTROMAGNETIC TRAPPING OF COLD ATOMS

Abstract. The review describes the methods of trapping cold atoms in electromagnetic fields and the fields combined of electromagnetic and gravity fields. We discuss first the basic types of the dipole radiation forces used for cooling and trapping atoms in the laser fields. We outline next the fundamentals of the laser cooling of atoms and classify the temperature limits for basic laser cooling processes. The main body of the review is devoted to discussion of atom traps based on the dipole radiation forces, dipole magnetic forces, combined dipole radiation-magnetic forces, and the forces combined of the dipole radiation-magnetic and gravity forces. Physical fundamentals of atom traps operating as the waveguides and cavities for cold atoms are also considered. The review ends with the applications of cold and trapped atoms in atomic, molecular and optical physics.

1. INTRODUCTION

The trapping of atoms in a restricted space volume is a fundamental physical problem of considerable interest from the standpoint of both the performance of the physical investigations with small amounts of atoms and the development of new technologies based on the localization of the spatial motion of atoms. Important physical applications of the methods of trapping atoms in three-dimensional spatial regions include studies into the spectral properties of small amounts of atoms, including counted numbers of radioactive atomic isotopes, improvement of the accuracy and sensitivity of spectral measurements, and studies of quantum-statistical effects in atomic ensembles at low temperatures, such as the Bose-Einstein condensation. No less important physical and technological applications may be associated with the trapping atoms in one or two dimensions, allowing atomic waveguides and cavities to be developed. Important technological applications are expected to ensue from the use of trapped atoms in the atomic frequency and time standards.

In the course of the many decades that this problem has been discussed, numerous physical ideas were put forward that could be used either for trapping atoms in three-dimensional regions of space or for trapping atoms in one or two dimensions. In essence, the practically developed methods appeared to be based on the use of the forces of electric dipole interaction of atoms with quasiresonance laser fields and (or) magnetic dipole interaction of atoms with static magnetic fields. In a sense, the main methods of trapping neutral atoms proved to be similar to those for trapping charged particles (electrons, protons, atom ions). To trap the latter, use is made of electromagnetic traps formed by inhomogeneous radio-frequency fields (Paul traps) or inhomogeneous stationary electric and magnetic fields (Penning traps) (Dehmelt, 1967, 1969; Paul, 1990).

From the physical standpoint, all the known techniques for trapping neutral atoms can be classed with but a few basic methods. These basic methods are: optical trapping using the forces of electric dipole interaction between atoms and laser fields, magnetic trapping based on the use of the forces of magnetic dipole interaction, mixed magneto-optical trapping using simultaneous interaction between atoms and magnetic and laser fields, and also mixed gravito-optical and gravito-magnetic trapping.

Historically, the first to be discussed were the methods of magnetic trapping. The very first suggestions on the possibility of electromagnetic trapping of atoms were already made when the first experiments were conducted on the deflection of atomic beams by a nonuniform magnetic field (Stern and Gerlach, 1921). The development of the idea of the magnetic deflection of atoms and molecules led to the appearance in the 1950s of the hexapole magnetic lenses and hexapole magnetic traps for particles with a permanent magnetic moment (Friedburg and Paul, 1951; Lemonick et al., 1955). These traps were successfully used to trap ultracold neutrons (Kugler et al., 1978; Golub and Pendlebury, 1979; Kugler et al., 1985). Many types of traps for particles with a permanent magnetic moment, starting with the most simple quadrupole trap and ending with the fairly complex Ioffe trap, were discussed in the works on plasma physics (Gott et al., 1962; Artsimovich, 1964; Krall and Trivelpiece, 1973). Concrete magnetic trap arrangements for trapping atoms started to be discussed in the 1960s (Vladimirskii, 1960; Heer, 1963; Letokhov and Minogin, 1980; Pritchard, 1983; Metcalf, 1984; Bergeman et al., 1987).

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"Education is what remains after one has forgotten everything he learnt in school" - Albert Einstein



...to be continued as soon as I figure out how to post Greek symbols and formulas on this blog template :-)...