Unlike synchronous stability 5, 21, which is useful for systematic evaluations but which does not demonstrate how a system would perform as an independent frequency standard, a self-comparison reproduces the short-term stability of an independent clock. ![]() A self-comparison approach compares two independent frequency locks operating on alternate experimental cycles 22. Both the quantum projection noise and the Dick effect have been confirmed to be correctly determined with a self-comparison, which agrees with the measurement from a two-clock comparison 21. With long-term drift under control at the low 10 −18 level, we can obtain a complete characterization of the clock stability with short-term stability measurements. Furthermore, stability data taken over the course of a month was robust and repeatable. ![]() We have demonstrated that after careful control of systematic effects, residual drifts did not affect clock stability at 2 × 10 −18 after thousands of seconds of averaging time 1. At long averaging times, the only mechanism that can limit the stability is drifting systematic shifts. Our clock stability at short averaging times is limited by the Dick effect 19-aliased high-frequency noise of the clock laser-that surpasses quantum projection noise 20 with 2,000 atoms. This corresponds to a gravitational redshift for a height change of 2 cm on Earth. With these developments, we achieve an overall systematic uncertainty of 2.1 × 10 −18, which is more than a threefold improvement over the previous best atomic clock 1. Here we describe a set of innovations implemented to improve the accuracy of the 87Sr clock: an optical lattice with no measurable ac Stark shift at 1 × 10 −18, blackbody radiation (BBR) thermometry with millikelvin level accuracy, atomic structure measurements that characterize the atomic response to BBR and active servo stabilization of electric and magnetic fields. In this work, we use an ultrastable laser with 10 s coherence time, referenced at 60% duty cycle to thousands of strontium atoms in an optical lattice, to achieve a record fractional frequency stability of 2.2 × 10 −16 at 1 s.īetter clock stability allows for faster evaluations of systematic uncertainties and enables the discovery of new physical effects 18. Clock stability can be extended from seconds to hours by referencing the ultrastable laser to a high-quality-factor optical transition of an atom 17. In an optical atomic clock, short-term stability originates from an ultrastable laser that serves as a local oscillator. ![]() The continued advances in clock stability and accuracy go hand in hand. Techniques developed for optical atomic clocks, such as advanced laser stabilization 13, 14, coherent manipulation of atoms 15 and novel atom trapping schemes 16, have given rise to new research opportunities in quantum physics. The pursuit of better atomic clocks has also had strong impact on many fundamental research areas, providing improved quantum state control 6, 7, deeper insights in quantum science 8, 9, tighter limits on fundamental constant variation 10, 11 and enhanced sensitivity for tests of relativity 12. Precise and accurate optical atomic clocks 1, 2, 3, 4, 5 have the potential to transform global timekeeping, enabling orders-of-magnitude improvements in measurement precision and sensor resolution for a wide range of scientific and technological applications.
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