Towards a Re-definition of the Second Based on Optical Atomic Clocks

TL;DR

This paper discusses the advancements in optical atomic clocks, their potential to redefine the second in SI units, and the necessary conditions and methods for such a redefinition, emphasizing recent developments at PTB.

Contribution

It provides a comprehensive overview of current optical clock technologies, their uncertainties, and the strategies for comparing and implementing them for a future SI second redefinition.

Findings

Optical clocks have achieved fractional uncertainties in the 10^-18 range.

Development of ultra-stable lasers supports optical clock precision.

Methods for remote clock comparison are advancing.

Abstract

The rapid increase in accuracy and stability of optical atomic clocks compared to the caesium atomic clock as primary standard of time and frequency asks for a future re-definition 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 of the current work at the Physikalisch-Technische Bundesanstalt (PTB). 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 via transportable standards, optical fibers, or transportable clocks is reported. Finally, the conditions, methods and next steps are discussed that are the prerequisites for a future re-definition of the second.