For a decade, single-ion optical clocks have been the world’s most accurate timekeepers. By counting the “ticks” of an oscillating laser field tuned to a transition of a lone, trapped atom, the clocks can measure time to 1 part in 1017—exquisite enough to detect the subtle gravity-induced time dilation resulting from a change in elevation of mere tens of centimeters. (See Physics Today, November 2010, page 16.) In recent years, however, some groups have been developing an alternative strategy, based on measurements not of single atoms but of ensembles of atoms trapped in optical lattices. Because the clocks probe many atoms at once, they more efficiently average out the statistical noise inherent in quantum measurements. Now a group led by Jun Ye (JILA, Boulder, Colorado) has built an optical-lattice clock, based on an electronic transition in strontium-87, that’s more accurate than even the best single-ion clocks. (The photo shows the clock’s viewport; a cloud of trapped, fluorescing Sr atoms is visible at the center.) Key to the achievement was the group’s precise characterization of systematic errors caused by atomic interactions, radiation, and other environmental factors. With a fractional uncertainty of just 6.4 × 10−18, the new timepiece is so accurate that it could run for 5 billion years—more than the age of Earth—without missing a second. One potential application: testing the variability of physical constants such as the fine-structure constant, which some celestial observations suggest is gradually changing over time. (B. J. Bloom et al., Nature, in press.)—Ashley G. Smart