Towards a redefinition of the second based on optical atomic clocks

The rapid increase in accuracy and stability of optical atomic clocks compared to the caesium atomic clock as the primary standard of time and frequency asks for a future redefinition of the second in the International System of Units (SI). The status of the optical clocks based on either single ions in radio-frequency traps or on neutral atoms stored in an optical lattice is described, with special emphasis on the current work at the Physikalisch-Technische Bundesanstalt (PTB, Braunschweig, Germany). Besides the development and operation of different optical clocks with estimated fractional uncertainties in the 10 − 18 range, the supporting work on ultra-stable lasers as core elements and the means to compare remote optical clocks with transportable standards, optical fibres, or frequency ratios is reported. Finally, the conditions, methods and next steps are discussed, which are the prerequisites for a future redefinition of the second.