Chinese team synchronize clocks with lasers over record distance

Physicists have found a way to very precisely synchronize the ticking of two clocks through the air over a record distance of 113 kilometers.

The feat is a step toward redefining the second using optical clocks—timepieces that are 100 times more accurate 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 one hurdle standing in their way is the need to find a reliable way to transmit signals between optical clocks in labs on different continents to compare their outputs. In practice, this probably means that the clocks’ time is transmitted through air and space to satellites. However, this is a challenge as 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, managed to send precise pulses of laser light between clocks at stations 113 kilometers apart in China’s Xinjiang province¹. That is seven times the previous record² of 16 kilometers.

The result published in Nature¹ on October 5, is “outstanding,” says David Gozzard, an experimental physicist at the University of Western Australia in Perth. Achieving such a high level of synchronization over this distance in the air “represents a significant advance in achieving that between a satellite and the ground,” he adds.

Synchronizing hyper-precise clocks in hard-to-reach places could also have benefits elsewhere in research, says Tetsuya Ido, director of the Space-Time Standards Laboratory at the Radio Research Institute in Tokyo. For example, the clocks could be used to test the general theory of relativity, which states that time should pass more slowly 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, the shifting of tectonic plates — he says.

Next generation watches

Since 1967, the second has been defined by atomic clocks using cesium-33 atoms: one second is the time it takes to traverse 9,192,631,770 cycles of the microwave radiation that the atoms absorb and emit as they transition between specific states. Today’s optical clocks use the higher frequency “ticking” of elements like strontium and ytterbium, allowing them to break time down into even finer fractions.

However, the official time cannot be generated with just one clock. Metrologists have to average the performance of hundreds of timepieces around the world. In cesium clocks, time can be transmitted by microwave signals, but microwave radiation is too low-frequency to transmit the high-frequency ticking of optical clocks.

Sending signals through air at optical wavelengths is not as easy as sending microwaves, because molecules in the air easily absorb the light, drastically reducing the signal strength. In addition, turbulence can deflect a laser beam off the target. To compare optical clocks, physicists have mostly relied on transmitting signals over fiber 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 managed to combine several smaller developments, says Gozzard. To generate their signal, the researchers used optical frequency combs — devices that generate extremely stable and precise pulses of laser light — and amplified their output with high-power amplifiers to minimize signal loss as the pulses traveled through the air. The team also tuned and optimized receivers so they could pick up weak signals and automatically track the direction of the incoming laser.

The group broadcast time intervals using two wavelengths of visible light and transmitted another over a fiber optic link. By comparing the tiny differences between the signals received at the receivers, the researchers showed that when measured over hours, they could propagate the ticking with a stability high enough to be only a second every about 80 billion years lose or win. The accuracy was on the level of optical clocks.

Not here yet

Although this transmission method is mankind’s most stable to date, Gozzard says it needs to be further improved to keep up with the stability of the best optical clocks.

Another limitation is that the experiment was conducted in a remote region with optimal atmospheric conditions, Ido says. “The humidity is fairly low and the air turbulence could be quieter than in traditional urban areas,” he says. Future studies need to verify how well the method works in other places.

But the experiment appears 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 turbulence to be expected 113 kilometers on the ground is comparable to that on the way from the ground to a satellite, she says.

Satellite-based transmission will face another hurdle — the clocks will orbit at high speeds, shifting the frequency of their signals, Gozzard says.

Pan says this is one of the challenges his team will face next. The team has previously developed technologies for a quantum communications satellite and is now using them to develop ways of transmitting between optical clocks in geostationary orbit and on the ground.

With optical clocks in space, it would also be “possible to provide new probes for fundamental physics, such as dark matter hunting and gravitational wave detection,” adds Pan.