Editor's Note: This story, originally published in the February 1992 issue of Scientific American, is being re-posted in light of Steven Chu's nomination as U.S. secretary of energy.
Before you turn another page of this magazine, consider your actions carefully. Every time you wish to grasp a page, you must place one finger above the paper and another below so that the distance between each finger and the paper is about equal to the diameter of an atom. At that point, the electrons at the surface of your fingers repel the electrons on either side of the page. This slight redistribution of charges produces an electric field that is strong enough to allow you to squeeze the page between your fingers. Remarkably, by applying electric forces at the atomic scale, you can hold onto objects that are, on the whole, electrically neutral.
In contrast, manipulating neutral objects that are atomic in size is a formidable technical challenge. Charged objects are much easier to control because electric and magnetic fields can exert much stronger forces over them. Indeed, for more than a century, scientists have applied electromagnetic forces to manipulate charged particles such as electrons and ions from a distance. But only in the past few years have researchers been able to move neutral particles at a distance with any facility. In particular, investigators have developed instruments that use lasers to trap and manipulate atoms and micron-size particles with astonishing control. These innovations have quickly led to a wide range of applications. My research group and others have cooled atoms to temperatures near absolute zero-conditions that allow us to examine quantum states of matter and unusual interactions between light and ultra-cold atoms. We have begun to develop atomic clocks and extremely sensitive accelerometers. Our techniques are being applied to handle such individual molecules as large polymers. In addition, we have devised an "optical tweezers" that uses laser beams to hold and move organelles inside of cells without puncturing the intervening membranes.
Almost a decade before scientists learned how to control neutral particles at a distance with laser light, they achieved some of the same tasks using magnetic fields. They applied fields to focus atoms in beams and trap them. After learning how to trap atoms with laser light, they turned to the vast arsenal of laser techniques to gain precise control over neutral particles. The first trap for neutral particles was developed by Wolfgang Paul of the University of Bonn. In 1978 he and his colleagues succeeded in trapping neutrons in a magnetic field. Seven years later, using the same basic principles, William D. Phillips and his colleagues at the National Bureau of Standards were able to trap atoms.
The magnetic trap can hold onto particles that have magnetic properties similar to those of a tiny bar magnet. To be more precise, the particle must carry a small magnetic dipole moment. If such a particle is placed in a magnetic field whose strength varies from region to region, it will move toward the weakest or strongest part of the field, depending on the particle's orientation [see illustration on next page]. Paul realized that it is possible to design a magnetic field with a local minimum in the field strength, and if the magnetic dipole is originally aligned to seek a position where the field is weakest, it will remain aligned in the "weak field-seeking" orientation [see "Cooling and Trapping Atoms," by W. D. Phillips and H. ]. Metcalf; SCIENTIFIC AMERICAN, March 1987].