Atomic clock

An atomic clock is a special kind of clock that measures time by watching the vibrations of atoms. Atoms have very steady patterns, and these patterns help scientists keep track of time very precisely. This exact way of measuring time helps define the second, which is the basic unit of time that we all use.

Because atomic clocks are so accurate, they are important for many things. They help keep our world time system, called UTC, matching the Earth's movement. This means that when you look at the time on your phone or watch, it stays correct over long periods.

Atomic clocks are also used in systems that help us find our way, like the GPS that cars and phones use. Even a tiny mistake in time, like one-billionth of a second, can make a big difference in knowing exactly where we are on Earth.

Today, many atomic clocks use a special type of atom called caesium, cooled to very cold temperatures to work at their best. For example, in the United States, there is a famous clock called NIST-F2 that is extremely precise, helping to set the standard for time around the world.

Recent advances

In July 2025, scientists in the United States made a big jump in timekeeping. They created a very precise clock using a single trapped aluminium ion. This new clock is much more accurate than older ones, improving the record by a lot and being even more stable than others like it.

Redefinition of the second

Because atomic clocks have become so accurate, experts around the world are thinking about changing the way we define a second. In June 2025, scientists in six countries compared their atomic clocks. This helped them work towards creating a new, global standard for measuring time using light-based atomic clocks.

Technological impact

Optical atomic clocks are helping create new technologies. They allow for very accurate sharing of time and frequency, making global navigation satellite systems even better. These clocks also help scientists measure differences in gravity and test important theories about how our universe works.

History

The Scottish physicist James Clerk Maxwell suggested using the vibrations of light waves to measure time in 1873. He believed this would be more accurate than using the Earth's rotation, which is how we usually tell time.

In the 1930s, the American physicist Isidor Isaac Rabi created tools to measure the vibrations of atoms. This idea led to the development of atomic clocks, which are much more accurate than older clocks. The first practical atomic clock, using caesium atoms, was built in the United Kingdom in 1955.

Definition of the second

Main article: Second

In 1968, scientists officially defined the second as the time it takes for a caesium-133 atom to vibrate 9192631770 times. This definition helps us keep very accurate time.

Metrology advancements and optical clocks

New technologies like lasers and optical frequency combs have made atomic clocks even more accurate. Scientists are now working on optical clocks, which use different atoms and can be even more precise. These clocks might help us redefine the second in the future.

Chip-scale atomic clocks

Scientists have also made very small atomic clocks, about the size of a grain of rice. These tiny clocks use less power and could be used in things like GPS devices.

Measuring time with atomic clocks

Clock mechanism

An atomic clock measures time by watching how atoms change energy levels. Scientists prepare a group of atoms in one energy state and then shine microwave radiation on them. If the microwave has the right frequency, some atoms switch to another energy state. By tuning the microwave to the exact frequency where the most atoms switch, scientists can use this frequency to keep very accurate time.

Unlike clocks that use the swing of a pendulum or the movement of gears, atomic clocks use the vibrations of atoms. These vibrations are not affected by temperature or other outside factors, making atomic clocks much more accurate. The frequency at which atoms vibrate is also the same everywhere, unlike other types of clocks. This makes atomic clocks the most reliable way we have to measure time.

Accuracy

Atomic clocks have become more and more accurate since the first one was made in the 1950s. The earliest clocks used atoms like caesium, rubidium, and hydrogen. From 1959 to 1998, scientists made seven caesium clocks, with the first being accurate to 10⁻¹¹ and the last to 10⁻¹⁵. These clocks were the first to use a caesium fountain and a method called laser cooling of atoms.

Scientists are now working to make clocks even more accurate, aiming for accuracies of 10⁻¹⁸ or even 10⁻¹⁹. This would allow them to redefine the second so that these clocks would not lose or gain more than a second over the entire age of the universe. New types of clocks using strontium, ytterbium, and optical lattice technology are being developed. These optical clocks have even higher accuracy because they use energy levels that vibrate much faster than traditional atomic clocks.

Two important clocks that reached an accuracy of 10⁻¹⁶ are the United Kingdom's NPL-CsF2 caesium fountain clock and the United States' NIST-F2. The increase in precision from NIST-F1 to NIST-F2 was achieved by using liquid nitrogen to cool the microwave interaction region, reducing effects from black-body radiation.

Comparing atomic clocks

Many special labs around the world keep very exact clocks called atomic clocks. These labs include places like the Paris Observatory, the National Institute of Standards and Technology in the United States, and others. They build clocks that use the vibrations of atoms to keep very accurate time. These clocks can be so exact that their accuracy is measured in tiny amounts — like one part in 10 to the power of 16.

These clocks need to be compared with each other very carefully. One way to do this is by using satellites in space. Systems like GPS, GLONASS, Galileo, and BeiDou help scientists check how well clocks match up over long distances. This is important because even tiny differences can add up.

Labs also work together to create a common time scale called International Atomic Time (TAI). This is made by averaging the time from many clocks around the world. To make everyday time match up with this, something called Coordinated Universal Time (UTC) is used. UTC adds or takes away seconds called "leap seconds" so that our clocks stay in sync with Earth's rotation.

Scientists are also finding new ways to compare clocks using special cables called fiber optics. These cables can carry time signals very accurately over long distances, helping labs check their clocks even better.

Microwave atomic clocks

Caesium

Main article: Caesium standard

Atomic clocks measure time by watching the vibrations of atoms. The official way we measure a second is based on the vibrations of caesium atoms. Caesium clocks, like the NIST-F1 clock from 1999 and the NIST-F2 clock from 2013, are the most accurate timekeepers we have.

Caesium works well for this because it moves slower than other atoms and vibrates at a very precise frequency, allowing us to measure time more accurately.

Rubidium

Main article: Rubidium standard

Rubidium atomic clocks are cheaper and smaller than caesium clocks. They are often used in portable devices and space tools. These clocks can last over ten years and cost much less. Some use signals from global positioning system to stay very accurate.

Hydrogen

Main article: Hydrogen maser

Hydrogen clocks change very little over short times, making them great for radio astronomy. However, they need regular checks to stay accurate. They are also used to test ideas about space and gravity.

Other types of atomic clocks

Nuclear clock concept

Main article: Nuclear clock

Scientists have thought about a new kind of clock called a nuclear clock. Instead of watching the movements of atoms, these clocks would look at changes in the heart of the atom, called the nucleus. One special type of thorium, written as ^229mTh, could help make such a clock. This thorium has a very low energy change that matches light waves we can measure today.

Researchers have measured this energy change very exactly, which means we could now build a working nuclear clock. One big benefit of a nuclear clock is that it could be more exact than today’s best clocks. The nucleus is tiny and protected by the rest of the atom, so outside forces would not affect it as much. Also, many atoms could be used together, making the clock even better.

Potential for redefining the second

In 2022, the best way to measure a second uses special clocks called caesium primary standard clocks. These clocks cool a group of caesium atoms and then measure how they react to certain types of light. These clocks are very accurate, losing only about 50 billionths of a second each day. Many of these clocks around the world work together to keep track of time for everyone.

Future clocks might use light with much higher frequencies, which could make our time measurements even more precise. These new clocks could "tick" much faster than today’s clocks. Scientists are also looking at ways to define the second using the properties of atoms, but this is very difficult and needs more research. Before changing how we define a second, scientists need to make sure these new clocks are very reliable and can share their information accurately.

Applications

Atomic clocks help us in many ways, such as in navigation satellite systems. These systems use atomic clocks to give very accurate timing and frequency signals. For example, the Global Positioning System (GPS), run by the United States Space Force, uses atomic clocks on its satellites to help us find our way anywhere in the world.

Atomic clocks are also used in time signal radio transmitters. These clocks help make sure that time is kept very accurate for many things, like in science and for keeping records in financial systems. They are important for making sure everything runs smoothly and fairly.

Atomic clocks are also used to test ideas in science, like how time changes in different places because of gravity, as described in general relativity. Scientists use these clocks to check how accurate these ideas are.

Manufacturers

Some companies make atomic clocks that help keep track of the world's time. These include Microchip, T4Science, Anritsu, Microsemi, and HP. These clocks are important for keeping our time very accurate.