Steven Chu was born in St. Louis, Missouri, on February 28, 1948. He attendedthe University of Rochester, from which he received a B.S. in physics and A.B. degree in mathematics. He then continued his education at the University of California at Berkeley, which awarded him a Ph.D. in physics in 1976. He remained at Berkeley for two more years, then accepted a position at Bell Laboratories in New Jersey.
In 1983, he was appointed head of the quantum electronics research departmentat the AT&T Bell Laboratories in Holmdel, New Jersey. It was during thistime that Chu conducted the experiments for which he was awarded the 1997 Nobel Prize for Physics.
This research was motivated by a growing interest on the part of scientists to study the properties of individual particles, such as atoms, molecules, andions. Such an objective would have seemed absurd only a few decades ago. Inthe first place, there are almost no mechanisms for viewing or otherwise measuring individual particles as small as atoms and molecules. In the second place, even if such instruments were available, they would be difficult to use with individual particles since such particles normally move at very great speeds. For example, the molecules of which air is composed move at speeds of about 2,480 miles (4,000 km) per hour at room temperature.
Chu developed a method for slowing down the motion of atoms and molecules sothat their properties could be studied more carefully. The technique that hedeveloped has been termed "atom traps" or "optical molasses" because it restrains atoms and molecules to a relatively small space and prevents them from moving out of that space for a period of time. The principle behind this technique is that a moving particle can be slowed down if it can be bombarded by apulse of radiation. For example, suppose that an atom is moving from left toright across this page with a speed of 2,480 miles (4,000 km) per hour. Thensuppose that one was to fire a pulse of laser radiation from the right to the left, aimed directly at the oncoming atom. When the laser pulse strikes theatom, the atom will absorb energy from the laser pulse and slow down (that is, have an increased energy in the backward direction). In theory, one mighthope to stop the atom simply by hitting it with repeated bursts of radiation.
In fact, the process is not quite that simple. Very shortly after the atom absorbs energy from the laser beam, it emits a photon that contains as much energy as was absorbed. That energy may be given off in any direction. As it does so, the atom recoils in some random direction. Its left-to-right speed hasbeen reduced, but a second laser pulse cannot be used to reduce its speed further in the same way.
Chu solved this problem by designing a "box" whose "walls" consisted of six laser beams arranged in three pairs. The beams were arranged along three parallel axes in such a way that an atom placed in the middle of the system was trapped. No matter which direction the atom tried to move, it was struck with one of the laser beams and was suspended in space. Even with this arrangement,atoms are not brought to a complete standstill. The system is able, however,to reduce the speed of particles to about 30 centimeters per second, a speedthat corresponds to a temperature of about 240 micro kelvin, or 240 millionths of a degree kelvin. Calculations suggest that this temperature may be thelowest that can be obtained by the process described here.
If the above experiment is continued over a period of time, a second effect is observed. Instead of trapping a single particle, groups of particles are collected, all with similar energies and properties. This collection of particles acts like a very thin gas consisting of slowly moving particles. It corresponds to a new state of matter predicted in 1927 by Albert Einstein and now known as a Bose-Einstein condensate (BSC). The BSC has properties distinct from those of the other four states of matter (solid, liquid, gas, and plasma) and appears to behave in some respects like a single "super-atom."
In 1987, Chu accepted a professorship at Stanford University. There, Chu hascontinued his work on atom traps and optical molasses, but has also found newapplications of these techniques. In these applications, laser beams are used to trap the ends of molecules and hold them in suspension long enough for their properties to be studied. The laser beams used in this way are sometimesreferred to as "optical tweezers."
One of the interesting aspects of Chu's work with optical tweezers has been the study of deoxyribonucleic acid (DNA) molecules. As with protein molecules,DNA molecules consist of very long polymeric chains that have complex levelsof organization. For example, they form spiral staircase-like coils whose structures, in that configuration, are sometimes difficult to study. Chu has found ways to use his "optical tweezers" laser beams to stretch DNA molecules out to a straight, spaghetti-like string, and then to study their properties in that form. One of the interesting discoveries he has made is that unraveledDNA molecules tend to vibrate, when agitated, in much the same way as a guitar string. His team had expected, Chu explains, that the vibration of a single DNA molecule might be more complex or, at least, different from that of a plucked string. Instead, the team has discovered that the behavior of a plucked strand of DNA can be described completely, simply by applying well known and traditional laws of simple harmonics.
In another series of experiments, Chu and his co-workers have found that identical DNA strands unravel when exposed to mild perturbations in completely different ways. This discovery is remarkable since one would assume that two molecules with identical chemical structures would behave in the same way. These results indicate that some factors are responsible for differences in the unraveling process that DNA molecules experience.