What Are Atomic Clocks, Anyway?

Atomic clocks. They almost sound like something out of science fiction, or an experiment confined to some elite physics lab, but in reality, they’ve been around since the 1950s in one form or another. These clocks aren’t coming to your house anytime soon, though: they can be the size of an armoire cabinet and consist of an interwoven mass of stainless steel, lasers, wires, and cables, all attached to a vacuum chamber that holds the microscopic stars of the show.

During World War II, scientists first cracked the secrets of the atom, birthing the study of quantum physics and the possibilities of using the tiniest building blocks of matter for practical applications, including the atomic clock.

At a minuscule scale, an atomic clock employs the same fundamental processes as a grandfather clock or a wristwatch: it offers a periodic phenomenon that you can count. As a clock pendulum swings back and forth, and a tuning-fork-shaped piece of quartz oscillates with an electric current in a watch, you count the periodic swings of electrons in an atomic clock as they jump between energy levels. (More on that later.)

Today, the exquisitely precise timekeeping of atomic clocks is used for measuring time and distance for everything from our Global Positioning System (GPS), online communications across the world, fractions of a second in trading stocks, and timed races in the Olympics. But scientists have developed even more advanced atomic clocks that could reveal more about the mysterious parts of the universe—like dark matter—than we have ever seen.

Here’s how atomic clocks work, and why we can’t imagine a world (or future discoveries about our universe) without them.

A Brief Subatomic History

After the advent of atomic physics, scientists developed a way to pass cesium atoms through radio waves, and then microwaves, a form of high-frequency electromagnetic radiation. In this early form of the atomic clock, the jolt of energy from the radiation caused electrons in the atoms to jump back and forth between energy levels, or orbits, around the atom’s nucleus. Because atoms of an individual element respond only to a specific and unique frequency (the number of waves that pass by a point in space in a given amount of time), scientists could measure this frequency to obtain a standard and accurate measurement of time.

“The fact that the energy difference between these orbits is such a precise and stable value is really the key ingredient for atomic clocks,” Eric Burt, an atomic clock physicist at NASA’s Jet Propulsion Laboratory, says in a 2019 blog post on NASA’s website. “It’s the reason atomic clocks can reach a performance level beyond mechanical clocks.”

Cesium was an early contender for creating a better timekeeper because atoms of this element have a much higher resonant frequency than the quartz used in wristwatches. Cesium’s outer shell has a single electron, making it chemically reactive to microwave radiation. Because you can count how many times electrons jump between two energy levels in an atom of cesium in a given amount of time, this frequency has become the official measurement tool for the length of a second. (Fun fact: These energy shifts show up as electromagnetic radiation in the form of visible light, such as the orange glow from sodium vapor lamps used for street lighting. The atoms in an atomic clock glow too.)

The cesium atomic clock at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland is so accurate that it would lose one second in 100 million years.

Today’s most accurate atomic clock would take around 30 billion years to lose even one second. In comparison, mechanical clocks can lose several seconds every month, scientists say. What’s the point of a timepiece with that kind of extreme accuracy? For one, it leads to jaw-dropping experiments that prove the principles of relativity ever more accurately, such as keeping two atomic clocks at different elevations and seeing that they “tick” at different rates because they experience different levels of gravity. A second reason is that it will open up new avenues of research scientists have only dreamed about, such as investigating dark matter by examining tiny shifts in gravitational waves.

Today’s Atomic Clocks Are Next-Level

Physicist Jun Ye of NIST, where scientists developed the first atomic clock, is working on a relatively new type of design—the optical atomic clock—in collaboration with the Joint Institute for Laboratory Astrophysics (JILA) in Boulder, Colorado. Instead of using microwaves, his lab setup releases lasers at a cloud of hundreds of thousands of supercooled strontium atoms. The lasers set these quantum oscillations in motion, and they also keep track of the electron jumps inside the atom. The lasers are tuned to precisely match the frequency of the light an electron emits each time it changes energy levels.

JILA’s experimental atomic clock based on strontium atoms held in a lattice of laser light. The image is a composite of many photos taken with long exposure times and other techniques to make the lasers more visible.
JILA’s experimental atomic clock based on strontium atoms held in a lattice of laser light. The image is a composite of many photos taken with long exposure times and other techniques to make the lasers more visible. JILA

Before they could start tracking time, however, scientists had to construct their clock. Ye’s team at NIST isolated and suspended the strontium atoms in a vacuum chamber using a laser. “It’s almost like an optical tweezer made of laser light,” Ye tells Popular Mechanics. The vacuum itself is not cold, but the atoms are cooled (again, by laser) to a fraction of a microkelvin. The kelvin is a unit of temperature that names zero as the absolute coldest point.

“This is incredibly cold, colder than any spot in the universe,” Ye says. The atoms need to be that cold in order to encourage them to behave less like particles and more like waves. “The quantum mechanical wave function of individual atoms overlap with each other and start to behave like a collective object,” he explains. It’s easier to hold them together this way. At this point, pairs of lasers go to work, trapping the cloud of atoms into a lattice formation.

The team started with a single pair of lasers beaming into the vacuum chamber. “Say you have a laser going from left to right. It gets reflected by a mirror. So now you have light coming from both directions, right, because the mirror will reflect the light. When the two waves come together, they have this interference that forms what’s called a standing wave. Essentially, just think of the wave as oscillating back and forth in a fixed location,” Ye says. Like water, the light wave has troughs and peaks. The highest intensity of light is at the peak, and light attracts the atoms; there is no light at the trough, and therefore no atoms. A pair of standing waves of light form what Ye calls a “stack of pancakes,” in which each pancake holds dozens of atoms. “That’s the simplest possible, one-dimensional lattice,” Ye says.

The team then shot another pair of laser lights into the vacuum chamber at right angles to the original laser beam. “Now the pancakes get sliced. Imagine you have a stack of pancakes this direction, and a stack of pancakes in that direction, when they slice into each other, they become like individual cigars,” Ye explains. Finally, the team added another set of lasers in a third spatial dimension. When three pairs of light interfere, it creates individual dots instead of cigars. “In this three-dimensional optical lattice, the atoms can be loaded one by one into individual dots,” Ye explains. Clock complete.

When the field of optical atomic clock research was new, each experiment involved only one atom. However, multiple atoms yield a clock many times more accurate, because of the strange properties of quantum mechanics. Many atoms oscillating together is similar to many coin flips—the more times you flip a coin, the closer you get to averaging to the correct overall probability.

You can assume that each atom acts like the pendulum of a standard clock, Ye says. “You want your pendulum to swing billions of times per second.” To further ensure its accuracy, his lab built multiple strontium clocks and also compared their optical lattice clock to another clock in a lab a mile away.

“One of the First Truly Quantum Technologies”

People get excited about the development of a quantum computer, “yet, atomic clocks are one of the first truly quantum technologies,” University of Wisconsin-Madison physicist Shimon Kolkowitz tells Popular Mechanics. While a laser is also a quantum technology, an atomic clock is the first example of the sort of tech leap that wouldn’t be possible without a deep understanding of quantum mechanics, he says. Quantum mechanics is the study of nature at the smallest scale: atoms and subatomic particles.

Kolkowitz’s research group recently measured differences between optical lattice atomic clocks, in which strontium atoms were separated into multiple clocks arranged in a line in the vacuum chamber. With one atomic clock, the laser could excite electrons in the same number of atoms for one-tenth of a second. However, when the laser hit two clocks simultaneously inside the vacuum chamber, the number of atoms with excited electrons stayed the same between the two clocks for up to 26 seconds.

To account for any differences due to changes in gravity or magnetic fields, the team ran more than 1,000 experiments. Ultimately, the researchers discovered that the clocks would match up their times perfectly before losing one second every 300 billion years. This study, published February 16 in Nature, set a world record for two spatially separated clocks. No other atomic clock had reached this level of accuracy, although the best single atomic clock in the world is 30 billion years.

Strontium is a better candidate for an optical clock than cesium, which lacks really narrow optical transitions, Kolkowitz says. “Cesium only has one valence electron, while strontium has two. This makes the level structure of strontium more complicated and rich, and results in extremely narrow optical transitions that simply don’t exist in cesium.”

The precision of atomic clocks is improving at a “mind-boggling” pace, Ye says. The first atomic clock would only gain or lose one second in three hundred years, a vast improvement over standard clocks and watches. Modern GPS timekeepers are a million times more accurate than this first clock. Today’s best optical atomic clocks can keep steady, accurate time, for far longer than the age of the universe, 13.8 billion years.

How We Use Atomic Clocks

Atomic clocks are now even being used to more accurately define all kinds of units—not just time, but also mass, length, electricity, and more, Kolkowitz says. For example, we can measure a meter with just the speed of light, which is scientifically accepted as a constant of 299,792,458 meters per second, and a clock. Now you can measure exactly how long a meter is because your atomic clock can register the amount of time it took for the light to travel one meter, Kolkowitz points out.

“We’re becoming more and more sensitive to new physics, even things we don’t know about,” Kolkowitz says. “Making better clocks is important, just because it determines how well we can measure the rest of the universe and actually quantify things.”

Multiple experiments involving two synchronized atomic clocks have proven that time can change depending on gravitational forces. In 2010, two atomic clocks ran side-by-side. Then, one was raised by 33 centimeters, and it began to run faster. The fraction of decrease in gravity in the higher clock made time slow down relative to the lower clock. Similar tests had been done before, by taking one of the atomic clocks high above Earth in an airplane. However, this more recent test proved to an astonishingly fine degree that the force of gravity affects the rate of time at a given point.

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