Corralling the Elusive Electron

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UW physicist Hans Dehmelt and colleagues have developed ways to trap and immobilize single atomic and subatomic particles, holding them suspended in space and carrying out extremely accurate measurments on them. The researchers were the first in the world to isolate a single proton, a positron, and a single barium atom, which when illuminated was visible to the naked eye as a tiny blue-white star. With research assistant professor Warren Nagourney and undergraduate student Jon Sandberg, Dehmelt was the first in the world to observe quantum jumps of the barium ion--discrete changes in the ion's electron energy states.

When Dehmelt came to the UW in 1955, he set about trying to isolate a single electron, to hold it, suspended in space, and to probe its deepest secrets. It was a quest inspired in him as a graduate student in Germany when one of his teachers drew a dot on the blackboard and announced, "Here is an electron."

"It was simple to draw that on the blackboard, but deceptive," reflects Dehmelt. "I began to wonder, 'How can I duplicate that feat in the laboratory?'" The quest led Dehmelt to a series of firsts in physics, measurements of extreme precision and elegance that were ultimately recognized by a Nobel Prize in 1989, and the 1995 National Medal of Science, the nation's highest scientific honor.

In 1973, Dehmelt and colleagues succeeded in observing a single electron in a trap, and two years later he introduced an advance for "cooling" the electron in order to improve the accuracy of measurements made on the particle. The special trap required "a fantastically good vacuum" so that the particle under study wouldn't collide with other interfering particles and go careening off into the chamber's walls. The combination of a strong magnetic field and a high electric voltage was used to "persuade the particle not to run away," notes Dehmelt. On top of that, the particle's motion was slowed way down by cooling the chamber to within a few degrees of absolute zero--about negative 460° Fahrenheit.

One characteristic the researchers were able to pin down with unprecedented detail was a quantity called the "g factor" of the electron--essentially the ratio of the magnetic moment and the angular momentum of the electron, which should be equal to two were it not for a slight deviation that arises from the effects of quantum electrodynamics, that is, the interaction of the electron with the surrounding radiation field. Dehmelt determined the magnitude of this anomaly with a precision of a few parts in a billion, a feat that also constitutes one of the most critical tests scientists have of the theory of quantum electrodynamics.

To put Dehmelt's achievements into perspective, consider the number of atoms in a speck of ink the size of a period on this page. The number of atoms in that speck of ink is greater than the number of people that have ever walked on this planet. The world around us is a huge sea of atoms and molecules--we are awash in them. It is hard enough to separate out and work with quantities of atoms as small as a speck of ink, though chemists and physicists manage to do so. But to trap and manipulate one single atom, or one particle inside an atom, such as the electron--that is a difficult feat indeed. Interestingly, Dehmelt's experiments resulted in revising our estimates of the size of an electron downward by a factor of 10,000. It is now believed an electron is on the order of 10-20 centimeter.

Dehmelt and his colleagues continued to refine their techniques for trapping isolated particles. Besides being the first to trap electrons, Dehmelt and colleagues were able to corral other fundamental particles: protons, positrons, anti-protons, and barium ions.

Barium ions were trapped using radio-frequency signals instead of an electric voltage to keep the ions from wandering off. The researchers then devised a way to observe quantum jumps--discrete changes in the electron energy states of the ion.

Quantum jumps are part of the mysterious world of quantum mechanics, which sometimes runs counter to human intuition. As Dehmelt puts it, at the atomic level, "nature is not continuous." If you put energy into an atom, by shining a light on it, for example, it can only absorb certain fixed quantities of energy, or "quanta." The same is true when an atom emits energy. The change from one energy state to another is like going up or down a staircase, instead of a ramp: the changes happen in certain fixed intervals, not by any variable amount. In 1986, the UW group was the first in the world ("by a nose" adds Dehmelt) to directly observe quantum jumps, the faint blinking on and off of a tiny atomic lighthouse.

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