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Published on: May 3, 2019
Atomic Clock Frequency Ratios with Fractional Uncertainty ≤3.2×10^{-18}
Alexander Aeppli1,2, Willa J Arthur-Dworschack2,3, Kyle Beloy3
1the University of Colorado, National Institute of Standards and Technology, JILA, Boulder, Colorado, USA.
Physical Review Letters
|July 31, 2026
Summary
High-precision atomic clock measurements using aluminum, ytterbium, and strontium achieve uncertainties below 3.2×10⁻¹⁸, a key step for redefining the second. New frequency comparisons highlight the need for independent laboratory validation and improved stability.
Area of Science:
- Atomic physics
- Metrology
- Quantum information science
Background:
- Optical atomic clocks are crucial for precise timekeeping and fundamental physics tests.
- Redefining the second requires atomic clocks with uncertainties below 10⁻¹⁸.
Purpose of the Study:
- To perform high-precision frequency ratio measurements between different optical atomic clocks.
- To assess the performance of atomic clocks against the criteria for redefining the second.
- To evaluate the impact of improved frequency transfer stability on clock comparisons.
Main Methods:
- Frequency ratio measurements using optical atomic clocks based on ^{27}Al^{+}, ^{171}Yb, and ^{87}Sr.
- Utilizing a common ultrastable reference delivered via a 3.6-km phase-stabilized fiber link.
- Employing three-corner-hat measurements to identify individual clock stabilities.
Main Results:
- Achieved total fractional uncertainties at or below 3.2×10⁻¹⁸ for clock comparisons.
- Observed discrepancies in ^{87}Sr and Al^{+}/Yb ratios compared to previous measurements.
- Demonstrated improved comparison stability by a factor of 2-3 using a cryogenic silicon cavity reference.
- Showcased the most stable optical lattice and trapped-ion clocks in a multispecies comparison.
Conclusions:
- The achieved uncertainties meet a critical milestone for the redefinition of the second.
- Discrepancies emphasize the importance of independent laboratory comparisons and validation.
- Enhanced frequency transfer stability improves accuracy and enables faster comparisons, pushing the limits of optical frequency standards.
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