WHAT’S THAT STUFF
atomic clocks These SENSITIVE DEVICES are beginning to do much more than just tell time THE RACE IS ON. Scientists across the
YE GROUP & BRAD BAXLEY/JILA
globe are hustling to build the latest, greatest—and most accurate—timekeeping device. But rather than working with high-precision gears and pendulums, these clockmakers are grappling with lasers and ultracold atoms. Today’s top-of-the-line timekeepers, dubbed atomic clocks, are improving so rapidly that “any ranked list of Best Clocks in the World would become almost immediately out of date,” says Thomas O’Brian, chief of the Time & Frequency Division at the National Institute of Standards & Technology (NIST). C&EN’s going to risk it, though: At press time, the world record holder was a strontium lattice optical clock housed at the University of Colorado, Boulder. The device is so accurate that if it ran continuously for 5 billion years, it wouldn’t gain or lose more than a second. Given this awe-inspiring level of performance, many people have asked O’Brian and his fellow clock researchers why they’re still pushing to do better. “We’re not just a bunch of geeks who want to measure things precisely,” O’Brian assures C&EN. As the clocks become more and more sophisticated, he explains, they’re able to do more than just tell time. One day, they might be exquisitely sensitive detectors of human brain activity or even black holes in outer space. Every clock, whether it’s a pendulumbased antique or an atom-based lab experiment, needs two components to keep time: an oscillator and a counter that keeps track of the oscillations. In prehistoric days, the oscillator was the sun’s periodic rise and fall in the sky. And the counter was a person. Observing the sun’s rays, “you would put a mark on your cave wall, and you would say, ‘That’s a day,’ ” says Jun Ye, leader of the research team that built the record-breaking strontium lattice clock. Atomic clocks, as their moniker suggests, use atoms as oscillators. Atoms have been naturally moving from one energy level to another since the beginning of the universe,
says Ye, who works at JILA, a joint institute of the University of Colorado and NIST. “They’re not going to change,” he adds, so they’re ideal for creating rock-steady timepieces. Pendulums and other mechanical oscillators, on the other hand, wear out over time.
finer markings, you’re going to get a more accurate reading.” Microwaves measure frequencies of about 1 billion cycles per second. Laser light has finer “markings” of up to 1 million billion cycles per second. High accuracy comes with a price, however. “Optical clocks are trickier,” Ye says. Because the devices are so sensitive, even the slightest movement in one of the atoms during measurement could cause errors in ticking rate. Researchers prevent this aberrant motion by using lasers to freeze an optical clock’s atoms in place. The strontium lattice clock, for example, has a network of intersecting lasers that trap a few thousand strontium atoms at a time, Ye says. Atomic motion isn’t the only thing that affects optical To count atoms’ oscillaclocks. They can be thrown TICKTOCK The strontium lattice optical clock tions, scientists can probe out of whack by changes in shown here is currently them with microwaves. gravity, electric field, magnetic the world’s most For instance, inside a cefield, and temperature. accurate timekeeper. sium fountain clock—the Rather than trying to shield U.S.’s current timekeeping or correct for those factors, as standard—a ball of ultracold they’ve done in the past, scicesium atoms gets flung through a highentists are beginning to turn the tables and vacuum microwave cavity. As the atoms use atomic clocks to sense shifts in gravity, pass through, microwaves flip the “spin” of magnetic field, and the like. For instance, an electron in their outermost shells. ReNIST collaborated with Germany’s metrolsearchers tune the cavity’s frequency until ogy institute, Physikalish-Technishe Bundea majority of the atoms in the ball make this sanstalt, to measure the magnetic fields proso-called hyperfine transition. The optiduced by a person’s brain with a miniature mized microwave frequency—9,192,631,770 rubidium atomic clock (Biomed. Opt. Express Hz—defines one second in time. 2012, DOI: 10.1364/boe.3.000981). Even though cesium fountain clocks Working with a team at the University around the world help set time on our digital of California, Berkeley, NIST also recently devices, they aren’t the most accurate timeproved that a similar device could hijack keepers on the planet. That distinction goes Earth’s weak magnetic field to perform nuto optical atomic clocks—experimental inclear magnetic resonance spectroscopy on a struments that probe atoms with laser light. hydrocarbon/water mixture (Angew. Chem. “Microwaves set a fundamental limit Int. Ed. 2014, DOI: 10.1002/anie.201403416). on how accurately you can measure time,” Says O’Brian, “It’s an exciting and NIST’s O’Brian says. Think of it like a gratifying time to be in the atomic clock ruler, he adds: “If you have a ruler with business.”—LAUREN WOLF
“We’re not just a bunch of geeks who want to measure things precisely.” CEN.ACS.ORG
29
DECEMBER 1, 2014
