William Daniel Phillips was born on November 5, 1948, in Wilkes-Barre, Pennsylvania. He grew up in Camp Hill, outside Harrisburg, and attended Juniata College. Phillips was awarded his B.S. degree from Juniata in 1970. He then continued his studies at the Massachusetts Institute of Technology (MIT), from which he earned a Ph.D. in physics in 1976. He then spent two years as a postdoctoral Chaim Weizmann Fellow at MIT. In 1978, Phillips accepted an appointment as a physicist at NIST, a post he retained for the next two decades.
Phillips has been interested in the problem of cooling and trapping atoms since his years at MIT. This line is of growing interest among physicists because it provides a way of studying the structure and properties of individual molecules and atoms. Traditionally, scientists have been able to study only thebulk properties of matter, that is, the average properties of, at best, trillions and trillions of atoms and molecules. Yet, it is known that these average properties mask the properties of individual particles that make up a sample. The study of atoms and molecules is of interest not only for theoreticalreasons, but also because of its potential practical applications on a macroscopic scale. Engineers are beginning to learn how to construct particles thatconsist of a relatively small number of particles, called nanostructures, with potentially revolutionary applications. In order to advance in this fieldof research, it has become necessary to understand how individual atoms and molecules behave.
The key to these studies is to find ways to cool atoms and molecules, that is, to reduce the speed at which they travel. At room temperature, the particles in a gas are typically traveling at speeds of a few thousand miles (kilometers) per hour. At these speeds, particles escape from a field of view so quickly that no meaningful measurements can be made. In order to keep the particles within a field of view, their velocity must be reduced. Simply cooling a gas by conventional methods (such as putting it in a refrigerator) does not solve this problem. Under those conditions, the gas particles do lose speed, but they eventually condense to form a liquid or solid. In both liquids and solids, particles are so close to each other that individual properties cannot be determined. The objective, then, is to cool down a gas without permitting it to condense.
Theoreticians were proposing methods for solving this problem since the 1950s. One of the most popular approaches was to bombard a moving particle with aphoton of energy moving towards that particle. Imagine, for example, a streamof ping pong balls fired from a gun. Those ping pong balls might be comparedto the particles in a gas. Then imagine that short puffs of air are directedat the stream of ping pong balls. The puffs of air correspond to photons ofenergy. As each puff of air strikes a ping pong ball, it transfers energy tothe ball and reduces the speed with which the ball is moving in the forward direction. If this process is repeated over and over again with any given pingpong ball, the ball eventually slows down and comes to a stop.
This analogy is greatly oversimplified, however. Real particles that are struck by a photon quickly give off the energy they have absorbed and recoil in adirection that cannot be predicted. Another photon aimed at the same particle will miss contact because the particle has changed its line of flight.
Another factor to be considered is that atoms and molecules absorb only certain quantized levels of energy. A photon with the wrong energy will have no effect on any particle that it strikes. Furthermore, the energy needed to excite a particle changes as the particle moves toward or away from the source ofenergy. This effect is known as the Doppler effect and is familiar to anyonewho has heard the pitch of a train whistle increase or decrease as it moves toward or away from an observer.
Still, given all these conditions, an experimenter should be able to design an apparatus by which bursts of energy can be used to slow down the movement of atoms and molecules. In the mid-1980s, Steven Chu and his co-workers at theBell Laboratories in Holmdel, New Jersey, designed an "atom trap" in which particles are bombarded by three pairs of laser guns arranged at right anglesto each other. This arrangement was able to slow atoms down to a speed of about 12 in per second (30 cm per second), equivalent to a temperature of about240 microkelvin. Theoretical calculations had shown that this temperature wasprobably the lowest that could be reached by the methods described above.
In 1988, Phillips attained a remarkable breakthrough by repeating the kinds of experiments carried out earlier by Chu and his colleagues. Phillips' groupfound that they were able to attain a minimum temperature of 40 microkelvin,six times lower than that predicted by theory and observed by Chu. Phillips later wrote that his results were "at first difficult to believe, especially considering the attractive simplicity of the Doppler cooling theory (and the generally held belief that experiments never work better than one expects). This turn of events," he went on, "was both welcome and unsettling." The theoretical explanation for these observations was later provided by Claude Cohen-Tannoudji at the École Normale Supérieure in Paris, an accomplishment for which he was awarded a share of the 1997 Nobel Prize in Physics.
Phillips produced other breakthroughs in the study of low temperature behavior. In 1985, for example, he found a way to trap atoms in a maze of laser beams. The beams were arranged so as to produce constructive and destructive interference patterns. When placed into this kind of maze, gaseous atoms at verylow temperatures fell into low-energy pockets in somewhat the manner that eggs fall into the depressions of the cartons in which they are sold.
Using devices such as these, Phillips has been able to create entirely new forms of matter that exist only under low temperature conditions and only for very brief periods of time. For example, in one experiment, he brought together a collection of sodium atoms that formed a molecular-like substance but that was much larger than an ordinary molecule. The collection of atoms broke apart after about 10 nanoseconds.