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Thanks to NIST-F4, a newly developed atomic clock at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, the accuracy of timekeeping on Earth has reached new heights.
This month, NIST researchers published findings in Metrologia, positioning NIST-F4 among the most precise timekeepers globally. The clock has also been proposed for recognition as a primary frequency standard by the International Bureau of Weights and Measures (BIPM), the global authority on time regulation.
NIST-F4 measures a consistent frequency inherent to cesium atoms, which has served as the basis for defining the second since 1967. The clock employs a “fountain” design, which is considered the benchmark for precision in timekeeping. Remarkably, if this clock had begun operation 100 million years ago, during the age of dinosaurs, it would only be off by less than a second today.
By entering an exclusive cohort of elite timepieces operated by only ten countries worldwide, NIST-F4 enhances the stability and security of global timekeeping. Additionally, it contributes to the clocks used by NIST to maintain the official time in the United States. This authorized time is crucial for various domains, including telecommunications, transport systems, and financial markets.
“NIST-F4 has elevated time signals that are utilized billions of times daily, from synchronizing clocks to ensuring precise timestamps for numerous electronic financial transactions,” stated Liz Donley, head of the Time and Frequency Division at NIST.
A Unique Timekeeping Mechanism
Cesium fountain clocks, such as NIST-F4, belong to a specialized category of atomic clocks, which are complex instruments that retrieve timing signals from atoms. These devices play an essential role in our interconnected world by serving as “primary frequency standards.” These standards collaborate to regulate Coordinated Universal Time (UTC), a system that synchronizes time using data from atomic clocks across the globe.
Measurement laboratories like NIST generate and distribute their versions of UTC; NIST’s variant is termed UTC(NIST). These national time scales are pivotal for synchronizing the clocks essential for our daily activities.
In fountain clocks, cesium atoms are initially cooled to near absolute zero via lasers. Afterward, two laser beams gently propel the atoms upward, allowing them to fall back down due to gravity.
During this descent, the atoms traverse a microwave-radiation chamber twice. The initial interaction with microwaves causes the atoms to assume a quantum state oscillating at a frequency known as the cesium resonant frequency, a constant dictated by natural laws.
Upon their return, a second microwave interaction assesses how closely the clock’s frequency aligns with this intrinsic resonant frequency, enabling further refinement of the microwave signal. Ultimately, the detector counts 9,192,631,770 cycles of the adjusted microwaves, defining the official international second.
This definition may soon evolve, as nations plan to deliberate redefining the second in terms of alternative atomic elements utilized in upcoming optical clocks, known for even greater precision. However, cesium fountain clocks will likely continue to play a significant, albeit reduced, role in timekeeping.
A Long-Awaited Development
There are fewer than 20 cesium fountain clocks currently in operation worldwide. Unlike commercially available atomic clocks designed for internet data centers and financial entities, most fountain clocks are crafted and managed by scientists in national measurement laboratories like NIST.
“It’s a remarkable technology that offers substantial performance benefits, but it is also quite delicate,” remarked Greg Hoth, a physicist involved in the clock’s development.
The journey to bring NIST-F4 to fruition involved many years of work. The original fountain clock, NIST-F1, was constructed in the late 1990s and operated for over 15 years. However, following its relocation in 2016, it required extensive restoration to regain its status as a primary frequency standard, a process that took longer than anticipated.
In 2020, physicist Vladislav Gerginov began reexamining NIST-F1’s frequency measurements. Along with Hoth and their team, they ultimately decided to rebuild the clock’s core components, focusing specifically on the microwave cavity, where cesium measurements are made. Achieving the desired precision required maintaining tolerances ranging from 5 to 10 microns—equivalent to about one-fifth the diameter of a human hair.
The team meticulously refined new electric heating coils, magnetic coils, optics, and microwave components, eventually leading to the naming of the new clock as NIST-F4, following NIST-F2 and NIST-F3.
The research team conducted extensive measurements to ensure NIST-F4 remained stable against fluctuations in pressure, temperature, and stray electric and magnetic fields. They continually compared the fountain’s timekeeping with that of hydrogen masers—conventional atomic clocks responsible for official U.S. time—to verify consistent performance.
“Fountain clocks should ideally be very stable and predictable,” Hoth noted. Evaluating a clock like NIST-F4 is a lengthy process, according to Gerginov, who emphasized the necessity of thorough understanding prior to operational deployment, as any timing inaccuracies could disrupt not just U.S. time, but the global timekeeping system as a whole.
Recently, the NIST team confirmed that NIST-F4’s frequency measurements are accurate to within 2.2 parts in 10 to the 16th (10 million billion), placing it among the world’s top fountain clocks. The data has also been submitted to the BIPM for expert evaluation prior to the official certification as a primary frequency standard.
“The success of NIST-F4 reinforces NIST’s leadership in the realm of primary frequency standards,” remarked Donley. “Vladi and Greg displayed exceptional skill and creativity in restoring the reliable, world-class operation of our atomic fountains.”
NIST-F4 and another fountain clock, NIST-F3, maintain operational status approximately 90% of the time, ensuring that at least one clock is functioning at any given moment. Data from NIST-F4 will be regularly transmitted to BIPM for UTC calibration, while both timepieces are actively contributing to the NIST time scale, UTC(NIST).
Donley noted that “the NIST time scale has greatly benefited from both the high uptime and the dependable performance of the fountain clock.”
More information: Vladislav Gerginov et al, Accuracy evaluation of primary frequency standard NIST-F4, Metrologia (2025). DOI: 10.1088/1681-7575/adc7bd
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phys.org