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
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
Second ‘Quantum Logic Clock’ Based on Aluminum Ion is Now World’s Most
Precise Clock.” [www.nist.gov/public_affairs/releases/logicclock_020410.html]
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
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.
Media Contact: Laura Ost, firstname.lastname@example.org, (303) 497-4880