Clock will keep time for 300m years

The atomic clock is twice as accurate as any similar device made previously.

Experts believe it could be used to measure distances to far away galaxies, or even track tiny movements in the Earth's crust that might give a warning of earthquakes.

An atomic clock consists of gas atoms trapped in a magnetic field and cooled almost to absolute zero, or minus 273 Celsius, the coldest temperature permitted by the laws of physics. Focused beams of laser light cause the atoms to switch between energy states as electrons swing from one orbit to another. The oscillations of the electrons make the clock "tick".

Researchers were previously frustrated when they hit an accuracy barrier of one second lost in 150 million years. This appeared to be because the atoms making up the clock, which belonged to a non-interacting class called "fermions", were colliding - something they were not supposed to do under the laws of quantum physics.

The scientists discovered that the laser light used to operate the clock was itself interfering with the atoms and causing the problem.

The solution was to "re-tune" the light frequency in a certain way that brought the atoms under control. Accuracy could now be increased to one second lost per 300 million years.

Professor Jan Thomsen, a nuclear physicist from the Niels Bohr Institute at the University of Copenhagen in Denmark, who worked on the clock, said: "It was a fascinating journey into the world of quantum mechanics. We found out that not all fermions were the same. At the very low temperatures the fermions begin to 'see' each other and interact and then the atomic clock begins to go awry."

Not content with what they have achieved, the scientists are continuing to search for even more accuracy. "We dream of getting an atomic clock with perfect precision," said Prof Thomsen.

The research was reported in the journal Science.