NIST has introduced NIST-F4, a new cesium fountain atomic clock that is among the most accurate timekeepers in the world. This clock measures the unchanging frequency of cesium atoms, which is the basis for defining the second. NIST-F4 is so precise that if it had been running since the time of the dinosaurs, it would still be off by less than a second today. The clock has been submitted to the International Bureau of Weights and Measures (BIPM) for approval as a primary frequency standard, which helps maintain global time standards like Coordinated Universal Time (UTC).
NIST-F4 was developed after rebuilding the core of an older fountain clock, NIST-F1, with extreme precision—down to 5 to 10 microns. The new clock is now operating reliably and contributes to the accuracy of official U.S. time, which is used in telecommunications, finance, and transportation. While NIST-F4 is currently under review for global recognition, it is already helping to improve the stability of UTC(NIST), the version of UTC maintained by NIST. The clock’s high accuracy and reliability are expected to support future advancements in timekeeping, including potential changes to how the second is defined by 2030.
Keywords: atomic clock, cesium fountain, primary frequency standard
