In the search for stable and accurate atomic clocks, many-atom lattice clocks have shown higher precision than clocks based on single trapped ions, but have been less accurate; here, a stable many-atom clock is demonstrated that has accuracy better than single-ion clocks. Whether for the definition of SI units, testing the laws of physics or for applications yet to be dreamt of, scientists will always want more stability and more accuracy in their atomic clocks. Many-atom lattice clocks have achieved better precision than clocks based on single trapped ions, but their accuracy has so far been relatively poor. This study from the National Institute of Standards and Technology (NIST) demonstrates a many-atom clock that achieves better accuracy than single-ion-based clocks, and at the same time reduces the required measurement time by two orders of magnitude. Based on thousands of neutral strontium atoms trapped in a laser beam, this new 'optical lattice' clock has the stability, reproducibility and accuracy that make it a prime contender for consideration as a primary standard. It would neither gain nor lose one second in about 5 billion years — although the Earth is unlikely to last that long. Progress in atomic, optical and quantum science1,2 has led to rapid improvements in atomic clocks. At the same time, atomic clock research has helped to advance the frontiers of science, affecting both fundamental and applied research. The ability to control quantum states of individual atoms and photons is central to quantum information science and precision measurement, and optical clocks based on single ions have achieved the lowest systematic uncertainty of any frequency standard3,4,5. Although many-atom lattice clocks have shown advantages in measurement precision over trapped-ion clocks6,7, their accuracy has remained 16 times worse8,9,10. Here we demonstrate a many-atom system that achieves an accuracy of 6.4 × 10−18, which is not only better than a single-ion-based clock, but also reduces the required measurement time by two orders of magnitude. By systematically evaluating all known sources of uncertainty, including in situ monitoring of the blackbody radiation environment, we improve the accuracy of optical lattice clocks by a factor of 22. This single clock has simultaneously achieved the best known performance in the key characteristics necessary for consideration as a primary standard—stability and accuracy. More stable and accurate atomic clocks will benefit a wide range of fields, such as the realization and distribution of SI units11, the search for time variation of fundamental constants12, clock-based geodesy13 and other precision tests of the fundamental laws of nature. This work also connects to the development of quantum sensors and many-body quantum state engineering14 (such as spin squeezing) to advance measurement precision beyond the standard quantum limit.

An optical lattice clock with accuracy and stability at the 10 −18 level

Atomic clocks are increasingly important for many applications in scientific research and technology. Here, Nicholson et al. present a series of developments allowing them to achieve a new record in atomic clock performance, with a systematic uncertainty of just 2.1 × 10−18 for their 87Sr atomic clock.

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Systematic evaluation of an atomic clock at 2 × 10 −18 total uncertainty

A pair of 87Sr optical lattice clocks with a statistical agreement of 2 × 10−18 within 6,000 s has been developed. To this end, the behaviour of the blackbody radiation—a major perturbation for optical lattice clocks—was directly investigated. The accuracy of atomic clocks relies on the superb reproducibility of atomic spectroscopy, which is accomplished by careful control and the elimination of environmental perturbations on atoms. To date, individual atomic clocks have achieved a 10−18 level of total uncertainties1,2, but a two-clock comparison at the 10−18 level has yet to be demonstrated. Here, we demonstrate optical lattice clocks with 87Sr atoms interrogated in a cryogenic environment to address the blackbody radiation-induced frequency shift3, which remains the primary source of systematic uncertainty2,4,5,6 and has initiated vigorous theoretical7,8 and experimental9,10 investigations. The systematic uncertainty for the cryogenic clock is evaluated to be 7.2 × 10−18, which is expedited by operating two such cryo-clocks synchronously11,12. After 11 measurements performed over a month, statistical agreement between the two cryo-clocks reached 2.0 × 10−18. Such clocks' reproducibility is a major step towards developing accurate clocks at the low 10−18 level, and is directly applicable as a means for relativistic geodesy13.

Cryogenic optical lattice clocks

The passage of time is tracked by counting oscillations of a frequency reference, such as Earth’s revolutions or swings of a pendulum. By referencing atomic transitions, frequency (and thus time) can be measured more precisely than any other physical quantity, with the current generation of optical atomic clocks reporting fractional performance below the 10−17 level1–5. However, the theory of relativity prescribes that the passage of time is not absolute, but is affected by an observer’s reference frame. Consequently, clock measurements exhibit sensitivity to relative velocity, acceleration and gravity potential. Here we demonstrate local optical clock measurements that surpass the current ability to account for the gravitational distortion of space-time across the surface of Earth. In two independent ytterbium optical lattice clocks, we demonstrate unprecedented values of three fundamental benchmarks of clock performance. In units of the clock frequency, we report systematic uncertainty of 1.4 × 10−18, measurement instability of 3.2 × 10−19 and reproducibility characterized by ten blinded frequency comparisons, yielding a frequency difference of [−7 ± (5)stat ± (8)sys] × 10−19, where ‘stat’ and ‘sys’ indicate statistical and systematic uncertainty, respectively. Although sensitivity to differences in gravity potential could degrade the performance of the clocks as terrestrial standards of time, this same sensitivity can be used as a very sensitive probe of geopotential5–9. Near the surface of Earth, clock comparisons at the 1 × 10−18 level provide a resolution of one centimetre along the direction of gravity, so the performance of these clocks should enable geodesy beyond the state-of-the-art level. These optical clocks could further be used to explore geophysical phenomena10, detect gravitational waves11, test general relativity12 and search for dark matter13–17. Improved techniques allow the measurement of a frequency difference with an uncertainty of the order of 10–19 between two independent atomic optical lattice clocks, suggesting that they may be able to improve state-of-the-art geodetic techniques.

Atomic clock performance enabling geodesy below the centimetre level

Progress in realizing the SI second had multiple technological impacts and enabled further constraint of theoretical models in fundamental physics. Caesium microwave fountains, realizing best the second according to its current definition with a relative uncertainty of 2-4 Â 10 A 16 , have already been overtaken by atomic clocks referenced to an optical transition, which are both more stable and more accurate. Here we present an important step in the direction of a possible new definition of the second. Our system of five clocks connects with an unprecedented consistency the optical and the microwave worlds. For the first time, two state-of-the-art strontium optical lattice clocks are proven to agree within their accuracy budget, with a total uncertainty of 1.5 Â 10 A 16. Their comparison with three independent caesium fountains shows a degree of accuracy now only limited by the best realizations of the microwave-defined second, at the level of 3.1 Â 10 A 16 .

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Experimental realization of an optical second with strontium lattice clocks.

We report on the observation of a dc Stark frequency shift at the 10-13 level by comparing two strontium optical lattice clocks. This frequency shift arises from the presence of electric charges trapped on dielectric surfaces placed under vacuum close to the atomic sample. We show that these charges can be eliminated by shining UV light on the dielectric surfaces, and characterize the residual dc Stark frequency shift on the clock transition at the 10-18 level by applying an external electric field. This study shows that the dc Stark shift can play an important role in the accuracy budget of lattice clocks, and should be duly taken into account.

https://hal.archives-ouvertes.fr/hal-00616423/document

M. Gurov

Papers

Observation and cancellation of a perturbing dc stark shift in strontium optical lattice clocks