The World’s Most Precise Clock Reveals the Nature of Time and the Universe
Jun Ye shows fellow scientist Victoria Orphan his work. Image: Screengrab/Motherboard

The World’s Most Precise Clock Reveals the Nature of Time and the Universe

Physicist Jun Ye built the world’s most precise clock and is part of the group of scientists who changed our understanding of time itself.

THE MOST UNKNOWN is Motherboard's love letter to the scientific process. For the next nine weeks, we'll be profiling the people trying to answer science's most difficult questions. Our feature-length documentary is now available on Netflix, and bonus episodes are available on YouTube.

The question Jun Ye gets asked the most is: why would you want to make the world’s most precise clock?

For many of us, who live our lives by the day, hour or, if we’re lucky, by the moment, it’s difficult to understand the use in splitting time up into smaller and smaller chunks.


But as the Colorado University, Boulder physics professor explained to me over Skype, the endeavor of studying time is anything but futile. Time, Ye said, isn’t just at the heart of everything we do, but at the very core of understanding how the universe functions.

“Time is one of the most fundamental tools that connects us to nature” Ye said. The passage of the sun across the sky allowed ancient Egyptians to track their work hours, the pull of gravity kept pendulum clocks ticking to allow seafarers to navigate the oceans, and the vibration of quartz under stress brought timekeeping on to people’s wrists. Measuring time has always been at the heart of human society, Ye explained, but how we do that has changed considerably.

As measurements became more precise, scientists discovered that these natural timekeepers were fickle and inconsistent. They searched for clock mechanisms that wouldn’t fall behind or need to be reset as often.

The search led physicists to atoms, which “tick” naturally thanks to their physical properties. Inside each atom are even smaller particles that are arranged like a solar system. In the core are protons and neutrons, the sun, and orbiting them at increasing distances are electrons, the planets. In the tiny, subatomic world, atoms obey the laws of quantum physics. Electrons can jump between orbits and, when they do, they give off or absorb a tiny jolt of microwave radiation and change into a different energy state. This happens many times every second and each jump back and forth is the atom’s transition frequency, or clock tick.


The physicists Louis Essen and Jack Parry found a way to harness the atoms of the soft, silvery-gold metal cesium to make one of the first atomic clocks. In doing so they literally redefined time. A second is now defined as the transition frequency of cesium 133 atoms, roughly 9.2 billion cycles. Essen and Parry’s clock, now a museum piece, used atoms to reset quartz clocks by giving the mineral a nudge if its vibrations slowed down. It would have neither gained nor lost a second in 300 years, if it had stayed in operation that long.

This chart shows the accuracy of various atomic clocks over time. Image: Engineering and Technology History Wiki

Over time, scientists found ways to use cesium atoms on their own. They’d prod the atoms with microwaves tuned to cesium’s exact frequency to make the atoms change to their high energy form. This new generation of clocks improved the stability (how much time is lost or gained) a million times over.

If you glance down at your cellphone or computer right now, the time you see has been synced with more than 500 atomic clocks all around the world. That means throwing in an extra leap second every 19 months or so to allow the time taken based on Earth’s rotation to catch up with the more precise atomic time.

Time isn’t just the subject of Ye’s research; good timing has played at least a small part in his path to building the world’s most precise clock.

He was born in Shanghai, right at the end of China’s Cultural Revolution and at the beginning of its education system revival. Soon after he decided to pursue an educational path emphasizing science and technology, over his other great love, literature, he was selected to represent his school at a national physics competition. That serendipitous event would set him on the road to life as a physicist.


Jun Ye in his office. Image: Lindsay Blatt/Motherboard

“I did reasonably well,” he recalled in an interview for his university’s webpage, “and, I discovered that physics was exciting for me.”

As timing would have it, Ye was in the right place at the right time when physicists figured out how to solve one of the biggest hurdles to making an even more precise clock. Precision depends both on how small you can make the chunks of time but also on how well you can read those chunks. Previous clocks relied on converting an atom’s super-fast microwave radiation to slower radio waves that the electronics of the time could read. That meant you could only see larger “chunks” of time, like having a clock you know ticks every second but with a face that only has hour and minute hands.

When Ye was hired as a physicist for the National Institutes of Standards and Technology and assistant professor at the University of Colorado, Boulder in 1999, he was meant to be taking over from eminent physicist, and Ye’s PhD adviser, John Hall. But Hall and his colleague Theodor Hänsch from the Max-Planck-Institute of Quantum Optics in Garching, Germany were on the verge of solving this clock-reading puzzle.

Around the turn of the century, they invented the optical frequency comb, a time ruler made out of light frequencies from a laser, then retired a few years after that. Now, scientists could read atoms that vibrated at even higher frequencies, including those in the optical range—frequencies between 430 and 770 trillion hertz (for comparison, a pendulum clock swings at a frequency of one hertz).


Ye described this as a “breakthrough” for science that opened up new avenues of exploration. He speaks about this discovery like a great explorer might speak of encountering a new land.

The blue atoms of Ye's clock. Image: Screengrab/Motherboard

“We were basically coming up to [the edge of] a slope where you look down and there’s just wildflowers everywhere in the valley and you can just go down and pick these beautiful wildflowers,” Ye told me.

Soon, Ye was leading breakthroughs of his own. He developed an atomic clock made from strontium 87, a soft, silver-white yellowish metal. Strontium 87’s atoms tick at femtoseconds—that’s 1 million billion times per second. His latest version of the clock has thousands of supercooled strontium atoms arranged in a three dimensional lattice (imagine M&M’s sitting on the peaks and troughs of an egg carton.) The atoms are prodded to start vibrating using a red laser tuned to strontium’s frequency and then an optical frequency comb reads out that vibration.

This is the most precise clock in the world. If it had been running since the big bang, 13.8 billion years ago, it would only have only strayed by one second. Ye shows off the clock in the Motherboard feature, The Most Unknown. He leads Caltech geobiologist Victoria Orphan through a maze of wires to a cavern illuminated by the blue glow of the strontium clock. To the average person, it looks a little like a Rube Goldberg machine.

I struggle to imagine the scale that Ye works on and how or even why he studies what he does. Ye explained that, just like the first sundials allowed people to plant and harvest crops at the right time, atomic clocks hide beneath the conveniences of everyday life.


Take GPS systems, for example. Whether or not your Uber driver brings you to the right place relies, at least in part, on atomic clocks. GPS devices work by calculating how long it took for messages from satellites around Earth to reach it. From there the device can figure out how far away it is from each satellite and then work out your position. Any little error in time measurement and you might end up on the other side of town. A single millisecond error translates to a distance error of 300 kilometers. Ye envisions that atomic clocks will underpin the navigation systems of autonomous vehicles or even the vehicles that are sent further afield to Mars and beyond.

Ye. Image: Screengrab/Motherboard

Ye also thinks there’s great scientific value in creating super-precise clocks.

“Measurement is at the heart of science,” he said “we should be able to reveal the very core of nature.”

In particular, he thinks atomic clocks will allow scientists to figure out how the weird properties of quantum physics, like particles being in two places at once or in superposition, relate to the classical physics we experience everyday.

“There has to be a place where quantum physics and classical physics connect” he reasoned. “Once the universe knows a quantum system is in superposition, somehow it will conspire to break that and turn it into a classical world. It sounds a little like superstition but quantum mechanics is a little bit like that.”


Understanding quantum systems better could help scientists could figure out how to make big things, not just tiny ones, act in a quantum way. That’s the idea behind quantum computing, to run computer bits in all their superpositions at once, instead of one after the other.

Ye thinks that would be possible if there was a clock precise enough to measure how time changes near the tiny dips in spacetime around a particle. Take it one step further and if you measure how other objects are pulled into those dips, you can measure gravity in a super precise way. That could allow geophysicists to predict volcanic eruptions, Ye said, by sensing when masses under the surface of the Earth move by a tiny fraction.

The more we delve into the depths of atomic time, the more endless the possibilities seem. Even Ye himself said he doesn’t believe there’s a limit to a clock’s performance and the leaps forward that accompany it.

“I’ve thought of many road blocks but fundamentally I couldn’t think of any limit,” he said.“My interest, although it’s always been there, is actually growing stronger because of the recent 10 to 15 years of advancement. It makes the whole community feel more and more courageous to ask those deep, insightful questions.”

It’s hard to imagine what a world with such a precise clock would be like but, perhaps, that’s the point. With every leap forward in how we measure time comes some notable technical or societal leap. The pendulum allowed people to cross the atlantic. Atomic clocks enabled GPS and the internet.

The beauty of these clocks is that they, as Ye described, give us the chance to peer over a new mountain and see what wildflowers might be on the other side.