Issue Date: December 1, 2014
What Are Atomic Clocks And How Can They Measure Your Brain Waves?
The race is on. Scientists across the 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clocks, are improving so rapidly that “any ranked list of Best Clocks in the World would become almost immediately out of date,” says Thomas O’Brian, chief of the Time & Frequency Division at the National Institute of Standards & Technology (NIST).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 pendulum-based 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.
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To count atoms’ oscillations, scientists can probe them with microwaves. For instance, inside a cesium fountain clock—the U.S.’s current timekeeping standard—a ball of ultracold cesium atoms gets flung through a high-vacuum microwave cavity. As the atoms pass through, microwaves flip the “spin” of an electron in their outermost shells. Researchers tune the cavity’s frequency until a majority of the atoms in the ball make this so-called hyperfine transition. The optimized microwave frequency—9,192,631,770 Hz—defines one second in time.
Even though cesium fountain clocks around the world help set time on our digital devices, they aren’t the most accurate timekeepers on the planet. That distinction goes to optical atomic clocks—experimental instruments that probe atoms with laser light.
“Microwaves set a fundamental limit on how accurately you can measure time,” NIST’s O’Brian says. Think of it like a ruler, he adds: “If you have a ruler with 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 clocks. They can be thrown out of whack by changes in gravity, electric field, magnetic field, and temperature.
Rather than trying to shield or correct for those factors, as they’ve done in the past, scientists are beginning to turn the tables and use atomic clocks to sense shifts in gravity, magnetic field, and the like. For instance, NIST collaborated with Germany’s metrology institute, Physikalish-Technishe Bundesanstalt, to measure the magnetic fields produced by a person’s brain with a miniature rubidium atomic clock (Biomed. Opt. Express 2012, DOI: 10.1364/boe.3.000981).
Working with a team at the University of California, Berkeley, NIST also recently proved that a similar device could hijack Earth’s weak magnetic field to perform nuclear magnetic resonance spectroscopy on a hydrocarbon/water mixture (Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201403416).
Says O’Brian, “It’s an exciting and gratifying time to be in the atomic clock business.”
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