The Best Clock on Earth

Physicists have created a 1-meter-high fountain of cold cesium atoms that could be used to build the most precise timepiece ever known. Described in the 7 June issue of Physical Review Letters, the new "clock" would be more than 10 times as accurate as any other known chronometer.

The electrons orbiting a cesium atom are like tiny magnets; each has a north and a south pole. Like two ordinary magnets, two electrons "repel" each other--and the total energy increases--when their magnetic orientations, or spins, are aligned. But unlike household magnets, quantum mechanics dictates that two electrons circling a nucleus in the same orbit of an atom must be either completely aligned or completely misaligned; there is no in-between. Quantum laws also say that the frequency of light required to make an electron "flip" into the higher energy state--that is, become aligned magnetically with another electron--is proportional to the energy difference between the states.

Conventional atomic clocks work by tuning microwaves until they are exactly the right frequency to flip the spins in a beam of cesium atoms. That provides a frequency standard, which can be electronically converted into lower frequencies for measuring time. But the atoms in the beam interact only fleetingly with the microwaves, limiting the precision with which the microwaves and the atoms can be matched.

Now a team led by physicist Andre Clairon of the Paris Observatory in France has stretched out the interaction time drastically by using a trick with two laser beams to launch a single "ball" of 600,000 cesium atoms into a vacuum. The atoms interacted with microwaves twice, half a second apart, during their slow rise and fall. By tuning the frequency of the microwave light so that all the atoms in the ball are flipped, the light's frequency can be held almost perfectly constant at 9 gigahertz, as Clairon's team verified by comparing the microwave frequency to a hydrogen laser and the microwaves from three other independent cesium fountains. "It's a three-cornered hat system," says team member Giorgio Santarelli.

Yale University physicist Kurt Gibble says Clairon's invention is so precise that the only limits are posed by the intrinsic quantum uncertainty of the atoms themselves. Atomic collisions in the ball cause uncertainty by shifting the frequency needed to flip the atoms, Gibble explains. "But there are still a few tricks left" to improve on accuracy, he adds. New "juggling fountains" that keep as many as 30 balls in the air and average the results will become 10 times more accurate than a simple fountain in about a year, he predicts.