Physicists have devised a way to synchronize the ticking of two clocks through the air with extreme precision, across a record distance of 113 kilometres.
The feat is a step towards redefining the second using optical clocks — timekeepers that are 100 times more precise than the atomic clocks on which coordinated universal time (UTC) is currently based.
Metrologists hope to use optical clocks to redefine the second in 2030. But a hurdle standing in their way is the need to find a reliable way to transmit signals between optical clocks in laboratories on different continents, to compare their outputs. In practice, this will probably mean transmitting the clocks’ time through air and space, to satellites. But this is a challenge because the atmosphere interferes with signals.
A team led by Jian-Wei Pan, a physicist at the University of Science and Technology of China in Hefei, succeeded in sending precise pulses of laser light between clocks at stations 113 kilometres apart in China’s Xinjiang province1. This is seven times the previous record2 of 16 kilometres.
The result, published in Nature1 on 5 October, is “outstanding”, says David Gozzard, an experimental physicist at the University of Western Australia in Perth. Achieving such a high level of synchronization over that distance of air represents “significant progress in being able to do this between a satellite and the ground”, he adds.
Synchronizing hyper-precise clocks in hard-to-reach places could also have advantages elsewhere in research, says Tetsuya Ido, director of the Space-Time Standards Laboratory at the Radio Research Institute in Tokyo. For instance, the clocks could be used to test the general theory of relativity, which says that time should pass more slowly in places where gravity is stronger, such as at low altitudes. Comparing the ticking of two optical clocks could even reveal subtle changes in gravitational fields caused by the movement of masses — for example by shifting tectonic plates — he says.
Since 1967, the second has been defined by atomic clocks using caesium-33 atoms: a second is the time it takes to cycle through 9,192,631,770 oscillations of the microwave radiation the atoms absorb and emit when they switch between certain states. Today, optical clocks use the higher-frequency ‘ticking’ of elements such as strontium and ytterbium, which allows them to slice time into even finer fractions.
However, official time can’t be generated using just one clock. Metrologists must average the output of hundreds of timepieces across the world. For caesium clocks, the time can be transmitted through microwave signals, but microwave radiation is too low-frequency to convey the high-frequency tick of optical clocks.
Sending signals through the air at optical wavelengths is not as easy as sending microwaves, because molecules in the air readily absorb the light, drastically reducing the strength of the signal. Furthermore, turbulence can send a laser beam off target. To compare optical clocks, physicists have so far relied mostly on transmitting signals through fibre-optic cables, or transporting the bulky, complex timepieces themselves, to compare them side by side. But these methods are impractical for creating the kind of global network needed to define the second.
Pan’s team succeeded by combining several minor developments, says Gozzard. To create their signal, the researchers used optical frequency combs — devices that produce extremely stable and precise pulses of laser light — and boosted their output using high-powered amplifiers, to minimize the signal lost when the pulses travelled through the air. The team also tuned and optimized receivers so that they could pick up low-powered signals and automatically track the direction of the incoming laser.
The group sent out time intervals using two wavelengths of visible light, and transmitted another through a fibre-optic link. By comparing the tiny differences between signals picked up at the receivers, the researchers showed that, when measured over hours, they could disseminate the ticking with a stability high enough to lose or gain only a second roughly every 80 billion years. The level of accuracy was on a par with that of optical clocks.
Not there yet
Although this transfer method is the most stable humanity has so far, it will need to be improved further to match the stability of the best optical clocks, says Gozzard.
Another limitation is that the experiment was done in a remote region with optimal atmospheric conditions, says Ido. “The humidity is quite low and air turbulence could be more quiet than in conventional urban areas,” he says. Future studies will need to check how well the method works in other locations.
But the experiment seems to be a good proxy for sending such signals into space, says Helen Margolis, a physicist at the National Physical Laboratory in Teddington, UK. The amount of turbulence expected over 113 kilometres on the ground is comparable to that on the way from the ground to a satellite, she says.
Satellite-based transmission will face a further hurdle — the clocks will be orbiting at high speed, which shifts the frequency of their signals, says Gozzard.
Pan says this is one of the challenges his team will take on next. The team previously developed technologies for a quantum-communications satellite, and is now using those to develop ways to transmit between optical clocks in geostationary orbit and on the ground.
Using optical clocks in space, it would also be “possible to provide new probes for fundamental physics, such as hunting for dark matter and detecting gravitational waves”, Pan adds.