An atomic clock that uses an aluminum atom to apply the logic of computers to the peculiarities of the quantum world now rivals the world's most accurate clock, based on a single mercury atom. Both clocks are at least 10 times more accurate than the current U.S. time standard.
The measurements, described in a new paper,* were made in a yearlong comparison of the two next-generation clocks, both designed and built at the National Institute of Standards and Technology (NIST). The clocks were compared with record precision, allowing scientists to measure the relative frequencies of the two clocks to 17 digits—the most accurate measurement of this type ever made. The comparison produced the most precise results yet in the worldwide quest to determine whether some of the fundamental constants that describe the universe are changing slightly over time, a hot research question that may alter basic models of the cosmos.
The aluminum and mercury clocks are both based on natural atomic vibrations and would neither gain nor lose one second in over 1 billion years—if they could run for such a long time—compared to about 80 million years for NIST-F1, the U.S. time standard based on cesium atoms.
The mercury clock, first demonstrated in 2000, continues its reign as the world's most accurate for now, by a margin of 20 percent over the aluminum clock, but the designers say both experimental clocks could be improved further. The aluminum clock has an advantage in being relatively insensitive to background magnetic and electric fields and temperature. It has the lowest known sensitivity of any atomic clock to temperature.
Both the clocks are based on ions vibrating at optical frequencies, which are 100,000 times higher than microwave frequencies used in NIST-F1 and other similar time standards. Because optical clocks divide time into smaller units, they can be far more precise. Highly accurate clocks are used to synchronize telecommunications networks and deep-space communications, and for satellite navigation and positioning. Next-generation clocks may also lead to new types of gravity sensors, which have potential applications in exploration for underground natural resources and fundamental studies of the Earth.
The new paper provides the first published evaluation of the operational quantum logic clock, so-named because it is a spin-off of NIST research on quantum computers. The NIST quantum logic clock uses two different kinds of ions, aluminum and beryllium, confined closely together in an electromagnetic trap and slowed by lasers to nearly "absolute zero" temperatures. Aluminum is a stable source of clock ticks, but its properties cannot be detected easily with lasers. The NIST scientists applied quantum computing methods to share information from the aluminum ion with the beryllium ion, so they could detect the aluminum clock's ticks by observing light signals from the beryllium ion.
The work was supported in part by the Office of Naval Research and Disruptive Technology Office. For more details, photos and video, see "NIST 'Quantum Logic Clock' Rivals Mercury Ion as World's Most Accurate Clock"
* T. Rosenband, D.B. Hume, P.O. Schmidt, C.W. Chou, A. Brusch, L. Lorini, W.H. Oskay, R.E. Drullinger, T.M. Fortier, J.E. Stalnaker, S.A. Diddams, W.C. Swann, N.R. Newbury, W.M. Itano, D.J. Wineland and J.C. Bergquist. Frequency ratio of Al+ and Hg+ single-ion optical clocks; metrology at the 17th decimal place. Science Express. Published online March 6, 2008.