Chinese team sync clocks over record distance using lasers

Physicists have come up with a way to make sure the clock on the other side of the world stays on time.

The optical clock is 100 times more precise than the atomic clock and could be used to redefine the second.

Metrologists want to use optical clocks in the future. There is a need to find a reliable way to transmit signals between laboratories on different continents in order to compare their outputs. The clock will probably be transmitted through air and space. The atmosphere is interfering 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 precisepulses of laser light between the two clocks. The previous record was two of 16 kilometres.

David Gozzard is an experimental physicist at the University of Western Australia in Perth. Significant progress has been made in being able to do this between a satellite and the ground

The director of the Space-Time Standards Laboratory at the Radio Research Institute in Tokyo says that hyper-precise clocks in hard to reach places could have advantages. The general theory of relativity states that time should be slower in places where gravity is stronger, such as at low altitudes. He says that it's possible to see subtle changes in the fields caused by the movement of the mass.

Next-generation clocks

Since 1967, the second has been defined by atomic clocks using caesium-33 atoms: a second is the time it takes for the atoms to switch between states. The higher-frequencyticking of elements such as strontium and ytterbium allows the optical clock to slice time into even fine fractions.

Official time can't be created using just one clock. Metrologists need to average the output of hundreds of watches. Microwave radiation is not high enough to convey the high-frequency tick of optical clock time.

Sending signals through the air at optical wavelength is not as easy as sending microwaves due to the fact that Molecules in the air absorb the light and reduce its strength. Turbulence can cause a laser beam to go off the target. Physicists have mostly relied on sending signals through fibre-optic cables to compare optical clocks. These methods are impractical for defining a global network.

Gozzard says that Pan's team succeeded by combining small developments. To create their signal, the researchers used optical frequency combs, devices that produce extremely stable and precisepulses of laser light, and boosted their output using high-powered amplifier, to minimize the signal lost when the pulse traveled through the air. They were able to pick up low-powered signals and automatically track the direction of the incoming laser with the help of the team's receiver tuning.

The group used visible light and a fibre-optic link to transmit time. The researchers showed that they could spread the tick with a stability high enough to lose or gain only a second every 80 billion years. The level of accuracy was close to that of opticalclocks.

Not there yet

Gozzard says that the transfer method needs to be improved to match the stability of the best optical clock.

The experiment was done in a remote area with good atmospheric conditions. He says that the air turbulence in the area could be more quiet than in a city. Future studies will have to check how the method works in other places.

Helen Margolis is a physicist at the National Physical Laboratory in Teddington, UK. She says that the amount of turbulence on the ground is similar to that on the way from the ground to a satellite.

The high speed of the clock will affect the frequencies of the signals.

Pan says this is a challenge his team will tackle next. The team previously developed technologies for a quantum-communications satellite and is now using those to develop ways to transmit between optical clocks.

It is possible to provide new probes for fundamental physics with the use of optical clocks in space.

The article is titled: "D41586 022-03297-0."