• NIST-F4 is a new cesium fountain atomic clock developed by NIST with accuracy to within 2.2 parts in 10 to the 16th, making it one of the world's most precise timekeepers.
• The clock has been submitted for acceptance as a primary frequency standard by the International Bureau of Weights and Measures (BIPM), solidifying global time stability and security.
• NIST-F4's accuracy impacts critical infrastructure like telecommunications, financial trading, and data centers by improving the reliability of official U.S. time signals.

The National Institute of Standards and Technology (NIST) in Boulder, Colorado, has introduced NIST-F4, a new cesium fountain atomic clock. NIST-F4 is designed to measure an unchanging frequency in cesium atoms, the standard for defining the second since 1967. The clock's precision is so great that it would only be off by less than a second if it had been running since the age of dinosaurs, approximately 100 million years ago. By joining the group of elite timekeeping devices run by only 10 countries, NIST-F4 enhances the stability and security of global time, and will contribute to official U.S. time.

• What: NIST-F4 is a cesium fountain atomic clock that serves as a primary frequency standard for timekeeping. It measures the unchanging frequency of cesium atoms to define the second.
• Who: The clock was developed by scientists at the National Institute of Standards and Technology (NIST), including Greg Hoth and Vladislav Gerginov, and overseen by Liz Donley, chief of the Time and Frequency Division at NIST. The International Bureau of Weights and Measures (BIPM) is involved in certifying the clock.
• When: NIST-F4 began running in April 2025. Details of the NIST-F4's workings were published April 15 in the journal Metrologia.
• Where: The clock is located at the NIST campus in Boulder, Colorado. Data from the clock will be sent to the BIPM.

NIST-F4 represents a significant advancement in atomic clock technology, improving the accuracy and reliability of timekeeping standards. Its integration into the global timekeeping infrastructure will enhance the precision of various systems that rely on accurate time signals. The development of NIST-F4 also highlights ongoing efforts to refine time measurement and potentially redefine the international second using optical clocks in the future, ensuring increasingly precise timekeeping for scientific and technological applications.

Time signals 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.'

'It’s a beautiful technology that has real performance advantages, but it’s very delicate.'

'Evaluating a fountain clock such as NIST-F4 is a slow process because we have to be very conservative. We should know everything about it' before putting it into service, because any error in the timing signals could corrupt not only U.S. time but also the global timekeeping infrastructure.

NIST-F4 marks a significant milestone in timekeeping technology, providing enhanced accuracy and stability to global time standards. With its pending certification by the BIPM, NIST-F4 will play a crucial role in maintaining precise time signals for various applications, from telecommunications to financial trading. The ongoing operation of NIST-F4 alongside NIST-F3 ensures continuous reliability, contributing to the advancement of timekeeping science and technology.