Amazingly Precise Optical Atomic Clocks Are More Than Timekeepers CORE CONCEPTS Adam Mann, Science Writer

CORE CONCEPTS

Amazingly precise optical atomic clocks are more than timekeepers CORE CONCEPTS Adam Mann, Science Writer

In March, physicists unveiled the world’s most accurate timepiece. But this was no pocket watch. At its heart is an atomic pendulum that swings 500 trillion times per second and can measure fractions of a second out to 19 decimal places (1). The clock is so precise that it loses at most a tenth of a second over the entire lifetime of the universe. And yet, the device, known as an optical atomic clock, will allow physicists to do a lot more than tell time. Such clocks can help investigate fundamental constants of the universe with higher precision than ever before, searching for discrepancies in our current theories of reality. And because time and space are intimately related, these ultra-accurate clocks can also act like measuring tape, mapping the size and shape of Earth with a resolution of a centimeter or less. This, in turn, can help climate researchers monitor sea level rise and geologists track the movement of tectonic plates. It may seem startling that precision clocks have such ramifications. But any time science gets a new tool to study nature, it throws up surprises, says physicist Jun Ye of JILA, a research institute jointly operated by the University of Colorado Boulder and Trapped atoms of strontium-87 produce a bluish glow within a shielding box the National Institute of Standards and Technology surrounded by thermal sensors. The synchronized atoms are used to count (NIST). Ye’s team constructed the record-holding infinitesimal ticks for the world’s most precise optical atomic clock. Image clock announced in March. He thinks major new in- courtesy of Ye Labs, JILA. sights are sure to come from the high-precision mea- surement of time. “I think you could argue that it’s oscillations at optical frequencies, corresponding to visible kind of at the core of everything,” he says. light, which are about 100,000 times faster. Although per- fect for even more precise clocks, these atoms posed a Time is Telling serious challenge. “Optical transitions are just way too “ Albert Einstein famously defined time as that thing fast,” says physicist Mukund Vengalattore of Cornell you measure using a clock.” Clocks need a moving University in Ithaca, NY. No one knew how to count part, like a pendulum, to mark the passage of time. In these lightning-fast ticks. 1955, physicists Louis Essen and Jack Parry developed A breakthrough came in 1999 when physicists the first atomic clock at the National Physical Laboratory developed what is known as a femtosecond optical in London by using ions of cesium-133 as tiny sub- frequency comb (3, 4). This device uses a laser to gen- atomic pendulums (2). These atoms emit radiation as they oscillate between two closely spaced energy lev- erate extremely short pulses of light that appear as els. The work led to the modern definition of a second spikes at regularly spaced intervals, creating what as the time it takes to complete 9,192,631,770 such looks like the teeth of a comb. Each pulse is of a dif- oscillations of the cesium-133 atom. ferent color or frequency, and the comb spans the The cesium-133 radiation is in the microwave portion entire visible spectrum. Researchers could line up one of the electromagnetic spectrum. But other atoms— of the teeth of the laser light to the oscillations of an such as aluminum, strontium, and ytterbium—have natural atom radiating in optical frequencies, lock the laser

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www.pnas.org/cgi/doi/10.1073/pnas.1809852115 PNAS | July 17, 2018 | vol. 115 | no. 29 | 7449–7451 Downloaded by guest on September 23, 2021 A mobile optical atomic clock has the potential to revolutionize geodesy, a field that entails measuring the Earth’s gravitational field with high precision. A team at Germany’s national metrology institute is developing prototypes approaching the accuracy of the best atomic clocks. Image courtesy of Physikalisch-Technische Bundesanstalt.

frequency to that line, and use sophisticated elec- some astrophysicists have claimed evidence for a tronics to tally the transitions. This opened the door minute variation in the fine-structure constant—not for creating optical atomic clocks. just in terms of past values compared with today’s In 2013, optical atomic clocks were shown to be value of α, but even when observations of α from dif- better than cesium atomic clocks, and new records in ferent regions of the universe are compared (6, 7) accuracy have arrived yearly since then (5). With each But astronomical observations have large uncer- order of magnitude in improvement, researchers have tainties. So some teams have taken a different ap- imagined new ways to exploit the extreme abilities of proach to measuring variations in α, trying to see if it these clocks. “When you have a new instrument, varies over the course of a year on earth. If so, it would whatever it is, you always think how can you use it to cause an extremely tiny but noticeable variation in the test the known laws of physics to high precision,” says ticks of atomic clocks—speeding them up or slowing physicist Michael Romalis of Princeton University in them down. New Jersey. Several teams have carefully monitored optical atomic clocks at different times during a year and de- Reality Check termined that α cannot be changing in today’s universe In recent years, researchers have been using optical by more than one part in 1018 (8). Physicist Victor atomic clocks to investigate why our universe is the Flambaum of the University of New South Wales Syd- way it is. The standard model of particle physics de- ney says this almost rules out the effect seen in the scribes all known particles and the forces (other than astrophysical data, but the best tests will require as-yet- gravity) that act on them. Physicists would like to ex- unrealized clocks based on a transition in thorium-229, plain the values of the fundamental constants in this which happens to be an especially sensitive probe of theory from first principles. To do so they have to variations in α. sometimes consider theories in which these constants The optical atomic clocks are also sensitive to the can vary from place to place in the universe and over mass ratio of electrons to protons, another dimen- time. “Tests of fundamental physics using these kinds of sionless quantity that is fine-tuned in the standard clocks is a far more viable way to go beyond the stan- model. But so far, searches for any fluctuations in this dard model than the giant colliders,” says Vengalattore. ratio have turned up empty (9). One such number is the so-called fine-structure constant, a dimensionless quantity denoted by the Clocks as Measuring Tape Greek letter α that characterizes the strength of the It’s not obvious that keeping time has anything to do electromagnetic force. By studying the spectra of light with gravity, but according to Einstein’s general rela- from distant, extremely bright objects called quasars, tivity a clock near a strong gravitational field, such as which existed approximately 10 billion years ago, near the surface of the Earth, will tick more slowly than

7450 | www.pnas.org/cgi/doi/10.1073/pnas.1809852115 Mann Downloaded by guest on September 23, 2021 one farther away, where gravity is weaker. The time plates rise, slide, and deform by centimeters per resolution of optical atomic clocks is such that raising year. Placed in protective casing and positioned them a single centimeter in height causes a noticeable close to a volcano, the clocks could track the move- change in their ticking rate. This makes them ideal for ment of magma and provide data that might help geodesy, which involves measuring the size and shape predict eruptions. Oceanographers and atmospheric of the Earth and its gravity field. researchers could use them to monitor enormous The data collected by geodesists so far is inconsis- oceanic storms and other large masses of water and tent. “They actually see huge discrepancies between air, which contribute to the total gravitational field in some of the data of geodetic surveys from different their vicinity. ” countries, says physicist Andrew Ludlow of NIST. Optical atomic clocks located aboard GPS satellites “ And even within a country, using different techniques would be able to determine positions on the Earth with ” for geodesy actually gets very different answers. the accuracy of a centimeter, helping to autonomously Optical atomic clocks could help with superior geodesy, but they currently remain stuck in precision laboratory environments, requiring a tabletop of per- “I think in a certain sense, we’re just scratching the “ fectly calibrated lasers and mirrors. You want to surface of accuracy.” miniaturize all that, put it in a car trailer, move it somewhere and reconstruct it, and make sure it still —Mukund Vengalattore works after 3 weeks,” says physicist Christian Lisdat of the Physikalisch-Technische Bundesanstalt, Germany’s national metrology institute in Brunswick. drive cars, land airplanes, or direct military missiles. That’s exactly what Lisdat’s team is developing, Geodesists could use such detailed mapping to de- though his portable devices are still an order of mag- velop an International Height Reference System that nitude or so less accurate than the best laboratory could be useful for monitoring the effects of climate clocks (10). He expects to have mobile optical atomic change such as sea level rise. clocks capable of sensing centimeter-scale changes in Even as researchers dream up applications for the height of earth’s surface within the next 3 years. optical atomic clocks, others continue to improve the ’ “This would be really a revolution for geodesy because clocks themselves. Ye s record optical atomic clock you could use a clock to measure the Earth’s [gravitational] contains a cube of 10,000 strontium atoms cooled to potential directly,” says geodesist Markus Rothacher fractions of a degree above absolute zero. Sitting in a at the Swiss Federal Institute of Technology in Zurich. 3D lattice of laser light, each atom is cradled like an The gravitational field at a particular location de- egg in an egg carton, stabilized and shielded from pends on the density of mass underfoot—the denser disturbances by its neighbors, the ticks of all the the mass, the greater its gravitational field. Portable atoms perfectly synchronized. Ye hopes to add more optical atomic clocks could potentially discover atoms to the configuration and believes he will changes in the subsurface mass and density and help achieve another order of magnitude in accuracy within identify deposits of oil and minerals without using 5years. traditional prospecting techniques (11). With such techniques in mind, Vengalattore and The clocks could also revolutionize earth sciences. others think optical atomic clocks could get even more Studies of changes in mass and density could help precise and spawn yet-unimagined applications. “I think geophysicists gain a more fine-grained understanding in a certain sense,” he says, “we’re just scratching the of the structure of the lithosphere and watch crustal surface of accuracy.”

1 Marti GE, et al. (2018) Imaging optical frequencies with 100 μHz precision and 1.1 μm resolution. Phys Rev Lett 120:103201–103207. 2 Essen L, Parry J (1955) An atomic standard of frequency and time interval: A cæsium resonator. Nature 176:280–282. 3 Reichert J, Holzwarth R, Udem Th, Hänsch TW (1999) Measuring the frequency of light with mode-locked lasers. Opt Commun 172:59–68. 4 Diddams SA, Ma LS, Ye J, Hall JL (1999) Broadband optical frequency comb generation with a phase-modulated parametric oscillator. Opt Lett 24:1747–1749. 5 Le Targat R, et al. (2013) Experimental realization of an optical second with strontium lattice clocks. Nat Commun 4:2109–2117. 6 Murphy M, et al. (2003) Further evidence for a variable fine-structure constant from Keck/HIRES QSO absorption spectra. Mon Not R Astron Soc 345:609–638. 7 Webb JK, et al. (2011) Indications of a spatial variation of the fine structure constant. Phys Rev Lett 107:191101–191106. − 8 Huntemann N, Sanner C, Lipphardt B, Tamm C, Peik E (2016) Single-ion atomic clock with 3 × 10 18 systematic uncertainty. Phys Rev Lett 116:063001–063006. + 9 Huntemann N, et al. (2014) Improved limit on a temporal variation of mp/me from comparisons of Yb and Cs atomic clocks. Phys Rev Lett 113:210802–210807. 10 Grotti J, et al. (2018) Geodesy and metrology with a transportable optical clock. Nat Phys 14:437–441. 11 Mehlstäubler TE, Grosche G, Lisdat C, Schmidt PO, Denker H (2018) Atomic clocks for geodesy. Rep Prog Phys 81:064401–064474.

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