An experimental atomic clock based on ytterbium atoms is about four times
more accurate than it was several years ago, giving it a precision comparable
to that of the NIST-F1 cesium fountain clock, the nation's civilian time
standard, scientists at the National
Institute of Standards and Technology (NIST) report in Physical Review Letters.*
NIST scientists evaluated the clock by measuring the natural frequency of ytterbium,
carefully accounting for all possible deviations such as those caused by collisions
between the atoms, and by using NIST-F1 as a "ruler" for comparison.
The results were good enough to indicate that the ytterbium clock is competitive
in some respects with NIST-F1, which has been improving steadily and now keeps
time to within 1 second in about 100 million years. (Because the international
definition of the second is based on the cesium atom, technically no clock can
be more accurate than cesium standards such as NIST-F1.) More importantly, the
improved ytterbium clock gives the time standards community more options in
the ongoing development and comparisons of next-generation clocks, says NIST
physicist Chris Oates, an author of the new paper.
The NIST ytterbium clock is based on about 30,000 heavy metal atoms that are
cooled to 15 microkelvins (close to absolute zero) and trapped in a column of
several hundred pancake-shaped wells-an "optical lattice"-made
of laser light. A laser that "ticks" 518 trillion times per second
induces a transition between two energy levels in the atoms. The clock's
enhanced performance was made possible by improvements in the apparatus and
a switch to a different form of ytterbium whose nucleus is slightly magnetic
due its "spin-one half" angular momentum. This atom is less susceptible
to key errors than the "spin-zero" form of ytterbium used previously.
NIST scientists are developing five versions of next-generation atomic clocks,
each using a different atom and offering different advantages. The experimental
clocks all operate at optical (visible light) frequencies, which are higher
than the microwave frequencies used in NIST-F1, and thus can divide time into
smaller units, thereby yielding more stable clocks. Additionally, optical clocks
could one day lead to time standards up to 100 times more accurate than today's
The best optical clocks are currently based on single ions (electrically charged
atoms), such as the NIST "logic clock" using an aluminum ion (see
"NIST 'Quantum Logic Clock' Rivals Mercury Ion as World's
Most Accurate Clock".) But lattice clocks have the potential for higher
stability because they simultaneously average signals from tens of thousands
of atoms. Ongoing comparisons of the ytterbium clock to that of the strontium
lattice clock located nearby at JILA, a joint institute of NIST and the University
of Colorado at Boulder, (see "Collaboration Helps Make JILA Strontium
Atomic Clock 'Best in Class'") should help enable worldwide
tests of optical clock performance with extremely high precision. JILA is At
this point it is far from clear which atom and clock design will be selected
by research groups around the world as a future time and frequency standard.
Advances in atomic clock performance support development of technologies such
as high data rate telecommunications and the Global Positioning System (GPS).
Optical clocks are already providing record measurements of possible changes
in the fundamental "constants" of nature, a line of inquiry that
has huge 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.
* N. D. Lemke, A. D. Ludlow, Z.W. Barber, T. M. Fortier, S.A. Diddams, Y. Jiang,
S. R. Jefferts, T. P. Heavner, T. E. Parker and C.W. Oates. Spin-1/2 optical
lattice clock. Physical Review Letters. Published online Aug. 3, 2009.