Draft:NIST-F4
Atomic clock used for US time standard
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NIST-F4 is a caesium fountain atomic clock developed and operated by the National Institute of Standards and Technology (NIST) in Boulder, Colorado. It is intended to serve as a next-generation primary frequency standard for the United States, succeeding earlier cesium fountain clocks while incorporating design improvements aimed at reducing uncertainty.[1]
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NIST-F4 was created by rebuilding and upgrading the microwave cavity and associated systems of the earlier NIST-F1 fountain clock, resulting in a substantially revised instrument rather than a simple continuation of previous designs. As of the mid-2020s, NIST-F4 is undergoing formal accuracy evaluation and certification for contribution to international atomic time.
Background
NIST maintains a series of cesium fountain primary frequency standards, including NIST-F1 and NIST-F2. Experience gained from long-term operation of those clocks identified several prominent sources of uncertainty, particularly those associated with microwave cavity phase variations and related Doppler effects.
Beginning around 2020, NIST undertook a major redesign of the fountain apparatus, leading to the construction of NIST-F4.[2] The project focused on addressing known limitations of earlier fountains by implementing a newly engineered microwave cavity and associated interrogation system.
Design and operation
Like other cesium fountain clocks, NIST-F4 operates by laser-cooling a cloud of cesium atoms and launching them vertically through a microwave interrogation cavity. The atoms pass through the cavity twice—once on the way up and once on the way down—forming an atomic interferometer sensitive to the microwave frequency.
NIST-F4 differs from its predecessors in several key respects, including a redesigned microwave cavity intended to suppress distributed cavity phase effects, improved control of electromagnetic fields, and upgraded diagnostics for measuring frequency shifts. These changes were specifically motivated by lessons learned from previous generations of fountain clocks.[3]
Accuracy
NIST-F4 is designed to achieve fractional frequency uncertainty comparable to or better than earlier state-of-the-art cesium fountain clocks.[4] The improvements in cavity design and systematic shift control are intended to reduce dominant uncertainty contributions that limited earlier evaluations.
At its target performance level, NIST-F4 would not gain or lose a second over hundreds of millions of years, consistent with the best cesium-based primary frequency standards worldwide.
Evaluated accuracy
Formal evaluated accuracy reports for NIST-F4 are submitted to the International Bureau of Weights and Measures (BIPM) as part of the process of certification as a primary frequency standard. As of 2025, NIST-F4 is undergoing BIPM evaluation and has not yet accumulated the long-term reporting history of earlier NIST fountains.
Once certified, NIST-F4 is expected to contribute directly to International Atomic Time (TAI) and to support the realization of Coordinated Universal Time (UTC).
Role in timekeeping
Although optical atomic clocks now surpass cesium fountains in achievable accuracy, cesium-based standards such as NIST-F4 remain essential because the SI second is still defined using the cesium hyperfine transition. NIST-F4 therefore plays a critical role in maintaining continuity between the current SI definition and future optical time standards.
See also
- NIST-F3