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Two strontium optical-lattice clocks—one at LNE-SYRTE in Paris, France, and the other at PTB in Braunschweig, Germany—were linked at a connection point in Strasbourg, France. The research team found that the frequencies of the two clocks differed with an uncertainly of only 5×10−17 across the 1,400-km fiber link. [Image: PTB]

Taking advantage of the superior performance of optical-lattice clocks will require ways to tie them together reliably across large distances, so that their precision is available to scientists outside of local metrology labs. Researchers in Germany and France now have now offered a spectacular demonstration of such a long-distance connection: Using a fiber optic link, they’ve tied together two separate strontium optical-lattice clocks separated by a span of 1,415 kilometers (Nat. Commun., doi: 10.1038/ncomms12443). And they’ve established that the frequencies of the two clocks agree with a fractional uncertainty of only 5×10−17, a level considerably better than previous long-distance comparisons.

According to the researchers, the finding opens up the prospect of building a Europe-wide, fiber-connected networks of optical clocks. Such a network, the team believes, that will finally allow such clocks to be leveraged on some key unanswered questions in geodesy, fundamental physics and other areas. The work also represents another step on the road to an all-optical redefinition of the SI second.

Current limitations

Progress in optical-lattice clocks has led to setups that can provide precision in the 10−18 area, several orders of magnitude better than the cesium microwave clocks that currently define the SI second. But those high-performance optical clocks reside in local metrology labs. Connecting and comparing distant clocks has heretofore entailed using microwave signals or satellite-based communications.

As a result, the accuracy of such distant links tends to be limited to the values of the cesium microwave clock, making it effectively impossible to test and compare the better optical-clock performance across long distances. That has held back efforts to establish the reliability of optical clocks as a workable timekeeping standard, and leverage them in the real world.

An all-optical link

In the recently published research, scientists at three European institutions—the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany; the Systèmes de Référence Temps-Espace (LNE-SYRTE) in Paris, France; and the Laboratoire de Physique des Lasers (LPL) in Villetaneuse, France—decided to try an all-optical link to connect clocks, and thereby overcome the limitation of the less precise microwave links.

The team used fiber (both private and commercial infrastructure) to tie two strontium optical-lattice clocks—one at SYRTE in Paris and one at PTB in Braunschweig—to a connection point in Strasbourg, for a total fiber run of 1,415 km (see map, above). At each of the two endpoints, femtosecond frequency combs compared the local clock’s laser frequency with that of a narrow-linewidth transfer laser, which allowed the frequency ratio to be determined with better than 10−18 uncertainty. The transfer laser signal from both clock labs was then sent down the fiber link to Strasbourg, where the beat note between the two signals could be recorded.

Particularly ingenious was the group’s method of overcoming phase noise across the long span of fiber. They did so by routing part of the light back through the fiber, detecting the phase noise, and actively canceling it out, to ensure that the transfer laser’s coherent phase was preserved over the full span. The team also corrected for the gravitational redshift between the clocks due to the 25-m elevation difference between the two institutes.

Striking frequency agreement

The result was an uninterrupted information transfer between the two clocks that allowed their frequencies to be compared in detail. And the comparison was impressive: The frequencies between the two clocks agreed with an uncertainty of only 5×10−17, and reached a fractional precision of 3×10−17 after 1,000 seconds of averaging time—“already 10 times better and more than four orders of magnitude faster than any previous long-distance clock comparison,” according to the paper. Also particularly significant, in the researchers’ view, is that the striking agreement came in a comparison that involved “two fully independent apparatuses and teams.”

The latter fact in particular, in the view of the team, points to the possibility of a continent-wide clock network linked by optical fiber, and involving other European metrology labs. Such a network could make reference signals with optical-clock precision available to a wide array of scientific and other researchers, and open new lines of inquiry in some interesting areas—ranging from the search for dark matter to the measurement of fundamental constants to highly precise measurements of the Earth’s gravitational potential field. And, by allowing more precise comparisons of different clocks over long distances, the work could clear one more hurdle in the effort to move to an optical-clock definition of the SI second.