| Abstract | Which periodic oscillator would make the best clock? Nobody really knows. But in 1982 Hans Dehmelt—who would later win the Nobel prize for his development of the ion trap—realized that the “ultimate” time keeper might be a single ion, trapped by electric fields and cooled with lasers. The frequency of light emitted by the ion as it transitions between two internal states could, Dehmelt estimated, be measured with a fractional uncertainty of 10⁻¹⁸, allowing a similarly precise definition of the second as a fixed number of cycles of the light wave. Nearly four decades later, researchers have fulfilled Dehmelt’s visionary prediction. Samuel Brewer and colleagues from the National Institute of Standards and Technology (NIST) in Colorado report an Al⁺ ion clock with a fractional uncertainty of 0.94×10⁻¹⁸, the most precise clock in existence today. Using different ions or neutral atoms in so-called optical lattices, other groups are expected to soon reach the 10⁻¹⁸ milestone, which would allow for increasingly precise tests of variations in fundamental constants. |
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