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The German team’s optical-clock design, which included a reference laser cavity carefully mounted to minimize sensitivity to vibration (top), was loaded on an automotive trailer and used for several measurement campaigns off campus (bottom). The clock was able to achieve fractional uncertainties of 7.4 × 10–17. [Images: Koller et al., Phys. Rev. Lett., doi: 10.1103/PhysRevLett.118.073601]

Optical lattice clocks have achieved remarkable gains in precision and stability in recent years—but they’ve been stuck in the lab. Now, research groups in Germany and China have reported the development of relatively compact, transportable optical clocks that can take their act on the road (Phys. Rev. Lett., doi: 10.1103/PhysRevLett.118.073601; arXiv:1607.03731).

While falling somewhat short of the super-fine accuracy and stability of the most advanced optical lattice clocks, the new transportable models have uncertainties nearly an order of magnitude better than the best microwave cesium clocks, currently used to calibrate clocks separated on distant continents and to define the SI second. The ability to truck these more precise optical clocks to far-flung locations, according to the scientists, could thus take the world closer to a more precise redefinition of the standard second. And, they suggest, it could also open applications in high-precision gravity experiments, and even opportunities for space-borne optical clocks.

Linking precise clocks, continents apart

Lab-based optical lattice clocks work by confining ensembles of cooled atoms in an optical trap, or lattice, and measuring their “tick”—the electronic transitions of excited atoms in the trap—with a narrowband laser tuned to the transition frequency. These clocks have reached fractional uncertainties on the order of only a few parts in 1018, and have stoked widespread discussion about a more precise definition of the SI second, currently tied to microwave clocks with uncertainties two orders of magnitude greater.

But there’s been a problem. The best optical clocks lie in laboratories situated literally continents apart. And checking the consistency of those clocks, even using standard microwave clocks, requires finding a way to tie them together on those length scales. While a German-French consortium last year used optical fiber to link two optical clocks separated by more than 1400 km on the European continent, clock comparisons on intercontinental length scales can take place only by satellite links. Such links are sufficiently noisy to be of limited use for a robust comparison of optical-clock frequencies on different continents.

A transportable optical clock of sufficient precision, by contrast, could be taken directly to the sites of lab-based optical clocks, allowing on-site calibration and consistency checking, and thus moving the world closer to a redefinition of the SI second based on these super-precise clocks. Creating an optical clock tough enough to travel, however, has proved a challenge. To be useful, such clocks would need to at least approach the uncertainty and stability levels of the best lab-based clocks. But they would also have to stand up to vibrations and temperature fluctuations never confronted by those pampered laboratory models.

Making optical clocks road-ready

Two research groups have now reported different approaches to achieving such a robust transportable optical clock. One group, headed by Christian Lisdat of the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany, and reporting its results in Physical Review Letters, used a clock based on 87Sr atoms trapped in a 1-D lattice, and interrogated by high-frequency-stability laser. The team focused on compact optics and other components to keep the size and weight manageable. Particularly impressive was the reference cavity for the interrogation laser, mounted on a finely adjusted array of wires tied to the symmetry planes of the cavity to minimize its sensitivity to vibration.

The German team mounted its clock assembly in an automotive trailer, and conducted two measurement campaigns away from the main PTB campus. By characterizing the mobile unit against a stationary lattice clock, the researchers found that the transportable clock’s systematic uncertainty came in at a mere 7.4 × 10–17—far better than microwave clock levels. Further, the team argues that it can reach uncertainties of less than 10–17 through better characterization of the laser system used to create the optical lattice, since much of the systematic uncertainty traces to uncertainties in the frequency shift of that component in particular.

A second research group, led by Xueren Huang of the Wuhan Institute of Physics and Mathematics in China, has reported a transportable optical clock with a comparable uncertainty in a paper posted on the arXiv preprint server, but not yet published in a peer-reviewed journal. The Huang group’s clock is based on transitions in a single trapped 40Ca ion, rather than an ensemble of atoms as with the Lisdat group in Germany.

That makes the Chinese group’s approach potentially simpler and more compact (it reportedly takes up a mere 0.54 m3, excluding electronics), but also may give the German team’s device the edge in signal-to-noise ratio and, thus, in cutting down the time it takes to get a good measurement. While the Chinese group bills its clock as transportable, the device has not yet been field tested outside of the lab in the same way as the German group’s trailer-mounted model.

Optical clocks in orbit?

The simultaneous appearance of several different options for transportable optical clocks could create some auspicious opportunities for taking the power of such clocks out of the lab. The ability to transport clocks with better-than-10–17 uncertainties to the far-flung locations of precise, lab-based clocks could vastly simplify the prospect of frequency comparisons for those clocks. That, in turn, could boost confidence in the reliability of the ultra-precise lab-based optical clocks, and bring the world that much closer to a redefinition of the SI second.

But beyond that fundamental bit of timekeeping, the transportable clocks could prove useful in other settings. For example, the transportable clocks could be taken to remote locations on Earth's surface to detect differences in gravitational redshifts, with precision sufficient to resolve height differences of less than 10 cm and, thus, to allow some previously impractical tests of fundamental physics. Clocks set up in remote locations could also help measure tiny changes in sea levels and the heights of ice sheets tied to climate change.

Perhaps most intriguing, the development of transportable optical clocks could improve the prospects, already being explored, for putting an optical lattice clock on the International Space Station. That’s a development that would likely open some intriguing new opportunities both in fundamental physics and in more practical realms, such as improving satellite-based navigation.