All atoms are identical, making ensembles of atoms ideal for universal timekeeping. In 1967, the SI definition of the second was changed to reference a hyperfine transition frequency in Cesium atoms. Atomic clocks based on alkaline earth atoms or alkaline earth like ions (such as Al) have now been developed making use of narrow-line optical transitions. To achieve high numbers of non-interacting atoms, which assists in the precision of these clocks, neutral atoms can be trapped in optical lattices. On the other hand, ion traps permit long interrogation times.