Redefining the second: the road from cesium to optical clocks

Since 1967 the SI second has been defined by a microwave transition in cesium-133. It is the most precisely realized unit in all of measurement — and it is scheduled for retirement. Optical atomic clocks, which tick at frequencies tens of thousands of times higher using atoms like strontium and ytterbium or single trapped ions, now outperform the best cesium fountains by roughly two orders of magnitude in fractional uncertainty.

The international time community, through the CIPM and its consultative committee for time and frequency, has published a roadmap of criteria that must be met before redefinition — among them: multiple optical clocks validated against each other at the target uncertainty, demonstrated continuity with the cesium second, and regular contributions of optical data to International Atomic Time. The often-cited target window for a decision is around the 2030 General Conference cycle, though the criteria, not the calendar, will decide.

If the kilogram's 2019 redefinition is any guide, the day after the second is redefined will look exactly like the day before at every working laboratory. Stopwatch calibrations, timer verifications and frequency-counter certificates will not change. What changes is the ceiling: the definition will no longer be the limiting uncertainty for the world's best time and frequency measurements — and history says applications eventually rise to meet whatever headroom metrology creates.