Scientists synchronise clocks to new level of accuracy – in a step towards changing the length of a second

Scientists have managed to synchronise two clocks 12km apart to within one quadrillionth of a second – bringing a historic change to the length of the basic unit of time one step closer.

Using a specialised laser, an international team of researchers, led by the US National Institute of Standards and Technology (NIST), managed to get one optical clock to show almost exactly the same time as another.

The technique is so accurate that they had to account for the minuscule swaying of the buildings when measuring how closely the clocks were showing the same time.

The changing atmosphere can also alter the time taken for light to travel between the two clocks by hundreds of picoseconds (a picosecond is one trillionth of a second).

This may mean nothing to ordinary humans, but it matters a great deal to scientists trying to create significantly better clocks.

A second has been the same length for nearly 50 years, but the increasing accuracy of optical clocks means scientists are considering changing this – although the difference between the "old second" and new one would hardly be noticed.

However it would enable GPS systems to become accurate to with a few centimetres and also be useful for computerised financial networks and electric power grids.

Dr Laura Sinclair, a NIST physicist, said: “The 12km of turbulent air results in massive distortions of the laser beams – yet the two clocks agree in time to 20 digits.”

She said they found there was “no degradation of the clock agreement with the increased distance and turbulence”.

“This suggests that we could go even greater distances, especially if the path isn't completely horizontal – like to a mountaintop or balloon,” she said.

The team, who reported their findings in the journal Applied Physics Letters, are now looking into two other problems.

“First: can we still sync clocks if one of them is moving?” Dr Sinclair asked.

“The same 'Doppler' effect that changes the pitch of an ambulance siren when it's coming toward us also impacts our clocks, so we need to correct for this effect to allow for the development of synchronised clock networks on mobile platforms.

“Second: how far – in distance – can we really go? If someday we want to redefine the second so that it’s based on an optical standard instead of a microwave standard, we'll need to be able to link up the world's best clocks and then distribute that time information.”

If the second were to be redefined, scientists would try to keep it as close to the existing length, but there would almost inevitably be a tiny, tiny change that would make it either longer or shorter.