NIST's Second 'Quantum Logic Clock' is Now World's Most Precise Clock

Physicists at the National Institute of Standards and Technology (NIST) have built an enhanced version of an experimental atomic clock based on a single aluminum atom that is now the world's most precise clock, more than twice as precise as the previous pacesetter based on a mercury atom. The new aluminum clock would neither gain nor lose a second in about 3.7 billion years, according to measurements to be reported in a forthcoming issue of Physical Review Letters.

The new clock is the second version of NIST's "quantum logic clock," so called because it borrows the logical processing used for atoms storing data in experimental quantum computing, another major focus of the same NIST research group. The second version of the logic clock offers more than twice the precision of the original. In addition to demonstrating that aluminum is now a better timekeeper than mercury, the latest results confirm that optical clocks are widening their lead—in some respects—over the NIST-F1 cesium fountain clock, the U.S. civilian time standard, which currently keeps time to within 1 second in about 100 million years.

Because the international definition of the second (in the International System of Units, or SI) is based on the cesium atom, cesium remains the "ruler" for official timekeeping, so technically no clock can be more accurate than cesium-based standards such as NIST-F1.

The logic clock is based on a single aluminum ion trapped by electric fields and vibrating at ultraviolet light frequencies, which are 100,000 times higher than microwave frequencies used in NIST-F1 and other similar time standards around the world. Optical clocks thus divide time into smaller units, and could someday lead to time standards more than 100 times as accurate as today's microwave standards. Higher frequency is one of a variety of factors that enables improved precision and accuracy.

Aluminum is one contender for a future time standard to be selected by the international community. NIST scientists are working on five different types of experimental optical clocks, each based on different atoms and offering its own advantages. NIST's construction of a second, independent version of the logic clock proves it can be replicated, making it one of the first optical clocks to achieve that distinction. Any future time standard will need to be reproduced in many laboratories.

Clocks have myriad applications. The extreme precision offered by optical clocks is already providing record measurements of possible changes in the fundamental "constants" of nature, a line of inquiry that has important implications for cosmology and tests of the laws of physics, such as Einstein's theories of special and general relativity. Next-generation clocks might lead to new types of gravity sensors for exploring underground natural resources and fundamental studies of the Earth. Other possible applications may include ultra-precise autonomous navigation, such as landing planes by GPS.

For more on this story, see the NIST Feb. 4 news release "NIST's Second 'Quantum Logic Clock' Based on Aluminum Ion is Now World's Most Precise Clock."  For additional info on the application of quantum logic to timekeeping, see the March 6, 2008, NIST news release "NIST 'Quantum Logic Clock' Rivals Mercury Ion as World's Most Accurate Clock." This work was supported in part by the Office of Naval Research.

C.-W. Chou, D.B. Hume, J.C.J. Koelemeij, D.J. Wineland and T. Rosenband. Frequency comparison of two high-accuracy Al+ optical clocks. Physical Review Letters. Forthcoming. A preprint is available at http://arxiv.org/abs/0911.4527.