Abstract:
In recent years the possibility to realize a so-called Nuclear Clock has attracted increasing attention, as today’s most precise timekeeping devices, based on optical atomic clocks, could be challenged in performance by a nuclear clock, based on a nuclear transition instead of an atomic shell transition.
Such a nuclear clock promises intriguing applications in applied as well as fundamental physics, ranging from geodesy and seismology to the investigation of possible time variations of fundamental constants and the search for Dark Matter [1,2].
Only one nuclear state is known so far that could drive a nuclear clock: the ‘Thorium Isomer 229mTh’, i.e. the isomeric first excited state of 229Th, representing the lowest nuclear excitation so far reported in the whole landscape of nuclear isotopes. Since its first direct detection in 2016 [3], considerable progress could be achieved in characterizing the properties and decay parameters of this elusive nuclear excitation: the half-life of the neutral isomer was determined [4], the hyperfine structure was measured via collinear laser spectroscopy, providing information on nuclear moments and the nuclear charge radius [5] and also the excitation energy of the isomer could be directly determined 8.28(17) eV [6]. In a recent experiment at CERN’s ISOLDE facility, the long-sought radiative decay of the Thorium isomer could be observed for the first time via implantation of (β decaying) 229Ac into a VUV transparent crystal and subsequent fluorescence detection in a VUV spectrometer. Thus, the excitation energy of 229mTh could be determined with unprecedented precision to 8.338(24) eV, corresponding to a wavelength of 148.71(42) nm [7]. Moreover, the observation of the radiative decay lays the foundation for a future solid-state based nuclear clock as a promising alternative to an ion-trap based configuration.
This recent breakthrough opens the door towards a laser-driven control of the isomeric transition and thus to the development of an ultra-precise nuclear frequency standard. The talk will review the present status together with recently completed, ongoing and planned activities towards the realization of a first nuclear clock.
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[2] P G Thirolf, B Seiferle and L von der Wense, Ann. Phys. 531, 1800391 (2019)
[3] L von der Wense, B Seiferle, M Laatiaoui, J B Neumayr, H-J Maier, H-F Wirth, C Mokry, J Runke, K Eberhardt, C E Düllmann, N G Trautmann and P G Thirolf, Nature 533, 47 (2016)
[4] B Seiferle, L von der Wense and P G Thirolf, Phys. Rev. Lett. 118, 042501 (2017)
[5] J Thielking, M V Okhapkin, P Głowacki, D M Meier, L von der Wense, B Seiferle, C E Düllmann, P G Thirolf and E Peik, Nature 556, 321 (2018)
[6] B Seiferle, L von der Wense, P V Bilous, I Amersdorffer, C Lemell, F Libisch, S Stellmer, T Schumm, C E Düllmann, A Pálffy and P G Thirolf, Nature 573, 243 (2019)
[7] S Kraemer, J Moens, M Athanasakis-Kaklamanakis et al., arXiv:2209:10276 (2022) and Nature (2023), in press
