The $$^{229}$$ 229 Th isomer: prospects for a nuclear optical clock
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Review
The 229 Th isomer: prospects for a nuclear optical clock Lars von der Wensea , Benedict Seiferle Ludwig-Maximilians-Universität München, 85748 Garching, Germany
Received: 9 March 2020 / Accepted: 19 September 2020 © The Author(s) 2020 Communicated by Nicolas Alamanos
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 ± 0.11 eV (152.7 ± 2.1 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.
Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . 2 A nuclear optical clock for time measurement . . . . 2.1 The general principle of clock operation . . . . 2.2 Operational principle of optical atomic clocks . 2.3 Accuracy and stability . . . . . . . . . . . . . 2.4 The idea of a nuclear optical clock . . . . . . . 2.5 Nuclear transition requirements . . . . . . . . . 2.6 The special properties of 229m Th . . . . . . . . 2.7 Different 229m Th-based nuclear optical clock concepts . . . . . . . . . . . . . . . . . . . . . 2.7.1 The single (or multiple) ion nuclear clock 2.7.2 The solid-state nuclear clock . . . . . . . a e-mail:
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[email protected] (corresponding author)
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3 Steps towards a nuclear clock . . . . . . . . . . . . . 3.1 Generation of 229 Th3+ ions . . . . . . . . . . . . 3.2 Paul trapping of 229 Th ions . . . . . . . . . . . . 3.3 Laser cooling of trapped 229 Th ions . . . . . . . 3.3.1 Direct laser cooling . . . . . . . . . . . . . 3.3.2 Sympathetic cooling . . . . . . . . . . . . 3.4 Laser systems for nuclear clock development . . 3.5 Steps towards a solid-state nuclear clock . . . . . 4 Potential applications . . . . . . . . . . . . . . . . . . 4.1 Search for temporal variations of fundamental constants . . . . . . . . . . . . . . . . . . . . . 4.2 Chronometric geodesy . . . . . . . . . . . . . . 4.3 Dark matter detection . . . . . . . . . . . . . . . 4.4 A 229m Th-based nuclear laser . . . . . . . . . . . 229m 5 Th: past and present research . . . . . . . . . . . 5.1 229 Th γ -ray spectroscopy . . . . . . . . . . . . . 5.1.1 First energy constraints .
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