A single atom, isolated and motionless, should be an excellent clock. It can radiate signals that are free of Doppler shifts and distortions introduced by its interactions with the environment. Just such thinking has motivated much work on ion traps, and that research is now bearing fruit: A group working at the National Institute of Standards and Technology in Boulder has measured the frequency of an ultraviolet transition in a single mercury atom with a linewidth under 180 Hz. The work was reported by James Bergquist, Frank Dietrich (now at Gsa¨nger Optoelectronik, FRG), Wayne Itano and David Wineland at the Ninth International Conference on Laser Spectroscopy, held in Bretton Woods, New Hampshire, last June. Before this recent feat, the narrowest width measured for an optical frequency had been on the order of a few kilohertz.