NIST logo

Mercury Atomic Clock Sets Time-Keeping Record

From NIST Tech Beat: July 20, 2006


Contact: Laura Ost
(301) 975-4034

NIST physicist Jim Bergquist with the world's most accurate clock. The silver cylinder in the foreground is a magnetically shielded cryogenic vacuum system, which holds the heart of the clock— a single mercury ion cooled to near absolute zero.
©Geoffrey Wheeler
For a high-resolution version of this photo contact

An experimental atomic clock based on a single mercury atom is now at least five times more precise than the national standard clock, according to a paper by physicists at the National Institute of Standards and Technology (NIST) in the July 14 issue of Physical Review Letters*.

The experimental clock, which measures the oscillations of a mercury ion (an electrically charged atom) held in an ultra-cold electromagnetic trap, 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. Higher frequencies allow time to be divided into smaller units, which increases precision.

A prototype mercury optical clock originally was demonstrated at NIST in 2000. Over the last five years its absolute frequency has been measured repeatedly with respect to NIST-F1. The improved version of the mercury clock is the most accurate to date of any atomic clock. 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.

Improved time and frequency standards have many applications, including improved synchronization in navigation and positioning systems, telecommunications networks, and wireless and deep-space communications. Better frequency standards also can be used to improve probes of magnetic and gravitational fields for security and medical applications, and to measure 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.

Funding for the research was provided by NIST and the Office of Naval Research. For more detail, see:

* 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.