Most Accurate Clock

Photo: NIST physicist Jim Bergquist holds a portable keyboard used to set up the world's most accurate clock. The single mercury ion is contained in the silver cylinder in the foreground ©Geoffrey Wheeler (Courtsey: National Institute of Standards and Technology)

A path-breaking research paper by physicists at the National Institute of Standards and Technology (NIST) in the July 14 issue of Physical Review Letters describes an experimental atomic clock based on a single mercury atom, which at present is at least five times more precise than the national standard clock. The experimental clock consists of a silver cylinder which acts as a magnetic shield that surrounds a cryogenic vacuum system. The heart of the clock, a single mercury ion (electrically charged atom) is brought to rest inside this chamber by laser-cooling it to near absolute zero. The optical oscillations of the essentially motionless ion are used to produce the "ticks" or "heartbeat" of the world's most stable and accurate clock.

The mercury ion ticks at “optical” frequencies—much higher than the microwave frequencies measured in cesium atoms in NIST-F1, the national standard and one of the world’s most accurate clocks. This achievement of shifting the operation to higher frequencies allows time to be divided into smaller units and reach greater precision.

The current version of NIST-F1 —if operated continuously—would neither gain nor lose a second in about 70 million years. The latest version of the mercury clock would neither gain nor lose a second in about 400 million years.

This improved time and frequency standards would eventually lead to improved synchronization in navigation and positioning systems, telecommunications networks, and wireless and deep-space communications and would allow designing improved probes of magnetic and gravitational fields for security and medical applications. This would also let physicists investigate whether “fundamental constants” used in scientific research might be varying over time—a question that has enormous implications for understanding the origins and ultimate fate of the universe.

Here is the reference for the paper:
W.H. Oskay, S.A. Diddams, E.A. Donley, T.M. Fortier, T.P. Heavner, L. Hollberg, W.M. Itano, S.R. Jefferts, M.J. Jensen, K. Kim, F. Levi, T.E. Parker and J.C. Bergquist. 2006. A single-atom optical clock with high accuracy. Physical Review Letters. July 14.