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The atomic transitions employed in the best of today's optical clocks are a strong foundation for the upcoming redefinition of the SI second. Including multiple transitions in the definition offers increased accuracy, a robust diversity of…
The current SI second based on the atomic hyperfine transition in the ground state of 133Cs is expected to be replaced by a new definition based on optical frequency standards, whose estimated uncertainty has now been established two orders…
The absolute frequency of the $^{87}{\rm Sr}$ clock transition measured in 2015 was reevaluated using an improved frequency link to the SI second. The scale interval of International Atomic Time (TAI) that we used as the reference was…
The pursuit of ever more precise measures of time and frequency is likely to lead to the eventual redefinition of the second in terms of an optical atomic transition. To ensure continuity with the current definition, based on a microwave…
The rapid increase in accuracy and stability of optical atomic clocks compared to the caesium atomic clock as primary standard of time and frequency asks for a future re-definition of the second in the International System of Units (SI).…
We report on a $^{88}$Sr$^{+}$ single-ion optical clock with an estimated fractional systematic uncertainty of $7.9\times 10^{-19}$. The low uncertainty is enabled by small rf losses, a thorough evaluation of the blackbody-radiation…
A two-photon transition in laser-cooled and trapped calcium atoms is proposed as the atomic reference in an optical frequency standard. An efficient scheme for interrogation of the frequency standard is described, and the sensitivity of the…
A new definition of the SI second based on optical transitions is expected to be adopted within the next 10 years. Several options for this redefinition are currently under consideration. Among them, a definition based on several…
With 87-Sr atoms confined in a one dimensional optical lattice, the frequency of the optical clock transition 5s^2 ^1S_0 - 5s5p ^3P_0 has been determined to be 429 228 004 229 872.9(5) Hz. The transition frequency was measured with the help…
We report the first contribution to the international atomic time (TAI) based on a secondary representation of the SI second. This work is done with the LNE-SYRTE FO2-Rb fountain frequency standard using the $^{87}$Rb ground state hyperfine…
Transition frequencies of atoms and ions are among the most accurately accessible quantities in nature, playing important roles in pushing the frontiers of science by testing fundamental laws of physics, in addition to a wide range of…
Atomic frequency standards are used to generate accurate and precise time and frequency, enabling many communications, synchronization, and navigation systems in modern life. GPS and other satellite navigation systems, voice and data…
We measured the absolute frequency of the $^1S_0$ - $^3P_0$ transition of $^{171}$Yb atoms confined in a one-dimensional optical lattice relative to the SI second. The determined frequency was 518 295 836 590 863.38(57) Hz. The uncertainty…
We report high-precision frequency ratio measurements between optical atomic clocks based on $^{27}$Al$^+$, $^{171}$Yb, and $^{87}$Sr. With total fractional uncertainties at or below $3.2 \times 10^{-18}$, these measurements meet an…
Time scales consistently provide precise time stamps and time intervals by combining atomic frequency standards with a reliable local oscillator. Optical frequency standards, however, have not been applied to the generation of time scales,…
The highly forbidden $^2$S$_{1/2} \rightarrow ^2$F$_{7/2}$ electric octupole transition in $^{171}$Yb$^+$ is a potential candidate for a redefinition of the SI second. We present a measurement of the absolute frequency of this optical…
A method is presented for analysing over-determined sets of clock frequency comparison data involving standards based on a number of different reference transitions. This least-squares adjustment procedure, which is based on the method used…
Optical clocks have achieved remarkable estimated fractional frequency uncertainties reaching the $10^{-18}$ level and below, enabling applications in fundamental physics, general relativity, and geodesy. However, the challenge of verifying…
The current methods for realization of the second with the highest accuracy through the frequency of the hyperfine transition between two levels of the ground state of the cesium 133 atom are considered. It is discussed possible ways for…
In the last ten years extraordinary results in time and frequency metrology have been demonstrated. Frequency-stabilization techniques for continuous-wave lasers and femto-second optical frequency combs have enabled a rapid development of…