China science

China to launch world’s first ‘cold’ atomic clock in space ... and it’ll stay accurate for a billion years

Device should lose only a second in one billion years

PUBLISHED : Thursday, 15 September, 2016, 8:02am
UPDATED : Monday, 12 June, 2017, 12:53pm

The clock is ticking for the world’s most accurate working time piece, the NIST-F2 atomic clock operated by America’s National Institute of Standards and Technology in Boulder, Colorado.

And fittingly, the challenge is coming from the country that invented the mechanical clock almost 1,300 years ago – China.

Now we’ve scooped them, it is understandable that some might feel a bit sour
Professor Xu Zhen

The US clock is a large, heavy machine, standing more than 2.5 metres high, with support facilities filling an entire room, but it is so accurate that it would lose just one second in 300 million years.

In contrast, the Cold Atomic Clock in Space (Cacs) recently developed by researchers in Shanghai can easily be lifted by two people and would fit comfortably in the boot of a car. But it is expected to be three times more accurate than NIST-F2, losing only a second in one billion years.

It will be able to beat the US clock because it will have escaped the negative grip of gravity.

“It is the world’s first cold atomic clock to operate in space ... it will have military and civilian applications,” said Professor Xu Zhen, a scientist involved with the Cacs project.

China’s Beidou satellite navigation network currently provides less precise guidance than the US GPS system, but Xu said that using Cacs as a time reference in space would give a “significant boost” to Beidou’s performance.

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Cacs, several thousand times more accurate than the clocks used in GPS satellites, will start its journey when Tiangong-2, China’s second space laboratory, is launched from the Jiuquan Satellite Launch Centre in Inner Mongolia Thursday night . Other instruments on Tiangong-2, some of them world firsts, include the Gamma-ray Burst Polarimeter (Polar), capable of studying the most powerful explosions in the universe, quantum communication devices, and a “forge” to generate extremely high heat for the creation of new materials such as armour in a microgravity environment. A “bodyguard” satellite will fly around the space lab to protect it from potentially deadly collisions with space debris.

Scientists said the launch of Tiangong-2 would mark China’s transition from a follower in space research to a pioneer.

Xu, a researcher with the Chinese Academy of Sciences’ Shanghai Institute of Optics and Fine Mechanics, said the space atomic clock project was a good example of that transition.

The idea of sending an atomic clock into space was first proposed by European scientists more than 20 years ago. But the European Space Agency’s Atomic Clock Ensemble in Space project (Aces) has faced numerous delays and, according to its latest schedule, won’t be mounted on the International Space Station until next year.

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Scientists in the United States also started their own space cold atomic clock project but it was cancelled due to federal government budget cuts, prompting some of the American researchers to switch to the European project.

An atomic clock runs on the principle that electrons orbiting the nucleus of an atom “leap” from one energy state to another under certain frequencies of microwave radiation. The atom could therefore serve as an extremely reliable “timekeeper” when generating and maintaining microwaves at a stable frequency for time reference.

The official definition of a second, for instance, is 9,192,631,770 cycles of the microwave that would cause an atom of the element caesium to swing between two energy states.

A cold atomic clock is more accurate than a normal – or “hot” – atomic clock because it uses a laser to slow down the atom from a speed of several hundred metres per second to just one centimetre per second. Because the atom is the timekeeper, a slower moving one lessens the likelihood of counting errors and results in a more accurate clock.

But atoms are also “distracted” by gravity, and the low level of gravity in space should be able to further improve the accuracy of cold atomic clocks.

In a paper last year, the Chinese team said that the ground prototype of Cacs had caught up with the performance of Pharao, the competing clock in Europe’s Aces project.

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Unlike Pharao, which used atoms of caesium, the Chinese clock used rubidium atoms, which could provide “superior performance” in terms of accuracy and reliability, they said in the paper.

German scientists have built an even more accurate atomic clock, using atoms of ytterbium, that should not lose a second in several billion years, but that is an experimental device, not used for real-world applications.

The Chinese clock still had room for improvement, admitted researchers led by Professor Liu Liang. For instance, Cacs produced more background noise than its European counterpart, which might affect the quality of the signals it broadcast.

But getting the machine into space first would put the Chinese team a significant step ahead of its overseas competitors, Liu said.

None of the scientists involved with the Aces project responded to requests for comment on the launch of China’s cold atomic clock.

Xu said he was not surprised by the lack of response.

“When I heard of the Aces project, I was still a student,” he said. “Now we’ve scooped them, it is understandable that some might feel a bit sour.”

The scientists were not to blame for the delay, Xu said, adding “it’s their government.”

The launch of Tiangong-2 has also been delayed, with the mission originally scheduled for last year.

But a space scientist involved with the Tiangong programme said the Chinese space industry and related research programmes had received sufficient, stable financial support from a centralised government with the ability and resources to “execute”, minimising the length of the delay.

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The funding, speed and efficiency of Chinese space research projects had drawn an increasing number of overseas scientists to join research projects on the mainland, said Professor Wu Bobing, a researcher with the Institute of High Energy Physics in Beijing.

Wu was a lead scientist in the development of Polar, Tiangong-2’s gamma-ray-burst detector, which was built by scientists from China, Switzerland and Poland.

He said gamma-ray bursts were the most violent and mysterious events in the universe after the big bang itself. It was believed they were produced by the enormous explosion before the creation of a black hole, and it had been confirmed that some very short gamma-ray bursts were generated by the merging of two neutron stars, but scientists were still not sure what caused the more frequently seen long gamma-ray bursts that could last up to a day.

It would Polar’s mission to find the answers, Wu said.

The box-like machine will be fitted outside Tiangong-2, with its back to the earth so that it can catch the signals of explosions billions of light years away.

Wu said Polar was a product international collaboration, with the European scientists contributing many important components such as electronic circuits and some detection sensors.

He had also been involved in other Chinese space research projects featuring the participation of foreign scientists and said that while Polar was the only scientific instrument built through such an international effort, “there will be many more of these kind of projects in the coming years.”

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By international standards, Tiangong-2 is still a small, primitive space facility.

It has room for only three astronauts, and its small size limits the amount of scientific equipment it can carry. And while it has a robotic arm for repairs, it is still in the experimental stage while similar technology has been used on the International Space Station for decades.

China plans to launch a full-sized international space station by around 2020, drawing on the lessons and experience from the Tiangong space labs, the first of which was launched five years ago.

“We are still a turtle in a race with rabbits,” said a mainland space scientists involved with China’s manned space project.