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Inside view of the trailer containing the transportable optical atomic clock. [Image: Physikalisch-Technische Bundesanstalt (PTB)]

Scientists can do only so much with an ultraprecise optical atomic clock in the laboratory. To measure Earth’s gravitational field, for example, the complex timekeepers must be deployed at multiple locations.

Now, researchers from several European laboratories have built an easily transportable strontium optical lattice clock that fits inside a car trailer (Nature Phys., doi:10.1038/s41567-017-0042-3). The team used the mobile device to measure the difference in gravitational potential between two locations, 90 km apart.

Making it mobile

Researchers at the Physikalisch-Technische Bundesanstalt (PTB), Germany’s national standards institute, built the clock, which cools 87Sr atoms to microkelvin temperatures inside a magneto-optical trap. Rubber dampers inside the trailer helped to insulate the rigidly mounted equipment from vibrations. The small interior space of the trailer, roughly 14.5 cubic meters, led to poor air circulation and hot spots, so the team had to place shielding around the optical equipment for temperature stability.

“Making everything more compact and robust was surely a big challenge,” says Christian Lisdat, who heads the PTB's working group on optical lattice clocks. Typically, such optical clocks fill a whole laboratory room. In addition, the teem needed to power up all the laser systems and optics and test clock shortly after transportation. “This makes it more difficult to identify problems compared to a setup that stays in a quiet lab for years,” Lisdat adds.

Test drive

For the gravitational test, the team took the clock’s trailer to the Modane Underground Laboratory, site of an ongoing particle-physics experiment within the FrĂ©jus road tunnel, deep inside a mountain on the French-Italian border. From there, the transportable clock was connected via a 150-km-long noise-compensated optical fiber to a cesium fountain clock and an ytterbium optical lattice clock at the Istituto Nazionale di Ricerca Metrologica (INRIM), Italy’s standards laboratory, in Turin. These clocks served as reference standards. At Modane, a frequency comb provided by the National Physical Laboratory in Great Britain measured the optical frequency ratio between a laser resonant with the strontium clock’s transition and an ultrastable laser located at INRIM.

Much of the experiment consisted of error analysis—measuring the “uncertainty budget” of the clocks. The scientists also had to contend with vibrations from periodic construction blasting inside the tunnel and air conditioning inefficiencies in the low-humidity environment of the underground lab.

Nevertheless, over seven days, the team accumulated about 31,000 seconds of common operation of the two optical clocks and the frequency comb and measured the systematic uncertainties of the clocks. The scientists hope that their proof-of-concept work leads to more accurate geodetic measurements across national borders and better monitoring of sea-level changes.

“To make a real impact in geodesy and metrology, we have to do more campaigns that have to demonstrate reliability and accurate measurement results,” Lisdat says. “Clearly, we have to improve the height resolution to 10 cm and better. Thus we will improve our clock and test it to see where we are. We think that we have already improved the clock, but it is still a lot of work to operate it.”

[Updated 2/22/2018 to add comments from author Christian Lisdat.]