Thanks to the first ever all-optical atomic clock, physicists can now divide time down to the femtosecond. In today's issue of Science, Scott A. Diddams and colleagues at the National Institute of Standards and Technology in Boulder, CO, the Max-Planck-Institut fr Quantenoptik in Garching, Germany and at the University of Colorado-Boulder, describe the instrument, which they say will "provide an even finer-grained view of the physical world" and help researcers "observe physical 'constants' evolve in time."

Since 1967, scientists have defined a single second according to microwave-frequency transitions in cesium, "ticks" that occur roughly once each nanosecond. Optical transitions in atoms take place more frequently and can thus meter out smaller divisions of time. But until recently, researchers had no way to tally the faster ticks. Now using a femtosecond laser, Diddams's team succeeded in counting optical transitions in a single, cooled mercury ion. These flips take place about 1.064 quadrillion times a second, or once per femtosecond. Not only is the new timepiece more precise, it's smaller too: in all, it occupies a compact space measuring 120 centimeters by 60 centimeters (see image).

Image: courtesy of SCOTT DIDDAMS