SUPERCOOL SCIENTIST: WILLIAM PHILLIPS
by Martha Davidson
An insatiable curiosity about the way things work led William Phillips to a career in physics and, eventually, to groundbreaking research on the atom. Phillips, a scientist in the Atomic Physics Division of the National Institute of Standards and Technology (NIST), shared the 1997 Nobel Prize in Physics with two other physicists in recognition of their development of new techniques to cool and trap atoms with laser light. By cooling atoms to nearly absolute zero, Phillips and his colleagues opened up a new field of subatomic research and technology that makes possible more accurate atomic clocks and other precise measuring and navigational devices.
Phillips spoke with seventh-grade students from three schools (the Queen Anne School in Upper Marlboro, Md.; the Ormond Stone Middle School in Centreville, Va.; and the Nysmith School in Herndon, Va.) at the Lemelson Center's April 2001 Innovative Lives program. He told the students how he came to be a physicist and described his research in supercooling atoms.
Phillips was born in 1948 and grew up in Camp Hill and other small towns in Pennsylvania. His curiosity about the world was evident from an early age, and his parents--both social workers--encouraged him with gifts of books and chemistry sets. "By the time I was five years old, I was completely fascinated by science," Phillips recalls. "My parents bought me a little microscope and I went around looking at all kinds of junk you can find around the house, to see what it would look like under the microscope." Another favorite toy was an Erector® set that included a little motor and gears. He enjoyed participating in science fairs and is still proud of an honorable mention he won for a junior high project on measuring radiation.
A straight-A student in high school, Phillips had no trouble selecting a major when he went to college. "It seems like I've been interested in physics as long as I can remember," he says. At Juniata College in Pennsylvania, he took courses in many areas of physics, including mechanics, electricity, and thermodynamics. "There's a whole lot of different subjects in physics you have to take, because physics encompasses so many different aspects of the way the world works. I have to understand all sorts of different kinds of physics--the physics of heat, the physics of electricity, the physics of motion, the physics of lasers. So I took all those kinds of classes when I was in college."
Phillips also studied French. Looking back, he realizes how important language skills have been to his career in science, enabling him to communicate his ideas clearly to an international community.
He received a B.S. degree in 1970, married Jane Van Wynen, whom he had known since high-school, and began graduate work at the Massachusetts Institute of Technology (MIT) that same year.
At MIT, he took advanced classes and carried out his thesis experiments under Professor Daniel Kleppner in the Physics Department and the Research Laboratory for Electronics. Although only one thesis was required, he did two. One was in magnetic resonance, a well-established field; the other was in an emerging field: the study of atoms with laser light.
Phillips was awarded a Ph.D. in physics in 1976 but remained at MIT as a Chaim Weizmann postdoctoral fellow. He carried out his postdoctoral research with Kleppner, Professor David Pritchard, and others, studying collisions of laser-excited atoms, among other projects. It was around this time that Phillips read a paper about laser cooling that captured his interest and set the direction of his work for the future.
The author of that paper, Arthur Ashkin, proposed that atoms could be slowed and trapped by laser beams. Phillips thought, "That sounds like a good idea. I've got a laser, I've got a beam of sodium atoms, I bet I could do that!" Phillips was also inspired by another paper in which David Wineland and colleagues reported the first laser cooling of ions, which are electrically charged atoms. Cooling electrically neutral atoms turned out not to be very easy, but when Phillips joined the staff of NIST (then called the National Bureau of Standards) in 1978, he began to attack the problem seriously.
Atoms at room temperature typically move at a speed of a few hundred meters per second. Light can be used to slow atoms down because it can exert a force that "pushes" against the atom if the color or frequency of the light is exactly right. "Each different kind of atom has particular colors that it will absorb," Phillips explains. "Atoms are exquisitely sensitive to the color or frequency of the light. . . . If you make the color just right, then the light will be absorbed and there'll be a push that can either slow or speed the atom."
To insure that the light slows the atoms requires the Doppler shift, a perceived change in the color depending on whether the observer (in this case, the atom) is moving toward or away from the source. An approaching light is detected by the atom as having a higher frequency (more blue), while a receding one seems to have a lower frequency (more red). Illuminating a gas (whose atoms normally all move in different directions) with multiple lasers from different angles, each tuned slightly red of the color the atoms would absorb if at rest, will in effect slow all the atoms. "No matter which way it goes, the atom thinks that the laser beam that is going in the opposite direction from the way the atom is going is the one that has the right color to be absorbed, and it absorbs it and slows down." Atoms moving more slowly are cooler than those moving more rapidly, so slowing the atoms means cooling them.
While this theory was understood in the late 1970s, it took nearly a decade of work by Phillips and others to develop the required techniques in the laboratory. Phillips knew that cooling atoms with lasers was an area of research that could have important applications for the kinds of precise measurements that were done at NIST, particularly for atomic clocks that set the standards of time. While working on other problems, Phillips used "stolen moments to dabble in laser cooling" with lab equipment he had brought with him from MIT. Fortunately, his research on atomic physics and lasers did not require special facilities. "Everything is built by a small team," he explains. "It's what we call table-top experiments." Phillips's supervisors at NIST supported and encouraged this research.
Phillips did not work alone. "Testing out ideas with other people, getting their feedback, getting their suggestions, asking questions, answering questions they pose, and then finally coming to some kind of idea of what can really make sense--for me it's always like that," Phillips says. "It's always with other people."
Working with Harold Metcalf, a physics professor at the State University of New York at Stony Brook, Phillips constructed an apparatus called a "Zeeman slower" that uses laser light and a spatially varying magnetic field to slow atoms. Around the same time, another physicist, Steven Chu, was working on a similar problem at Bell Labs in New Jersey. Chu and his team also succeeded in slowing and cooling a beam of atoms with radiation pressure from a laser. They then observed that the atoms, when acted on by the forces of intersecting laser beams, behaved as if they were in a viscous fluid, a condition they described as "optical molasses." At NIST, Phillips's team had independently theorized that the lasers would create this effect, which, by coincidence, they called a "molasses trap."
NIST recognized the importance of Phillips's experimental work and made it the focus of a new program in atomic physics. In this program, Phillips and his team continued to make important discoveries. For example, in the late 1980s they proved they could cool sodium atoms well below what was then believed to be the lowest limit, 240 microkelvin, or 240 millionths of a Celsius degree above absolute zero.
At first the NIST team questioned their own results and devised three different methods to replicate the measurement. The results were consistent--and groundbreaking: the temperature of the trapped atoms was the coldest ever measured, "about 100,000 times colder than the coldest thing that there is anywhere in the natural universe."
This amazing discovery met with skepticism at first in the scientific community, but two other teams, led by Steven Chu and a French physicist named Claude Cohen-Tannoudji, replicated the experiment with the same results. Then they worked to develop new theories of physics to explain the phenomenon. That new understanding ultimately allowed Phillips and his colleagues to laser cool atoms to less than a millionth of a degree above absolute zero!
Phillips had spent a month in Cohen-Tannoudji's lab the year before the discovery of "supercooling," and he returned to work with Cohen-Tannoudji the following year. Athough Phillips and Chu never worked together directly, all three physicists were constantly in communication with one another, sharing information or results. "That collaboration continues today," Phillips says, "and the information exchange includes many other groups."
Not only did supercooling lead to a new theoretical framework for understanding laser cooling, it also helped to confirm some earlier theories that had never been tested. In 1995, a team of scientists from NIST and the University of Colorado used supercooling techniques as a first step to create a new state of matter that had been predicted in the 1920s by Albert Einstein, using some new ideas from the Indian physicist Satyendra Nath Bose. This new state of matter, a new kind of gas called a "Bose-Einstein condensate," occurs when the gas is very cold and dense and a large fraction of the atoms essentially stops moving.
Slowing atoms by laser cooling has practical applications, too. One that particularly interests Phillips is the creation of new generations of atomic clocks. Atomic clocks are the most accurate timekeepers that can be made because they use the "ticking" frequency of cesium atoms as a standard for measuring seconds, minutes, and hours. Free cesium atoms tick at the same frequency everywhere, but in order to use this regular ticking rate to control an atomic clock, one must synchronize the "ticking" of some ordinary device, like a quartz clock, with the cesium's ticking. If the cesium atoms are moving too fast (they normally move at about 200 meters per second), they don't stay in the apparatus very long, and it is difficult to get an accurate measurement. By slowing the atoms' movement to one centimeter per second, scientists have made atomic clocks that are a billion times more accurate than an ordinary wristwatch.
Highly accurate clocks are essential to navigation instruments and other devices that use the Global Positioning System (GPS). The GPS depends on atomic clocks that circle the earth in satellites. By comparing time information from several satellites, GPS receivers in cars, airplanes, or hand-held instruments can determine their location on earth with an accuracy of just a few meters.
In April 1997, Phillips was elected to the National Academy of Sciences, one of the highest honors that a scientist in the United States can attain. Six months later, Phillips was notified that he had been selected, along with Chu and Cohen-Tannoudji, to receive the 1997 Nobel Prize in Physics. The Royal Swedish Academy of Sciences awarded it to them jointly as "leaders and representatives" of their respective groups.
At the award ceremony in Stockholm, they were instructed in the protocol to be followed in receiving the Nobel Prize. After bowing to the Swedish king, the queen, the members of the Academy, and the audience, Phillips spontaneously blew a kiss to his wife. That gesture "was not part of the instructions we were given," he recalls, "but apparently it was not too severe [a breach of etiquette], because the queen later told me it was very sweet."
Phillips's great enthusiasm for his work is evident to all who meet him, but work doesn't keep him from enjoying many other activities. His coworkers describe him as "one of the greatest guys in the world." He is devoted to his wife and two daughters, teaches Sunday school, and is active in his church's gospel choir. He also is "a card-carrying Girl Scout" and participates in Girl Scout programs bringing science to girls.
The awesome success of winning the Nobel Prize has not diminished Phillips's innate curiosity. To make atomic clocks that are even more accurate than the best instruments on earth, a team at NIST is preparing to conduct experiments on an atomic clock that is scheduled to be orbited on the International Space Station in 2005. By avoiding the effects of gravity, they hope to slow supercooled atoms even further and measure time more precisely than has ever been done before. From his childhood explorations of the world around him, Phillips has come to the scientific study of things that are, in his words, "literally out of this world"-- still feeling, as he did when he first looked through a microscope, the excitement of discovery.
That excitement of discovery can be infectious. Students who attended Phillips's Innovative Lives presentation returned to their classrooms eager to learn more about physics. One teacher reported that Phillips "certainly inspired the students. Several are already talking about next year's Science Fair project." They are following news about the International Space Station, taking apart clocks and watches to use the pieces in new machines they invent, and learning to be more patient and precise in their work. Perhaps some of these students will become physicists and even--inspired by Phillips--candidates for a Nobel Prize.
Teachers, explore the history of Nobel Prize further with your students with our "The Nobel Prize: 100 Years of Creativity and Innovation" interactive CD-ROM and teachers' guide. Request the CD-ROM package.
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