The extreme precision of optical atomic clocks has led to an anticipated redefinition of the second by the International System of Units. Furthermore, accuracies pushing the boundary of 1 part in 10 18 and beyond will enable new applications, such as in geodesy and tests of fundamental physics. The 1 S 0 to 3 D 1 optical transition in 176 Lu + has exceptionally low sensitivity to external perturbations, making it suitable for practical clock implementations with inaccuracy at or below 10 −18 . Here, we perform high-accuracy comparisons between two 176 Lu + references using correlation spectroscopy. A comparison at different magnetic fields is used to obtain a quadratic Zeeman coefficient of −4.89264(88) Hz/mT for the reference frequency. With a subsequent comparison at low field, we demonstrate agreement at the low 10 −18 level, statistically limited by the averaging time of 42 hours. The evaluated uncertainty in the frequency difference is 9 × 10 −19 and the lowest reported in comparing independent optical references.
The extreme precision of optical atomic clocks has led to an anticipated redefinition of the second by the International System of Units. Furthermore, accuracies pushing the boundary of 1 part in 10 18 and beyond will enable new applications, such as in geodesy and tests of fundamental physics. The 1 S 0 to 3 D 1 optical transition in 176 Lu + has exceptionally low sensitivity to external perturbations, making it suitable for practical clock implementations with inaccuracy at or below 10 −18 . Here, we perform high-accuracy comparisons between two 176 Lu + references using correlation spectroscopy. A comparison at different magnetic fields is used to obtain a quadratic Zeeman coefficient of −4.89264(88) Hz/mT for the reference frequency. With a subsequent comparison at low field, we demonstrate agreement at the low 10 −18 level, statistically limited by the averaging time of 42 hours. The evaluated uncertainty in the frequency difference is 9 × 10 −19 and the lowest reported in comparing independent optical references.
Optical atomic frequency references1 have far surpassed their microwave frequency predecessors, leading to an anticipated redefinition of the SI (International System of Units) second2. However, as the scientific community seeks consensus on a new standard, and several state-of-the-art optical standards now report evaluated fractional uncertainties below 10−18, verification by same-species comparisons to comparable levels remains an outstanding challenge. Here we report two 176Lu+ single-ion optical frequency references, each with evaluated fractional frequency uncertainty near 1 × 10−19, which are directly compared by correlation spectroscopy and demonstrate agreement, with a measured relative frequency difference of [−0.1 ± (5.7)stat ± (1.0)sys] × 10−19, where ‘stat’ and ‘sys’ indicate the statistical and systematic uncertainty, respectively. Our optical references have been comprehensively assessed with evaluated uncertainties below 10−18, supported by a same-species comparison of independent systems to the 5.7 × 10−19 level. This accuracy, achieved in practical room-temperature systems, will contribute to improving international timekeeping and towards chronometric levelling3,4 at the millimetre scale, as well as tests of fundamental physics5 such as Lorentz invariance6, general relativity7, searches for dark matter8,9 and variation of fundamental constants10,11. Two independent 176Lu+ single-ion optical clocks achieve fractional frequency uncertainties near 10−19 and agree at 5.7 × 10−19, advancing precision timekeeping.
Comparisons of high-accuracy optical atomic clocks \cite{Ludlow2015} are essential for precision tests of fundamental physics \cite{Safronova2018}, relativistic geodesy \cite{McGrew2018, Grotti2018, Delva2019}, and the anticipated redefinition of the SI second \cite{Riehle2018}. The scientific reach of these applications is restricted by the statistical precision of interspecies comparison measurements. The instability of individual clocks is limited by the finite coherence time of the optical local oscillator (OLO), which bounds the maximum atomic interrogation time. In this letter, we experimentally demonstrate differential spectroscopy \cite{Hume2016}, a comparison protocol that enables interrogating beyond the OLO coherence time. By phase-coherently linking a zero-dead-time (ZDT) \cite{Schioppo2017} Yb optical lattice clock with an Al+ single-ion clock via an optical frequency comb and performing synchronised Ramsey spectroscopy, we show an improvement in comparison instability relative to our previous result \cite{network2020frequency} of nearly an order of magnitude. To our knowledge, this result represents the most stable interspecies clock comparison to date.
Atomic clock frequency ratios with fractional uncertainty ≤3.2×10−18
We report high-precision frequency ratio measurements between optical atomic clocks based on 27Al+, 171Yb, and 87Sr. With total fractional uncertainties at or below 3.2×10−18, these measurements meet an important milestone criterion for redefinition of the second in the International System of Units. Discrepancies in 87Sr ratios at approximately 1×10−16 and the Al+/Yb ratio at 1.6×10−17 in fractional units compared to our previous measurements underscore the importance of repeated, high-precision comparisons by different laboratories. A key innovation in this work is the use of a common ultrastable reference delivered to all clocks via a 3.6 km phase-stabilized fiber link between two institutions. Derived from a cryogenic single-crystal silicon cavity, this reference improves comparison stability by a factor of 2 to 3 over previous systems, with an optical lattice clock ratio achieving a fractional instability of 1.3×10−16 at 1 second. By enabling faster comparisons, this stability will improve sensitivity to non-white noise processes and other underlying limits of state-of-the-art optical frequency standards.
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 3.2×10^{-18}, these measurements meet an important milestone criterion for redefinition of the second in the International System of Units. Discrepancies in ^{87}Sr ratios at approximately 1×10^{-16} and the Al^{+}/Yb ratio at 1.6×10^{-17} in fractional units compared to our previous measurements underscore the importance of repeated, high-precision comparisons by different laboratories. A key upgrade from our previous work is the use of a common ultrastable reference delivered to all clocks via a 3.6-km phase-stabilized fiber link between two institutions, enabling better accuracy and stability in frequency transfer. Derived from a cryogenic single-crystal silicon cavity, this reference improves comparison stability by a factor of 2-3 over previous systems, with an optical lattice clock ratio achieving a fractional instability of 1.3×10^{-16} at 1 s. Identifying individual clock stabilities with a three-corner-hat measurement, we demonstrate the most stable optical lattice and trapped-ion clocks used in a multispecies comparison. By enabling faster comparisons, this stability will improve sensitivity to nonwhite noise processes and other underlying limits of state-of-the-art optical frequency standards.
International atomic clock comparison achieving 5×10−18 uncertainty
Two single-ion optical clocks, located in the United Kingdom and in Germany, are compared remotely via a 2220\,km long optical fiber link. Both clocks are based on the 2S1/2(F=0)↔2F7/2(F=3) electric octupole (E3) transition in 171Yb+. Using more than 106 seconds of measurement data, we determine a fractional frequency difference of 1.0(4.8)×10−18, demonstrating agreement of the independently developed systems. To our knowledge, this constitutes the first international clock comparison reaching a fractional uncertainty below 5×10−18, an important milestone towards an anticipated optical redefinition of the SI second. The measurement uncertainty is dominated by that of the relativistic redshift correction between the two sites, which includes time-varying gravity potential differences. We exploit the large baseline between the two clocks to search for violations of Lorentz invariance. In the absence of statistically significant modulations, we improve constraints on the Robertson-Mansouri-Sexl parameter α by approximately a factor of five in the uncertainty, and provide limits on corresponding parameters within the Standard-Model Extension framework.
Laboratory optical atomic clocks based on quantum-logic spectroscopy of the1S0↔3P0transition in27Al+have reached a systematic uncertainty of 8.6×1018[1], enabling table-top tests of fundamental physics [2] as well as measurements of gravitational potential differences [3]. New applications in geodesy and hydrology will require that such clocks be ruggedized and packaged so that they can be deployed to and operated at sites of interest outside the laboratory. Before a high-precision global optical-frequency transfer network is realized, a portable optical clock with high accuracy could also facilitate comparison between high-accuracy clocks developed around the globe.