US physicists have made an atomic clock that is so accurate it will neither gain nor lose a second over the next 200 million years. The clock outperforms the official atomic clock used by the U.S. Commerce Department's National Institute of Standards and Technology (NIST), which promises to keep accurate time down to the second for 80 million years. The secret to making an extremely accurate clock is speeding up how fast it ticks. "If you make a mistake, you can know about that mistake very fast," said Jun Ye, who developed the atomic clock at the Joint Institute for Laboratory Astrophysics, a collaboration between NIST and the University of Colorado in Boulder. Its pendulum uses thousands of strontium atoms suspended in grids of laser light. This allows the researchers to trap the atoms and measure the movement of energy inside.

The advance was made possible by Boulder's critical mass of state-of-the-art timekeeping equipment and expertise. The JILA strontium clock was evaluated by remotely comparing it to a third NIST atomic clock, an experimental model based on neutral calcium atoms. The best clocks can be precisely evaluated by comparing them to other nearby clocks with similar performance; very long-distance signal transfer, such as by satellite, is too unstable for practical, reliable comparisons of the new generation of clocks. In the latest experiment, signals from the two clocks were compared via a 3.5-kilometer underground fiber-optic cable.

The strontium and calcium clocks rely on the use of optical light, which has higher frequencies than the microwaves used in NIST-F1. Because the frequencies are higher, the clocks divide time into smaller units, offering record precision. Laboratories around the world are developing optical clocks based on a variety of different designs and atoms; it is not yet clear which design will emerge as the best and be chosen as the next international standard. The work reported in Science Express is the first optical atomic clock comparison over kilometer-scale urban distances, an important step for worldwide development of future standards.