Tick-Tock, a Super Atom Clock

WASHINGTON, D.C.--Physicists have made the first clock based on the quantum ticks of balls of atoms known as Bose-Einstein condensates. The unusual timepiece is extremely primitive, but similar devices could one day replace the traditional atomic clocks that keep the world on time, physicists announced here yesterday at the 1998 Conference on Precision Electromagnetic Measurements.

Today's atomic clocks are pegged to the time it takes for an atom (typically cesium) to switch back and forth between two slightly different configurations--one in which the spin of the electron and the nucleus point in the same direction, and one in which they point in opposite directions. To get an observable signal, clocks usually watch many hundreds of thousands of these atoms. But in such large collections, the atoms can interfere with each other and blur the regular ticking slightly. Ideally, a clock would watch a single, still atom.

Carl Wieman, a physicist at the University of Colorado, Boulder, and the Joint Institute for Laboratory Astrophysics in Boulder, and colleagues managed something similar. Instead of a single atom, the team confined a half-million rubidium atoms in a magnetic trap and cooled them until they coalesced into a Bose-Einstein condensate in which all the atoms fall into the same state and act as a giant "super atom." Using a burst of laser light they formed a second, overlapping condensate in which the atoms were in a slightly different state. Over time, the atoms flip-flopped regularly between the two states like the ticks of a clock. Using another laser they could probe how many atoms were in each state, and essentially keep time. "You can think of this as the first Bose-Einstein Condensate clock, " Wieman says, but "I wouldn't want to push the accuracy on it much."

One problem is that the laser burst that reads the time destroys the clock. And the magnetic fields of the trap will probably also degrade its precision, points out Christopher Oates, a physicist at the National Institute of Standards and Technology in Boulder, Colorado. Condensate clocks may keep time for future generations, he says, but for now the idea is "still in diapers."