The $^{229}$Th isomer: prospects for a nuclear optical clock

TL;DR

This paper reviews the progress and prospects of developing a nuclear optical clock based on the low-energy isomer of thorium-229, highlighting its potential for high precision timekeeping and redefinition of the second.

Contribution

It provides a comprehensive overview of current research, challenges, and future directions in the development of a nuclear clock using $^{229}$Th.

Findings

Nuclear clock could achieve systematic frequency uncertainty of about 10^{-19}

Significant progress has been made toward nuclear frequency standard development

$^{229}$Th's low nuclear excitation energy enables nuclear laser spectroscopy

Abstract

The proposal for the development of a nuclear optical clock has triggered a multitude of experimental and theoretical studies. In particular the prediction of an unprecedented systematic frequency uncertainty of about $10^{-19}$ has rendered a nuclear clock an interesting tool for many applications, potentially even for a re-definition of the second. The focus of the corresponding research is a nuclear transition of the $^{229}$Th nucleus, which possesses a uniquely low nuclear excitation energy of only $8.12\pm0.11$ eV ($152.7\pm2.1$ nm). This energy is sufficiently low to allow for nuclear laser spectroscopy, an inherent requirement for a nuclear clock. Recently, some significant progress toward the development of a nuclear frequency standard has been made and by today there is no doubt that a nuclear clock will become reality, most likely not even in the too far future. Here we present a comprehensive review of the current status of nuclear clock development with the objective of providing a rather complete list of literature related to the topic, which could serve as a reference for future investigations.