NIST has also submitted the clock for acceptance as a primary frequency standard by the International Bureau of Weights and Measures (BIPM), the body that oversees the world’s time.
NIST-F4 measures an unchanging frequency in the heart of cesium atoms, the internationally agreed-upon basis for defining the second since 1967. The clock is based on a “fountain” design that represents the gold standard of accuracy in timekeeping. NIST-F4 ticks at such a steady rate that if it had started running 100 million years ago, when dinosaurs roamed, it would be off by less than a second today.
By joining a small group of similarly elite time pieces run by just 10 countries around the world, NIST-F4 makes the foundation of global time more stable and secure. At the same time, it is helping to steer the clocks NIST uses to keep official U.S. time. Distributed via radio and the internet, official U.S. time is critical for telecommunications and transportation systems, financial trading platforms, data center operations and more.
NIST-F4 has improved time signals that are “used literally billions of times each day for everything from setting clocks and watches to ensuring the accurate time stamping of hundreds of billions of dollars of electronic financial transactions,” said Liz Donley, chief of the Time and Frequency Division at NIST.
Cesium fountain clocks such as NIST-F4 are a type of atomic clock – a complex, high-precision device that extracts timing pulses from atoms. These clocks play a critical role in our globally connected society: They serve as “primary frequency standards” that work together to calibrate Coordinated Universal Time, or UTC (an agreed-upon system for keeping time using data from atomic clocks around the world, known as a time scale).
National measurement labs such as NIST produce and distribute versions of UTC using their own time scales; NIST’s version, for example, is known as UTC(NIST). Those national time scales are then used to synchronize the clocks and networks we rely on in our daily lives.
In fountain clocks, a cloud of thousands of cesium atoms is first cooled to near absolute zero using lasers. Then, a pair of laser beams toss the atoms gently upward, after which they fall under their own weight.
During their journey, the atoms pass twice through a small chamber full of microwave radiation. The first time, as the atoms are on their way up, the microwaves put the atoms into a quantum state that cycles in time at a special frequency known as the cesium resonant frequency — an unchanging constant set by the laws of nature.
About one second later, as the atoms fall back down, a second interaction between the microwaves and the atoms reveals how close the clock’s microwave frequency is to the atoms’ natural resonant frequency. This measurement is used to tune the microwave frequency toward the atomic resonance frequency.
A detector then counts 9,192,631,770 wave cycles of the fine-tuned microwaves. The time it takes to count those cycles defines the official international second.
(That may change as early as 2030, when nations plan to consider redefining the second in terms of one or more different atomic elements used in so-called optical clocks that can measure time even more precisely than fountain clocks can. Even after that, cesium fountain clocks will still play an important, though diminished, role in timekeeping.)
