The world’s most precise clock sits on a table in Jun Ye’s lab in Boulder, Colorado. A tangle of electronics, fiber optic cables, and laser beams, the clock is still a prototype, so no one actually uses it to tell time. Ye, who is a physicist at the research institute JILA, and his team have demonstrated that the clock can produce a second with precision in the parts per quintillion—that’s 10-19, some hundred billion times more precise than a quartz wristwatch. Put another way, if the clock had started ticking at the Big Bang, by today it would have lost or gained no more than a second. It’s not just the most precise clock in the world—it’s the most precise device in the world.
The heart of the clock is a chamber of around 100,000 strontium atoms that Ye has trapped using lasers. These atoms, when hit with a certain laser, emit red light with a wavelength of exactly 698 nanometers, which corresponds to some 430 trillion cycles of an electromagnetic wave per second. The rate of the oscillation depends on the fundamental structure of the atom, which means Ye’s sheltered strontium atoms tick with exceptional consistency. Compare that to the pendulum of a grandfather clock, which expands and contracts with changes in temperature and humidity to speed up or slow down.
In the future, the US government will likely use some iteration of Ye’s clock to set the time across the country, so you can get to your social engagements on time. But that’s probably the least interesting use for this clock. Astrophysicists have their eye on these tools too. They think that this clock’s near-perfectly spaced ticks can help them venture deeper into space.
That’s right: By studying time, they can study space. The concept relies on a postulate in Einstein’s theory of special relativity, which says that light travels at a fixed speed of 299,792,458 meters per second in the vacuum of empty space. If you can precisely measure how long it takes light to travel from point A to point B, you can figure out the distance between A and B. This is actually how GPS works. Satellites pinpoint your location on Earth by precisely measuring how long it takes a radio signal to bounce from your phone back to space. Hence, the word “spacetime”—measuring time is equivalent to measuring spatial distances, and vice versa. A clock doesn’t just count seconds; because the speed of light is predictable, a clock is also a cosmological tape measure.
Engineers already use early versions of these clocks to remotely steer spacecraft through our solar system. For example, if a spacecraft is making its way to Mars, NASA checks its trajectory by pinging it with a fleet of Earth-based radio antennae. When the radio signal reaches the spacecraft, it immediately bounces back to Earth. The Earth-based antennae, hooked up to atomic clocks that have precisely recorded when the signal departed, then time the signal’s arrival back on Earth. That time measurement allows NASA engineers to calculate the location and speed of the spacecraft to then instruct it how to move.
But this process is cumbersome. NASA has a limited number of space antennae, which means sometimes its operational spacecraft have to wait in line to talk to ground control. For example, a spacecraft near Mars has to wait up to 40 minutes sometimes to communicate with the antennae. This lag time increases NASA engineers’ likelihood of making maneuvering errors. So they want to speed up this process by putting atomic clocks directly on spacecraft. In this setup, the spacecraft could calculate its trajectory on board autonomously after receiving an initial ping from the Earth radio antennae. They think that this would enable more spacefaring missions. “We would be able to service more users than we are able to do today,” says navigation engineer Todd Ely of NASA’s Jet Propulsion Laboratory.
This June, in a first step toward these future self-steering spacecraft, Ely’s team will launch a toaster oven-sized atomic clock into orbit in a mission called the Deep Space Atomic Clock. Theirs should be the most precise clock in space, which they’ve designed to keep time to nearly a quadrillionth of a second per day. (It’s still about 10,000 times less precise than Ye’s record-holding clock.) They will keep the clock in space for a year to monitor its functionality, and eventually, they hope to put a version of this clock on future NASA orbiters.
Better clocks also improve astronomical imaging. A type of atomic clock known as a hydrogen maser was key to producing the first image of the black hole released in April. The black hole is so small in our sky—literally the size that a doughnut on the moon would appear from Earth—that astrophysicists needed eight observatories on four different continents looking simultaneously to see it. They had to synchronize their observatories to within a billionth of a second using these clocks, says astrophysicist Dan Marrone of the University of Arizona, a member of the Event Horizon Telescope team that took the first black hole image. Without the atomic clocks, they would have been unable to compare the data at each site, and the picture of the black hole would have ended up a smear.
Marrone’s atomic clocks also served a second role: to filter the sky for a specific radio frequency from gas swirling around the black hole. While this gas emits light of all colors, only certain frequencies can make it all the way to Earth mostly undisturbed. Marrone’s team has chosen to look for 221 megahertz. But to filter for just that frequency, they need the precision of the atomic clock. It essentially produces a reference tone, like a singer who plays middle C on the piano to start singing on the right note. They then mix a radio wave signal from the sky with the clock’s tone. When they match a radio frequency from the sky to the one produced by the clock, they know they’ve filtered for the right light. “We need an extremely pure tone to compare to the sky,” says Marrone.
Researchers could also adapt this capability to look for gravitational waves in space. Ye and his colleagues have written about a scheme that would involve future, miniaturized versions of his strontium clock. The scheme involves putting two super precise clocks in separate satellites in orbit, and beaming a laser between them. If a gravitational wave came through, it would briefly compress the distance between the two satellites. This compression would also change the frequency, or color, of laser light. By comparing the laser light to the pure tone of the atomic clock, they could determine when a gravitational wave came through.
These clocks could help solve scientific problems closer to home too. According to Einstein’s theory of general relativity, a clock experiencing stronger gravity will tick more slowly. Because a clock at sea level—closer to Earth—experiences slightly stronger gravity than a clock in the Himalayas, the sea level clock should tick at a more sluggish pace. Ye’s record-setting clock is precise enough that you could in theory detect a change in elevation of less than a centimeter, although you can’t really move it around in its current form.
Some researchers think they could actually use these clocks to precisely map elevation around Earth. For example, physicists at PTB, a German national laboratory, have developed a portable strontium clock that they’ve driven in a trailer to the France-Italy border. The precision of their clock isn’t good enough yet, but they’re hoping that eventually if they bring the trailer by the coastline, they can monitor how much the sea level rises.
Meanwhile, Ye is working on improving his clock—regardless of the applications. Since he started building the timepieces nearly 20 years ago, he has improved their precision by a thousandfold. He set the latest precision record last March, and he has clear ideas on how to make his clock even better. “I don’t see the progress slowing down yet,” he says. And by measuring the tiniest fraction of time possible, scientists hope to perceive the smallest changes in the universe.