On a campus in Boulder, Colorado, time just became a little more exact.
Inside the National Institute of Standards and Technology, or NIST, a new atomic clock named NIST-F4 has begun to tick — not with the sound of gears or bells, but with the quantum pulse of cesium atoms. So precise is its rhythm that, had it been running when dinosaurs walked the Earth, it would only now be off by less than a second.
NIST-F4 doesn’t just keep time. It defines it. It’s the latest primary frequency standard — what scientists consider a reference clock.
“NIST-F4 is an operational [primary frequency standard] with a type B frequency uncertainty of 2.2×10⁻¹⁶,” wrote Vladislav Gerginov and colleagues in a new paper published in Metrologia.
This month, scientists at NIST announced that NIST-F4 has joined the elite ranks of the world’s most accurate timekeepers. The agency has submitted the clock for official recognition to the International Bureau of Weights and Measures, or BIPM, the body that oversees global timekeeping.
What Makes NIST-F4 So Special?
NIST-F4 is a type of cesium fountain clock — an ultra-sensitive instrument that turns nature’s most reliable metronome into the definition of a second.
To understand how it works, imagine thousands of cesium atoms cooled by lasers to a temperature just above absolute zero. Two laser beams gently toss this cloud of atoms into the air like a fountain. As they rise and fall under gravity’s pull, the atoms pass through a chamber bathed in microwave radiation — twice.
The first pass excites the atoms into a quantum state that oscillates at a natural, immutable frequency. The second measures how closely the microwave frequency matches that atomic rhythm. Adjustments are made until the two align perfectly.
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The clock then counts exactly 9,192,631,770 microwave cycles — the number of wave peaks that define one second, according to international agreement since 1967.
The design of NIST-F4 draws from its predecessor, NIST-F1, which retired in 2022. Engineers recycled much of its vacuum hardware but introduced refinements to the laser and microwave systems, including a new copper Ramsey cavity — the heart of the fountain where atoms interact with precisely timed microwave pulses.
“Fountain clocks are supposed to be very boring,” said Greg Hoth, a NIST physicist who helped build NIST-F4.
And yet the implications of this device are anything but boring.
Time signals derived from NIST-F4 flow into everything from GPS navigation and high-frequency trading to power grids and smartphone networks. The clock’s regularity underpins the systems that keep modern life in sync — often without us ever noticing.
“These signals are used literally billions of times each day,” Donley said, “for everything from setting clocks and watches to ensuring the accurate time stamping of hundreds of billions of dollars of electronic financial transactions.”
Reducing Every Possible Source of Error
Measuring time this accurately means confronting every known quirk of physics that might influence time keeping, and then accounting for it.
There’s the quadratic Zeeman shift, where magnetic fields nudge the clock’s frequency ever so slightly. There’s blackbody radiation, the ambient thermal bath from surrounding hardware. Even gravity plays a role: the exact height of the microwave cavity above sea level affects the clock via Einstein’s general relativity.
NIST-F4’s designers left no systematic effect unexamined. They quantified cold collision shifts — where atoms subtly affect each other’s quantum states. They tracked microwave leakage, lensing effects from the microwave fields, and even “distributed cavity phase” shifts, where spatial variations in the electromagnetic field distort the timing.
This is how the scientists were ultimately able to achieve a total systematic uncertainty of 2.2×10⁻¹⁶ — a precision equivalent to losing less than a second every 140 million years.
In addition to precision, clocks must be stable — resistant to drift. In high-density mode, NIST-F4 achieves a frequency stability of 1.5×10⁻¹³ per √τ (tau), where τ is the measurement time in seconds.
This performance is limited mainly by quantum projection noise — the randomness inherent in quantum measurements — and by phase noise from its oven-controlled crystal oscillator (OCXO). Both factors could be improved with better oscillators and refined laser cooling, the team notes.
To validate its performance, NIST-F4 was compared against other national frequency standards, including the global average reported in the BIPM’s Circular T reports. It passed the test.
“NIST-F4 agrees with the [primary and secondary frequency standards] ensemble within the measurement uncertainties,” the authors report.
The Long Road to Perfect Time
NIST-F4 didn’t emerge overnight. It is the fourth in a series (hence its name) of fountain clocks developed by NIST over the past quarter century.
The agency’s first fountain clock, NIST-F1, began ticking in the late 1990s and served for over 15 years. But in 2016, after a move to a new building, it faltered. Bringing it back required more than a tune-up.
In 2020, physicist Vladislav Gerginov and his colleagues realized they needed to rebuild the clock’s core: the microwave cavity where atomic measurements are made. The precision they aimed for was staggering — tolerances of just 5 to 10 microns, thinner than a human hair.
They reengineered heating elements, magnets, optics, and microwave sources. The result was a clock so stable it could serve as a primary frequency standard — the benchmark against which all other timepieces are checked.
“Evaluating a fountain clock like NIST-F4 is a slow process,” Gerginov said. “We have to be very conservative. We should know everything about it.”
Because a small error doesn’t just throw off a wristwatch — it threatens the synchronization of power stations, aircraft navigation, and data flows that crisscross continents.
Now, NIST-F4 operates nearly 90% of the time, alongside a second fountain clock, NIST-F3. Together, they help guide UTC(NIST), the official U.S. time, and feed data into the international time scale maintained by BIPM.
Why This Matters — And What Comes Next
Only about 20 cesium fountain clocks exist worldwide, and they are the bedrock of global timekeeping. Each contributes to Coordinated Universal Time (UTC), a single clock woven from the work of national labs around the planet.
By contributing to UTC, NIST-F4 helps ensure that every phone call, every stock trade, and every satellite in Earth’s orbit runs on the same beat.
Yet change is coming. In the next few years — perhaps as soon as 2030 — timekeeping may shift toward optical clocks, which use different atoms and tick even faster. These new clocks promise even greater accuracy. Still, cesium fountains like NIST-F4 will remain vital as reference points and tools for international coordination.
In an age defined by speed, NIST-F4 is a reminder that sometimes, the most extraordinary progress is measured in billionths of a second.
🕰️ Evolution of Atomic Clocks
1949 – The First Atomic Clock
- Harold Lyons at the U.S. National Bureau of Standards (now NIST) developed the first atomic clock using the ammonia molecule.
- While groundbreaking, it lacked the precision needed for time standards.
1955 – First Cesium Atomic Clock
- Louis Essen and Jack Parry at the UK’s National Physical Laboratory built the first cesium-beam atomic clock.
- This clock was accurate enough to redefine the second.
1967 – Redefinition of the Second
- The International System of Units (SI) redefined the second based on the cesium-133 atom’s vibrations: “The duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.”
1993 – NIST-7 Cesium Beam Clock
- NIST introduced NIST-7, achieving an accuracy where it wouldn’t gain or lose a second in 6 million years.
2014 – NIST-F2 Cesium Fountain Clock
- NIST unveiled NIST-F2, operating at extremely low temperatures to reduce errors caused by radiation.
- It achieved an accuracy of one second in 300 million years.
2025 – NIST-F4 Cesium Fountain Clock
- NIST’s latest, NIST-F4, boasts an accuracy where it would be off by less than a second over 100 million years.
- It contributes to the U.S. time scale and supports global timekeeping.