Optical fibres keep clocks in sync

"The ability to compare multiple optical clocks will allow us to perform high precision experiments on very large scales … and is a prerequisite to any redefinition of the second based on an optical standard," says lead-author Katharina Predehl from the Max Planck Institute of Quantum Optics(MPQ) in Germany. But new research, reported in the journal Science, demonstrates for the first time that optical fibre networks can allow optical clocks to be synchronised over long distances. The breakthrough paves the way for a network of super-precise time-keeping devices that could revolutionise technology and fundamental physics experiments. The clocks are so accurate they can only be set if they're linked together, something that can't be done using the noisy satellite technology that connects today's atomic clocks. Optical clocks are similar to atomic clocks, but their 'tick' mirrors the vibrations of atoms at optical frequencies, rather than microwave frequencies, making it possible to measure times as small as a quadrillionth of a second. However, the clocks often take up entire labs spread out across the globe, and their signals become weak and distorted when travelling over long distances German scientists have used optical fibres to send a timing signal from one of the world's most accurate clocks over a distance of more than 900 kilometres. Bridging the distance

Researchers have been trying to create a fibre link between optical clocks for several years.

The previous record was 146 kilometres. "In the short term it won't mean much. But in the same way we depend on today's atomic clocks for telecommunications and navigation devices, we will come to depend on optical clocks. A feedback loop was also used to minimise distortion. Warrington is part of Australia's National Time and Frequency Network (NTFN), a collaboration that aims to build an optical clock in South Australia and develop optical fibre technology that will send its signal to researchers around the country. Revolutionising technology

Extending fibre connections further could allow researchers to build a global network of optical clocks, which could one day change the way the world keeps time, says Dr Bruce Warrington of Australia's National Measurement Institute in Sydney. "We can use exactly the same piece of fibre to communicate an optical clock signal, as long as we can overcome the challenge of co-existing with normal data," he says These techniques allowed the researchers to send a clear signal along 920 kilometres of optical fibre from Germany's national metrology institute in Braunschweig, to the MPQ with extremely high accuracy. "

According to Warrington, an optical clock network would also greatly benefit radio astronomy, such as the Square Kilometre Array project, and would allow researchers to test fundamental physical concepts, such as the speed of light and the theory of relativity. In this study, the team staggered nine amplifiers along the optical fibres in order to keep the frequency strong without introducing noise. According to Professor Andre Luiten, the leader of the NTFN at the University of Western Australia, Australia has a head start, as the implementation of the National Broadband Network will ensure the country is well connected with optical fibres.