WASHINGTON ­ US scientists said Thursday they have built the world's most precise clock, whose ticking rate varies less than two parts in one quintillion, or 10 times better than any other.

The clock, made from the element ytterbium
The physicists built their ytterbium clocks using about 10,000 rare-earth atoms cooled to 10 microkelvin (10 millionths of a degree above absolute zero) and trapped in an optical lattice made of laser light.