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The Chinese team’s experiment could be a stepping stone to building high-precision ground-to-satellite links, according to experts. Photo: Shutterstock

Chinese team says laser test could improve satellite navigation

  • Experiment could lead to discoveries in testing general relativity, the search for dark matter and other breakthroughs in fundamental physics, researchers say
  • It could also help scientists reach a new, more precise definition of the second as a unit of time

Chinese scientists say they have succeeded in an experiment that could lead to breakthroughs in fundamental physics, improve satellite navigation and redefine the second as a unit of time.

The scientists performed the experiment in Urumqi, capital of Xinjiang Uygur autonomous region in western China. They placed two terminals in laboratories 113km (70 miles) apart. Each terminal was equipped with a laser, a telescope and two optical frequency combs that measure exact frequencies of light.
They sent laser pulses carrying information between the two telescopes, allowing them to establish and confirm what time it was.
Using lasers to send signals through the open air comes with challenges, including wind turbulence and other atmospheric disturbances that cause loss of signal. Photo: CCTV

Previous attempts at sending signals through the open air were limited to a few dozen kilometres, said the research team led by quantum physicist Pan Jianwei at the University of Science and Technology of China (USTC), writing in the peer-reviewed journal Nature on Wednesday.

Scientists need to be able to send signals over long distances to build a global network of optical clocks – ultra-precise time measurement devices that can help improve the accuracy of satellite navigation services.
Optical fibres have been used to send signals over 1,000 km with high precision, according to the Chinese team. But it is difficult to stretch cables between certain locations, so researchers have tried to send signals through the open air.

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David Gozzard, a fellow at the International Centre for Radio Astronomy Research in Australia who was not involved in the study, said links through the open air were needed wherever a fibre-optic cable was too expensive or impractical to be installed, such as from the ground to a spacecraft.

But this approach came with its own challenges, such as loss of power and atmospheric turbulence that could lead to frequent signal dropouts, said Gozzard, who researches laser systems for high-precision measurement.

“Whenever a time signal is transmitted from one location to the other, turbulence causes the light to randomly speed up or slow down, so the time signal arrives after different amounts of flight time, ruining the timing synchronisation,” he said.


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Gozzard said the Chinese team’s efforts to suppress the effects of turbulence could be a stepping stone to creating high-precision ground-to-satellite links.

“By demonstrating their technology over 100km on the ground, the team have shown that it is able to handle the enormous power loss and huge amount of turbulence that will be experienced on a link from the ground to a spacecraft,” he said.

Sending time signals across long distances could open the doors to discoveries in fundamental physics, including testing of general relativity, the search for gravitational waves and dark matter and long-range quantum networks, the Chinese researchers said.

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They also said their methods could help scientists reach a new definition of the second as a unit of time.

The current definition of the second dates to 1967 and is based on a measurement of the number of cycles of radiation emitted by caesium atoms as they transition between quantum states. But as atomic clock technology advances, scientists are working to refine the unit to make it more precise.

In addition to researchers from USTC, the team included scientists from Ningbo University in Zhejiang province, Xinjiang Astronomical Observatory, Shanghai Institute of Technical Physics, the National Time Service Center and Jinan Institute of Quantum Technology in Shandong province.