How Do Atomic Clocks Work?

Credit: N. Hanacek/NIST

Our modern world depends on clocks that tap the secrets hidden inside atoms — the tiny building blocks of matter that make up everything from our bodies to the device you’re reading this on to the planet Earth. 

If you’ve checked the time on your phone, computer or smartwatch today, you have aligned yourself with the steady, unchanging beats of atoms. If you’ve used GPS for directions or flown on a plane, atomic clocks have helped deliver you safely to your destination. If you’ve bought or sold a stock, that transaction was stamped in atomic time. If you’ve made a phone call or searched the web, you have tapped into networks synced by atoms.

But how do we use these tiny particles to tell time? To peer into atoms and extract their beats, we must wield light with extreme and exquisite precision. And we must ingeniously hack into the strange and wondrous world of quantum physics, the rules that govern the world at its tiniest scales — including inside the atom.

What, Exactly, Is a Clock?

To understand atomic time, we must first understand something about clocks.

A clock is, at its core, a marriage of two components: a mechanism that oscillates, or ticks with a steady beat, and a device that counts those beats and displays the time. For most of history, the oscillating mechanisms have been human-made objects: the swinging pendulum of a grandfather clock; the quartz crystal in your wristwatch or laptop. 

Credit: Mountain Brothers/Shutterstock

But no two pendulums or crystals are exactly alike — so neither are the ticks of time they produce. Human-made timepieces can never be perfectly in sync. 

Manufactured clocks have a second problem: They drift. One tick of time is slightly different from the next, and the next, and so on. Human-made clocks are inherently unstable.

Atoms, by contrast, can create almost “perfect” beats. Atoms don’t have any manufactured parts, and they don’t wear out or slow down over time. And all atoms of a given type are identical. 

Quantum physics tells us that atoms absorb and emit light waves of specific colors, or frequencies. Light waves tuned to these atomic frequencies produce ticks of time that are billions of times more stable and precise than those of any other kind of clock.

Indeed, as far as we know, atomic frequencies offer the purest, most stable beats in the universe. If another advanced civilization has evolved on a distant planet, it probably measures time using atoms, as we do. 

So how do these fabulous atomic clocks work?

The Inner Life of the Atom

Atoms are all around us and inside of us. But when bound by the billions in solid objects like our bodies, atoms behave very differently than when they’re on their own. To make a clock, we must first separate and isolate individual atoms from almost everything else. 

We then immerse our atoms in light. Light is nature’s way of sending energy from place to place.  It travels in the form of waves that rise and fall as they speed through space, like the alternating crests and troughs of water waves traveling across the ocean. The frequency of a light wave is the number of times it cycles from crest to trough to crest again in one second.  

To turn atoms into a clock, we can’t use just any light: We need light that has been finely tuned to a special frequency known as the atoms’ resonant frequency. When the light frequency is near resonance, atoms can absorb some of the light’s energy and change their internal energy state — often called a “quantum jump.”

Credit: J. Wang/NIST

You can find an everyday example of resonance on a playground. If you push a child on a swing only when they reach a maximum height, the child will swing higher and higher. That’s because you’ve tuned your pushing rate to the swing’s natural resonant frequency. If, on the other hand, you push at random intervals, the swing won’t go higher and might even come to a stop. 

Another familiar example involves an opera singer and a wine glass. When the sound waves from the singer’s voice reach the natural resonance frequency of the glass, the glass can quickly absorb a lot of energy — and shatter.  

More metaphorically, we often say an idea resonates when it seems to harmonize with something inside of us. 

Now you may be wondering, what provides the steady tick in an atomic clock — and what does any of this talk about resonance and energy states have to do with telling time? 

What ticks in an atomic clock are the light waves used to energize atoms. As we tune the light frequency toward the atoms’ resonant frequency, more and more atoms will absorb energy from the light and make a quantum jump. 

By measuring how many atoms have absorbed energy, we can detect when the light frequency has reached the resonant frequency — or at least come very close. Then we count the crests and troughs of the resonant light and convert those counts into ticks of time.

How many cycles of crests and troughs do we need to count to tick off one second? It depends on the type of atom we’re using, because each has a different intrinsic resonant frequency. For one atom often used in clocks — cesium — the answer is 9,192,631,770 cycles. In fact, in 1967, the world’s timekeepers defined the official international second as the time it takes exactly that number of cycles of light tuned to the cesium resonant frequency to pass through a detector.

Why do we use atoms to keep time?

Why Do We Use Atoms To Keep Time?

The Atomic Clock Age

Because atoms themselves are perfect, an atomic clock’s accuracy depends only on how good its lasers, electronics and other components are.  The first atomic clock, invented in 1949 by scientists working in the Washington, D.C., lab of the National Bureau of Standards (now NIST), was hardly better than existing electronic clocks. But atomic clocks have improved at an astonishing rate, and they have transformed our world. 

Today’s best atomic clocks are billions of times more accurate and precise than any other type of clock. If they had been running since the Big Bang, 13.8 billion years ago, they would have lost or gained less than a second. 

In the past 75 years, scientists and engineers have built atomic clocks for timekeeping, navigation, telecommunications, scientific exploration and countless other uses. New kinds of clocks may soon open up capabilities that previous generations could only dream about.

This website offers a tour through atomic time. Read on to learn how different atomic clocks work, how atomic clocks help us tell time, the history of this remarkable technology and how atomic clocks have opened up profound and sometimes unexpected advances in both technology and science.