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American physicist Steven Chu shared the 1997 Nobel Prize in physics with two other researchers. Chu was honored for his work in cooling and trapping atoms and other small particles.
Steven Chu, born in 1948, American physicist and Nobel laureate. Chu led a team of physicists in the mid-1980s who were the first to trap atoms (one of the basic units of matter) with special beams of light called lasers. These laser traps (see Particle Trap: Neutral Particle Traps) allow scientists to study atoms more closely and use atoms more efficiently in devices such as atomic clocks. Chu’s research helped pave the way for important discoveries in atomic physics. It also led to the development of many new practical applications, including more accurate atomic clocks and more precise devices for measuring the pull of gravity. He shared the 1997 Nobel Prize for physics with two other researchers who made separate and complementary advancements in the field, French physicist Claude Cohen-Tannoudji and American physicist William D. Philips.
Chu was born in St. Louis, Missouri, and grew up on Long Island, New York. He earned dual undergraduate degrees in physics and mathematics at the University of Rochester in New York in 1970. In 1976 he received his doctoral degree in physics from the University of California, Berkeley. Chu spent two years conducting post-doctoral research at Berkeley, then served as a member of the technical staff of American Telephone and Telegraph (AT&T) from 1978 to 1983.
In 1983 Chu was named head of the quantum electronics research department at AT&T’s Bell Laboratories and moved to the lab’s complex in Holmdel, New Jersey. That year he began discussing the possibility of trapping atoms with American physicist Arthur Ashkin. Over the next few years, Chu, Ashkin, and fellow American physicists John Bjorkholm and Alex Cable conducted experiments in which six lasers bombarded atoms from six sides in a vacuum chamber at Chu’s lab.
At room temperature, atoms move at speeds of about 4000 km/h (2500 mph), much too fast for scientists to easily study them. The speed of atoms is related to their temperature—atoms at a higher temperature move faster than cooler atoms. Slowing a sample of atoms will therefore make the atoms cooler. Chu pioneered a technique for slowing and cooling atoms that uses lasers to immerse the atoms in photons (packets of light wave energy). The photons strike the atoms in a way that is roughly analogous to raindrops hitting a beach ball. The photons have no mass, but because they move at the speed of light, they carry momentum and can affect a small mass, such as an atom. In Chu’s trap, the impact of enough photons hitting atoms slowed the atoms down. The atoms moved so slowly that they seemed to be stuck, so Chu’s team named the process “optical molasses.”
In 1985 Chu and his team cooled atoms to 240 millionths of a Celsius degree (430 millionths of a Fahrenheit degree) above absolute zero, the point at which all matter stops moving (–273.15° C, or -459.67° F). By 1988 William D. Philips and his research team had cooled atoms to 40 millionths of a Celsius degree (70 millionths of a Fahrenheit degree) above absolute zero, which was lower than scientists believed to be theoretically possible at the time. Claude Cohen-Tannoudji and his team achieved a temperature of 0.2 millionths of a Celsius degree (0.4 millionths of a Fahrenheit degree) above absolute zero in 1995.
The ability to manipulate atoms and study them more closely led to many real and potential applications, including increased accuracy in atomic clocks, which improved their use in space navigation and global positioning systems. Atomic clocks keep track of time by counting waves of radiation emitted by special atoms in traps inside the clock. If the traps can hold the atoms at lower temperatures, the traps and mechanisms inside the clock can exercise more control over the atom, reducing the possibility of error. Atom manipulation also contributes to increased accuracy of the measurement of gravitational force, which is useful in, among other things, oil exploration. This is because a deposit of oil or other substance beneath the earth’s crust has a different density from the areas around it. Changes in density produce changes in the local gravitational field, because the gravitational force of an area is related to the mass of that area. Advances in the manipulation of atoms have also raised the possibility of using atoms to etch electronic circuits, thereby increasing the circuits’ capabilities.
In 1987 Chu joined the faculty at Stanford University as a professor of physics and applied physics. While at Stanford, he and his colleagues continued the study of cooled atoms. In 1992 Chu was named a fellow of the American Academy of Arts and Sciences and a year later a member of the National Academy of Sciences.