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METROLOGY ond, “probably correct is not good enough.”
He notes that something unexpected—
Better atomic clocks herald perhaps a stray electric field—could throw
off two clocks in the same lab by the same
new era of timekeeping amount, making the drift undetectable.
calls
a number
for
map
The BIPM road
of cross-checks, including reaching the
Metrologists move to redefine the second with a visible required accuracy with clocks in differ-
ent labs. Another check involves compar-
light standard that would boost accuracy by a factor of 100 ing the ticking of different types of atomic
clocks. The NIST scientists and colleagues
at JILA, a research institute down the road
By Edwin Cartlidge frequencies are a challenge to build. Im- in Boulder, are doing just that, comparing
provements in laser technology led BIPM the ytterbium clocks to others that rely
he atomic clocks that mark offi- a few years ago to begin reviewing the ac- on strontium atoms and aluminum ions.
cial time lose the equivalent of just curacy of optical clocks. On 14 February it Ludlow says the measurements are not
1 second every 200 million years. But published a paper in its journal, Metrolo- far off the desired accuracy, and that once
metrologists are not satisfied. A more gia, setting out five milestones that should they’ve finished they can compare their re-
precise time standard might improve be met before the second can be redefined sults with labs in Europe or Asia.
T the navigation of spacecraft and help based on visible light. And in unpublished One issue the road map does not ad-
experimenters look for variations in fun- work, Andrew Ludlow and colleagues at dress, Ludlow says, is how to choose which
damental constants that would signal new NIST appear to have reached the accuracy atomic transition—and hence type of
physics. So the push is on to replace current stipulated by BIPM’s first milestone. clock—will ultimately come to define the
clocks, which are tuned to a specific micro- The NIST team operated two optical second. He prefers clocks made from lat- Downloaded from
wave frequency, with even better clocks that clocks, using several lasers to cool and tices of neutral atoms because they need
exploit higher-frequency visible light. trap a few thousand ytterbium atoms in only run for a few hours before reaching
In a paper out last month, their stated accuracy, whereas
a group of experts set up by rival devices made from single
the International Bureau of trapped ions might need weeks
Weights and Measures (BIPM) to do that. But he acknowl-
in Sèvres, France, lays out a edges that the jury is still out.
road map for the steps needed “Ion clocks have made great http://science.sciencemag.org/
to redefine the unit of time— advances in the last decade
the metric second—in terms of and they keep getting better,”
optical radiation. Already, phys- he says.
icists at the National Institute Patrick Gill, a laser physicist
of Standards and Technology at the National Physical Labo-
(NIST) Boulder Laboratories in ratory in Teddington, U.K., says
Colorado appear to have satis- the switchover to optical clocks
fied one of the road map’s key shouldn’t happen while they on March 1, 2018
requirements—a 100-fold im- are all improving so quickly. He
provement in accuracy over the says that officials might agree
best microwave clocks—using a on a new definition when the
pair of optical clocks. world’s top metrological body—
Clocks mark time by track- the General Conference on
ing a periodic action. A grand- A cloud of cold strontium atoms, glowing with a blue light, is trapped in the vacuum Weights and Measures—meets
father clock relies on the regu- chamber of an optical clock at Germany’s National Metrology Institute. in 2026. (The organization
lar swings of a pendulum, meets every 4 years and this
and the original definition of the second an “optical lattice” and then excite a par- year is expected to approve new definitions
was based on the length of a day as fixed ticular energy transition in those atoms. for four other metric base units: the kilo-
by Earth’s spin. Current atomic clocks de- The researchers found that the two clocks gram, the ampere, the kelvin, and the mole.)
pend on the oscillations of a microwave ticked at the same rate to within 1.4 parts Ekkehard Peik, head of the time and
18
beam at the precise frequency needed to in 10 —just over 100 times better than the frequency group at Germany’s National
excite atoms of cesium-133 to a higher top cesium devices. “It would be the first Metrology Institute in Braunschweig, is
energy level. In 1967, the second was de- time that two clocks of the same species more cautious still. Arguing that cesium
fined as 9,192,631,770 cycles of a beam have been shown to agree at that level,” clocks are accurate enough for today’s ap-
tuned to the cesium standard. Today, the Ludlow says. plications, he says a redefinition can prob-
best cesium clocks have accuracies of Being a cautious bunch, metrologists will ably wait until the early 2030s, offering PHOTO: PHYSIKALISCH-TECHNISCHE BUNDESANSTALT/CC-BY
1.6 parts in 10 16. not accept the NIST result at face value. more time for scientists to duke it out with
The frequency of visible light is about Jérôme Lodewyck, a physicist working on competing clocks. Such rivalry, he says, “is
100,000 times higher than that of micro- strontium lattice clocks at the Paris Obser- what also drives progress and one should
waves, promising even more precision. vatory’s Time-Space Reference Systems lab, not be afraid of it.” j
However, the lasers needed to cool atoms says the NIST result “is probably correct,”
and provide a stable reference at these but that when it comes to changing the sec- Edwin Cartlidge is a journalist in Rome.

968 02 MARCH 2018 • VOL 359 ISSUE 6379 sciencemag.org SCIENCE
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