The performance boosts chances that the world's timekeepers will one day adopt it for defining the second, they said.

The atomic clock, introduced in 1955, measures time by using microwaves to probe atoms as they transit between two energy levels.

The high accuracy of these clocks lies behind the success of the Global Positioning System (GPS), which requires orbiting satellites to be synchronized so that their signals, received on Earth's surface, can be triangulated to give the receiver's location.

Impressive as these clocks are, they can -- in theory -- be outperformed by lasers.