Researchers in China use lasers to sync optical clocks over record distance

Everything is a step towards defining a second.
Ameya Paleja
Red laser beam and reflections
Red laser beam and reflections


A team of researchers in China created a new record when they synced their optical clocks over a distance of 70 miles (113 km). This is a major improvement from their last attempt, which was carried out across a distance of just under 10 miles (16 km), Nature reported.

The team was led by Jian-Wei Pan, a physicist at the University of Science and Technology of China in Hefei, and this moment marks an important milestone as meteorologists look to shift to optical clocks to redefine a second before the end of this decade.

What is a second anyways?

While a second or even thousands of them are spent lazily every day, scientists have been looking for ways to break down this unit of time into even finer sections. For over five decades, atomic clocks have been used to define a second. It is the amount of time that is needed to cycle through 9,192,631,770 oscillations of microwave radiation that cesium-33 atoms absorb and emit as they change states.

Optical clocks on the other hand use the 'ticking' of atoms of strontium and ytterbium that can provide even smaller fractions of time. However, official time is not the product of one clock but needs to be averaged from hundreds of timepieces that are located across the world. To do so, these clocks need to be synced from time to time, and it is here that meteorologists have faced difficulties in distances.

While cesium clocks can be synced using microwave radiation, optical clocks have a much higher frequency, which these low-frequency waves cannot match. Therefore, researchers have turned to waves in the visible light spectrum to get the job done. In previous attempts, researchers have used fiber optic cables for transmitting the signals, but the scale of the network that would be needed to define a second is impractical for deployment, the Nature report said.

Researchers in China use lasers to sync optical clocks over record distance
Clock dispersing

This is why the researchers turned to sending the signals via air. To achieve the record feat, they used recent advances such as the use of optical frequency combs that can produce precise pulses of lasers, high-powered amplifiers to ensure that the signals were not lost, as well as highly tuned receivers to automatically detect even the low-powered incoming signals.

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When the best is not good enough

The verify their work, the researchers sent another wavelength of light through the optical fiber link and then compared the difference between the signals that were sent through the air and through the optic fiber link. The work showed that over-the-air transmission had high ticking stability such that it would only lead to a loss or gain of one second over 80 billion years.

Yet, the researchers are far from deploying the system. The research work was conducted in a remote location that had the most optimal atmospheric conditions and now needs to be repeated in other locations to determine if it works equally well.

The current experiment is comparable to sending optical signals into space, where they will meet turbulence, experts told Nature. However, the high-speed orbits of satellites will present another challenge to signal transmission since there will be a shift in the frequency of the signal.

The research team has developed technologies for quantum communications satellites and will now work on transmitting signals between the ground and optical clocks in a geostationary orbit. This will help us in our search for dark matter as well as detecting gravitational waves, Pan told Nature.

The research findings were published earlier this month in Nature.


Networks of optical clocks find applications in precise navigation1,2, in efforts to redefine the fundamental unit of the ‘second’3,4,5,6 and in gravitational tests7. As the frequency instability for state-of-the-art optical clocks has reached the 10−19 level8,9, the vision of a global-scale optical network that achieves comparable performances requires the dissemination of time and frequency over a long-distance free-space link with a similar instability of 10−19. However, previous attempts at free-space dissemination of time and frequency at high precision did not extend beyond dozens of kilometres10,11. Here we report time–frequency dissemination with an offset of 6.3 × 10−20 ± 3.4 × 10−19 and an instability of less than 4 × 10−19 at 10,000 s through a free-space link of 113 km. Key technologies essential to this achievement include the deployment of high-power frequency combs, high-stability and high-efficiency optical transceiver systems and efficient linear optical sampling. We observe that the stability we have reached is retained for channel losses up to 89 dB. The technique we report can not only be directly used in ground-based applications, but could also lay the groundwork for future satellite time–frequency dissemination.

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