2019-09-02 Bloomberg Businessweek

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The lutetium-based atomic clock at the National University of Singapore

radiation.Soif thenumberofcycles

ofradiationnecessarytostimulatean

atomcanbecountedduringthebest

existingnon-atomicdefinitionofa sec-

ond,theworldwouldhavea newdefi-

nitionofthesecondgroundedinthe

unchangingatom.Ineffect,it’dhavean

atomicclock.

Overa periodofalmosttwodecades,

scientists determined that cesium,

whosevibrationsarecomparatively

easyto observeandwhichexistsin

onlyonestableform,andthuswouldn’t

needtobepurified,wasthebestele-

ment to use. In 1967, the General

ConferenceonWeightsandMeasures,

anintergovernmentalorganization,

replacedtheoldcelestialdefinition

ofa secondwiththeamountoftimeit

takestomeasure9,192,631,770oscilla-

tionsofthemicrowaverequiredtostim-

ulatevibrationofcesium-133.

Evenbeforetheatomicsecondwas

defined,therewerescientific,military,

andcommercialventuresdeeplyinter-

estedintakingadvantageofmorepre-

cisetiming.TheU.S.militarywasone

oftheearliestpatronsofatomicclocks,

and in 1973 thearmedforcesintro-

ducedtheglobalpositioningsystem,

a super-accuratenavigationnetwork

thatusessatellitesoutfittedwithatomic

clockstomeasurehowlongit takesa

signaltoreachandbouncebackfrom

a receiveronEarth.

Missiles andothermilitary tech-

nologieswere theimmediate bene-

ficiaries of GPS.Theprivatesector

benefited,too,fromGoogleMapsto

moderntelecommunicationnetworks

thatsynchronizeusingGPS.According

toa June 2019 studysponsoredbythe

National Instituteof Standardsand

Technology(NIST)inColorado,GPS

wasresponsiblefor$1.35trillionineco-

nomicbenefitstotheU.S.from 1984 to

2017.China,Europe,andJapanhave

theirownsatellitenavigationsystems

relyingonatomicclocks;a collective

outage,asunlikelyasthatis,would

crippletheglobaleconomy.

Fornearlytwodecadesscientists

aroundtheworldhavebeenworking

onimprovedatomicclocksthatstim-

ulateatomsusingvisiblelight,which

oscillates roughly 100,000 times faster

than the microwaves in a cesium clock.

Creating these optical clocks isn’t just a

matter of switching out microwaves for

lasers and cesium for another element.

Instead, scientists must overcome a

range of scientific and technical chal-

lenges, including the fundamental ques-

tion of which element is best suited for

the work of telling time.

There’s no lack of candidates, includ-

ing aluminum, mercury, and strontium.

For now, the front-runner appears to

be ytterbium, which is being investi-

gated by large, well-funded teams at

NIST and labs in Europe and Asia. In late

2018, NIST announced that a pair of

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