The universal need to be on time keeps everyone on the move, shaping society into a permanently well-oiled machine. Whether you find yourself glancing at a clock on the wall or checking your phone, the time you constantly see is the product of a meticulous system upheld by the world’s timekeepers. In the U.S., a new atomic clock called NIST-F4 has already proven to be one of the most precise timekeepers yet. 

Assembled by scientists at the National Institute of Standards and Technology, NIST-F4 is now up and running as of April 2025, as recently announced by the agency. In addition to synchronizing the clocks we see on our computers and smartphones, NIST-F4 sets the standard for an essential unit of time: the second.

The Golden Standard of Timekeeping

NIST-F4 is a cesium fountain clock, which is considered the cream of the crop — there are fewer than 20 of its kind operating in the entire world. Fountain clocks are not constantly running like normal clocks that show us hours and minutes.  Instead, they act as “primary frequency standards”, benchmarks for the world’s time. Altogether, these clocks decide Coordinated Universal Time (UTC), the global system for measuring time. 

According to a statement from NIST, the new clock is so reliable that if it started to run when dinosaurs existed 100 million years ago, “it would be off by less than a second today.” 

Read More: What Is Time?

How Do Cesium Fountain Clocks Work?

To achieve such exact timekeeping, NIST-F4 goes through a process involving cesium atoms. In cesium fountain clocks, a cloud of around 100,000 cesium atoms is first gradually cooled with lasers. After this, the atoms are lobbed upwards and exposed to microwave radiation. This causes the atoms to take on a quantum state that reflects their resonant frequency (in this case, the “cesium resonant frequency”). 

When the atoms fall back down, they’re hit with another round of microwaves. The interaction allows for a comparison to be made between the clock’s microwave frequency and the atoms’ natural resonant frequency. The microwave frequency is then tuned toward the atomic resonance frequency.

Lastly, a detector counts 9,192,631,770 wave cycles of the tuned microwaves. According to NIST, the time it takes to count these cycles defines what we know as a second. 

Calibrating Global Clocks

NIST-F4 is now the fourth iteration in NIST’s series of fountain clocks, following a legacy that started decades ago. The agency’s first fountain clock (NIST-F1) started operation in 1999, serving continuously as a primary standard frequency until 2016, when the clock required restoration. 

In 2020, NIST scientists figured out that there was an issue with the clock’s microwave cavity, a small chamber where the cesium atoms are subjected to microwave radiation. As a result, they had to rebuild the core from scratch. The restoration process also required them to add several new components and account for various factors that could disturb the clock’s frequency measurements (like pressure fluctuations and stray electric/magnetic fields). The revitalized clock became known as NIST-F4. 

Although the clock is supposed to be “boring”, NIST scientists say, this is exactly the quality they want to see in a fountain clock since it signifies stability. 

Data from NIST-F4 will be sent to the International Bureau of Weights and Measurements (BIPM), the global body in charge of coordinating UTC. The clock’s measurements will join data from around 450 other clocks worldwide. BIPM calculates a weighted average of all the data and lets timekeeping institutes know how their clocks compare to UTC, giving them an opportunity to adjust their time scales.

NIST-F4 and all the other atomic clocks are the pillars that support timekeeping as we know it. Without them, transportation and communication systems would be thrown into disarray. The timekeepers of the world, thankfully, are always making sure that everything is right on time, as it should be.

Read More: How Could Atomic Clocks Be Used to Detect Dark Matter?

Article Sources

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Jack Knudson is an assistant editor at Discover with a strong interest in environmental science and history. Before joining Discover in 2023, he studied journalism at the Scripps College of Communication at Ohio University and previously interned at Recycling Today magazine.