30 April 2012—Scientists have taken a major step toward creating a worldwide network of the most accurate atomic clocks. In results published last week in Science, Katharina Predehl and her team in Germany demonstrated that it’s possible to connect two so-called optical atomic clocks over a distance of 600 kilometers using 920 kilometers of optical fiber.
An atomic clock is useful on its own, because it keeps an ultraprecise rhythm. But for GPS, cellular networks, and other important technologies, it needs to be synced with other, distant atomic clocks. The most common type of timekeeper, the microwave atomic clock, is checked against other microwave clocks using satellite links. But for newer, more precise optical atomic clocks, a satellite link is too noisy.
The oscillating tick of an optical atomic clock is in the form of a laser, which shines through a vapor of atoms. The laser’s frequency is matched to the transition rate between energy states in the atoms, and the laser is locked and held stable using a feedback loop. The atoms transition trillions of times per second in some clocks making for amazingly accurate timekeeping.
But that accuracy is hard to translate across great distances. Predehl and two teams of researchers—one at the Max Planck Institute of Quantum Optics, in Garching, and one at the Physikalisch-Technische Bundesanstalt, in Braunschweig—took an essential step in building a link good enough for optical clocks, though their methods aren’t groundbreaking.
“Conceptually, it is relatively simple. Practically, it’s really not,” says Bruce Warrington, a metrologist at Australia’s National Measurement Institute who published a companion article in Science this week on the history and future of connecting atomic clocks. “The amount of work required is impressive,” he says.
Earlier experiments have shown that optical clocks can be synced by optical-fiber connections, but this has been accomplished only within the same laboratory, or at most within 120 km—basically at a citywide level.
The experiment in Germany put the theory to the test on a larger scale. The challenge is compensating for frequency shifts in the system, Predehl explains. If the fiber cable changes beyond the scale of an atom—expanding or contracting from, say, a difference in temperature—the frequency will change during transit.