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Since 1967, the SI second has been defined by the microwave hyperfine transition of caesium-133. Today’s optical clocks reach statistical and systematic uncertainties at the mid-to-low 10⁻¹⁸ level, around 1000 times better than the best caesium clocks, and are driving the upcoming redefinition of the second. The seminar starts with a short history of clocks and timekeeping, then introduces the state-of-the-art optical clocks running at major National Metrology Institutes: neutral-atom and trapped-ion clocks based on strontium, ytterbium, calcium, aluminium and mercury, as well as the emerging thorium nuclear clock. As a concrete example, Mirza Akbar Ali follows the aluminium-ion (Al⁺) quantum logic optical clock at PTB, which operates at an uncertainty in the low 10⁻¹⁸ range, from the first ion-loading stages through to full clock operation, together with a complete evaluation of its error budget. The talk also explains why a higher frequency alone does not make a better clock, and why a clock’s uncertainty is what ultimately sets its performance.
Doctoral researcher.