But make way, Cesium:
An experimental atomic clock based on ytterbium atoms is about four times more accurate than it was several years ago, giving it a precision comparable to that of the NIST-F1 cesium fountain clock, the nation's civilian time standard, scientists at the National Institute of Standards and Technology (NIST) report in Physical Review Letters.
This photo shows about 1 million ytterbium atoms illuminated by a blue laser in an experimental atomic clock that holds the atoms in a lattice made of intersecting laser beams. The photo was taken with a digital camera through the window of a vacuum chamber. NIST is studying the possible use of ytterbium atoms in next-generation atomic clocks based on optical frequencies, which could be more stable and accurate than today's best time standards, which are based on microwave frequencies. (Credit: Barber, NIST)