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 (Photo: Barber, NIST)
Technically, no clock can be more accurate than cesium standards such as NIST-F1 – the cesium fountain atomic clock that serves as the United States' primary time and frequency standard. But researchers have managed to develop an experimental atomic clock based on ytterbium atoms that boasts precision comparable to that of NIST-F1. The humble second was chosen as the International System of Units' (SI) base unit of time since it is based on the properties of the cesium atom (one second is the duration of 9,192,631,770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom).
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