Draft:NIST-F3
Optical atomic clock
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NIST-F3 is an optical clock operated by the National Institute of Standards and Technology (NIST) in Boulder, Colorado. It is based on an ytterbium optical lattice and serves as one of NIST’s highest-accuracy frequency standards. NIST-F3 contributes to international timekeeping through evaluations reported to the International Bureau of Weights and Measures (BIPM) and represents improvement over [1]microwave-based primary frequency standards.
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NIST-F3 was developed by the Time and Frequency Division of NIST’s Physical Measurement Laboratory as part of NIST’s transition from microwave cesium standards to optical frequency standards. Unlike cesium fountain clocks, NIST-F3 operates at optical frequencies, enabling substantially lower systematic uncertainty and higher frequency stability.
Background
For several decades, NIST maintained cesium-based primary frequency standards, culminating in the NIST-F1 and NIST-F2 cesium fountain clocks. Advances in laser stabilization, optical frequency combs, and atomic control enabled the development of optical clocks with orders-of-magnitude improvements in performance. NIST-F3 was designed to exploit these advances using neutral ytterbium atoms confined in an optical lattice.[2]
Although the SI second is still defined using the cesium hyperfine transition, optical clocks such as NIST-F3 are recognized by the BIPM as candidates for a future redefinition of the second and are reported as high-accuracy frequency standards.
Frequency measurement
NIST-F3 traps thousands of neutral ytterbium atoms in a one-dimensional optical lattice formed by counter-propagating laser beams. The lattice operates at a “magic wavelength” that minimizes perturbations of the atomic transition frequency. A highly stabilized laser probes a narrow optical transition of ytterbium, and the resulting excitation probability is used to lock the laser frequency to the atomic resonance.
An optical frequency comb links the optical reference frequency of NIST-F3 to microwave timescales, allowing direct comparison with other atomic clocks and enabling contributions to international time and frequency standards.
Accuracy and stability
NIST-F3 achieves fractional frequency uncertainties in the low 10−18 range, representing more than two orders of magnitude improvement over the best cesium fountain clocks. At this level of performance, the clock would neither gain nor lose a second over the age of the universe. Its short-term stability also enables rapid averaging to extremely low uncertainty, making it well suited for precision timekeeping, fundamental physics tests, and geodesy.
Evaluated accuracy
Evaluated accuracy reports for NIST-F3 are submitted to the BIPM and published as part of international time scale documentation. These evaluations account for systematic effects specific to optical lattice clocks, including blackbody radiation shifts, lattice light shifts, density-dependent interactions, and relativistic frequency shifts due to gravity.
NIST-F3’s evaluations place it among the most accurate atomic clocks in operation worldwide and provide key data supporting international comparisons of optical frequency standards.
Role in timekeeping
While cesium fountains remain the formal basis of the SI second, NIST-F3 plays a critical role in validating and cross-checking existing standards and in advancing the case for a future optical redefinition of the second. It operates alongside other NIST optical clocks and contributes to the broader international effort to transition global timekeeping to optical standards.