62
ytterbium-based clocks had set records
for precision, stability, and reproducibil-
ity of results. Lutetium was a late entry to
this intense and expensive competition.
Yet, in a relatively short time, it’s proven
itself a worthy and intriguing dark horse,
thanks to Murray Barrett.B
arrett didn’t aspire to be a clock-
maker. After completing his
Ph.D. at Georgia Tech in 2002,
where he studied atomic physics, he
did a two-year postdoctoral fellow-
ship at NIST in the quantum computing
and information program. He returned
briefly to New Zealand before taking
up his current position in Singapore.
His research initially focused on what
he calls “quantum information stuff,” a
field that marries computing to the tiny
world of quantum physics. According
to Barrett, he was in his office one day
and “just happened to look at lutetium
and got an interest in precision measure-
ment.” He pauses at the skepticism on
my face, then adds: “It really is as arbi-
trary as that.”
Even temperature changes so minute
they wouldn’t register on earlier genera-
tions of atomic clocks can be a source of
error for super-sensitive optical clocks.
Lutetium, it turns out, is uniquely insen-
sitive to temperature shifts. “And I would
argue,” Barrett says, “whatever atom has
the least sensitivity to its environment
will make the better clock.”
Standing in his lab, surrounded by
home-built lasers and electronics, some
of which sketch ghostly red lines in the
darkness, Barrett invokes two rulers of
similar length as a way of explaining
why a more accurate clock is necessary.
The rulers, he says, can be thought of
as clocks. The cesium ruler measures in
centimeters; but an optical clock based
on lutetium “would measure in milli-
meters,” and therefore allow the world
to be measured with greater accuracy.
It might even allow the measurement
of things that could never be measured
before, thereby revealing and resolving
fundamental questions of physics.
For example, Albert Einstein’s gen-
eral theory of relativity posited that
the faster you travel, the more gravity
pulls on you and the more time slows
down. To verify this, scientists have
flown atomic clocks in space and found
that they ran slower, relative to clocksonEarth.Similarly,anatomicclockona
mountaintoprunsfasterthanoneatthe
mountain’sbase,wheregravityis stron-
ger.If opticalclockscouldbeshrunkto
portablesize,somethingBarrettand
others areresearching, those mea-
surableshiftsintimecouldbeusedto
maptheEarth’stopographyinunprec-
edenteddetail.Intheory,theymight
alsobeabletorevealmineraldeposits
beneaththattopography.
Thescientificmotherlodefromopti-
calclockswouldbemeasurementsso
precisetheycouldrevealdarkmatter,
themysterioussubstancethatmakes
upabout27%oftheuniverse’smass.
Itsexistencehasonlybeeninferred,
becauseordinarymatteralonecan’t
explaintheevolutionandstructureof
massiveobjectssuchasgalaxies.NIST’s
ytterbium-based clocks are reaching
levels of precision that may eventually
prove sufficient to detect signatures of
dark matter.
In Boulder, Andrew Ludlow, the proj-
ect leader on the record-setting NIST
clocks, isn’t celebrating yet. “Right now
it’d be difficult to imagine a full-fledged
optical clock developed at a price point
it could be deployed all over,” he says.
Making matters even more difficult, the
intricacy of optical clocks means they’re
subject to significant downtime. For
them to serve as references for a new
second, much less detect dark mat-
ter signatures, they’ll need to be able
to run uninterrupted, 24/7. “Because
of the complexity of these clocks—
and the cost—a lot of the applications
are situations where you don’t need a
lot of them, but you need them to be
really good under special conditions,”
Ludlow says. “Various military applica-
tions fall into that category.” Ludlow’s
funders include NASA and the Defense
Advanced Research Projects Agency,
better known by its acronym, Darpa.
But a market for optical clocks is
percolating. “I’ve been contacted by a
couple of people in the financial indus-
try with generic curiosity about optical
clocks,” Ludlow says. An optical clock
could potentially allow an exchange to
time-stamp and order many more trades
in ever smaller increments of time, a
quality that would be of particular inter-
est to high-frequency traders and those
who like to foil them. In 2016 the hedge
fund Renaissance Technologies filed apatent on an exchange-trading system
that relies upon “atomic clocks, optical
clocks, quantum clocks, Global Position
System (GPS) clocks, or any clock capa-
ble of measuring time, accurate within
microseconds.”B
arrett smiles uncomfortably
when I ask him about the finan-
cial industry’s interest in optical
clocks. He’s seated at his desk in a spa-
cious office across from his lab, a white-
board covered in arrows and equations
beside him. He didn’t get into phys-
ics to find ways to time-stamp more
trades. Instead, the search for preci-
sion “suits my personality,” he says, and
he’s devoted to improving it through the
endless tinkering necessary to make any
clock better. “The hard part is knowing
when to stop and publish,” he says.
Barrett happily acknowledges that
fornowhislabandlutetiumaren’tthe
leadersintheracetobuildthesecond-
defining optical clock. For one thing,
he isn’t nearly as well-funded as other
groups. But what he lacks in resources
is made up for by the properties of “my
atom,” he says. “My argument is, if you
can make an accurate clock in ytter-
bium, then you must be able to make an
accurate clock in lutetium, and it should
be more accurate because the proper-
ties are just better.”
Singapore’s Agency for Science,
Technology and Research, a government
office that sponsors R&D that could ben-
efit the city-state competitively, is help-
ing bankroll him. “Why do they fund it?”
Barrett asks. “If you look at the history
of clocks, as we make them better and
better, new applications pop or the ones
we have get better and better.”
To anyone outside the clock labs,
the first evidence of progress will be a
new definition of a second. The soonest
that can happen, officially, is the next
gathering of the General Conference on
Weights and Measures, scheduled for- But most observers believe there
won’t be a global consensus on which
element or clock design should serve as
the basis for a new second until at least
the 2026 meeting. For now, Barrett, like
other clockmakers, remains focused on
the ticking. “That’s what you really do in
the clock business,”hesays. “You ask,
‘How do we makethesemeasurements
better and better?’”
Bloomberg Businessweek / SEPTEMBER 2, 2019 THE ELEMENTS