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 one second in about 3.7
billion years, according to measurements to be reported in 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 logic process is described at http://www.nist.gov/public_affairs/releases/logic_clock/logic_clock.html#background.)
The second version of the logic clock offers more than twice the precision of
"This paper is a milestone for atomic clocks" for a number of reasons,
says NIST postdoctoral researcher James Chou, who developed most of the improvements.
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
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, and no clock can be more accurate than cesium-based
standards such as NIST-F1.
The logic clock is based on a single aluminum ion (electrically charged atom)
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.
NIST scientists evaluated the new logic clock by probing the aluminum ion with
a laser to measure the exact "resonant" frequency at which the ion
jumps to a higher-energy state, carefully accounting for all possible deviations
such as those caused by ion motions. No measurement is perfect, so the clock's
precision is determined based on how closely repeated measurements can approach
the atom's exact resonant frequency. The smaller the deviations from the true
value of the resonant frequency, the higher the precision of the clock.
Physicists also evaluate the performance of new optical clocks by comparing
them to older optical clocks. In this case, NIST scientists compared their two
logic clocks by using the resonant laser frequency from one clock to probe the
ion in the other clock. Fifty-six separate comparisons were made, each lasting
between 15 minutes and 3 hours.
The two logic clocks exhibit virtually identical "tick" rates—differences
don't show up until measurements are extended to 17 decimal places. The agreement
between the two aluminum clocks is more than 10 times closer than any previous
two-clock comparison, with the lowest measurement uncertainty ever achieved
in such an evaluation, according to the paper.
The enhanced logic clock differs from the original version in several ways.
Most importantly, it uses a different type of "partner" ion to enable
more efficient operations. Aluminum is an exceptionally stable source of clock
ticks but its properties are not easily manipulated or detected with lasers.
In the new clock, a magnesium ion is used to cool the aluminum and to signal
its ticks. The original version of the clock used beryllium, a smaller and lighter
ion that is a less efficient match for aluminum.
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.