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【Science】China’s Atomic Clock Passes Space Test

Sep 21, 2017

Clocks that use cold atoms form the backbone of the international time system here on Earth. Now, scientists in China have successfully demonstrated a cold atom clock in space, an achievement that could lead to more accurate terrestrial timekeeping and better tests of fundamental physics.

Most atomic clocks rely on a very steady tick: the frequency of fluorescent light emitted by cesium atoms after being excited by a microwave field. The frequency is steadier when the atoms move slowly, and so scientists first trap the atoms using intersecting laser beams and cool them down to a few millionths of a degree above absolute zero.

Because the laser beams would impair the frequency measurement, the atoms must be released from the trap before they are excited by the microwaves. Typically, they are nudged upward by another laser and zapped with microwaves as they rise and then fall back down to Earth. But the briefness of this free fall limits how long the atoms can be probed and, hence, the clock’s accuracy and stability. In orbit, however, the atoms are in continuous free fall and can in principle be probed over longer periods of time.

The Cold Atom Clock Experiment in Space (CACES) involves trapping, cooling, and probing rubidium atoms within a box that could fit in the trunk of a car. In orbit at an altitude of 400 kilometers, the experiment was launched on board China’s Tiangong-2 space laboratory last September. Now, a year later, it is performing just as expected, according to a paper posted to the arXiv server by scientists at the Chinese Academy of Sciences’s Shanghai Institute of Optics and Fine Mechanics.

Shanghai team leader Liang Liu says that he and his colleagues “had to work night and day” to get CACES ready for launch. They “encountered tremendous technical difficulties,” he recalls, in shrinking the bulky and complex equipment needed to trap and cool atoms and also ensuring that the kit withstands the rigors of space. “Fortunately we did it, and after a year in orbit CACES is still working perfectly,” he says.

As such, Liu and co-workers have overtaken scientists working on a mission for the European Space Agency known as the Atomic Clock Ensemble in Space (ACES). Proposed in 1997, ACES has faced a series of funding and technical delays that mean it won’t arrive at the International Space Station for at least another year. In contrast, CACES has gone from conception to operation in just a decade.

For more details, please refer to http://www.sciencemag.org/news/2017/09/china-s-atomic-clock-passes-space-test.

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