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Panoramic view of the clock hall at PTB with the four caesium clocks CS1, CS2, CSF1 and CSF2.

Laser Nuclear Spectroscopy

Working Group 4.44


We are pursuing a novel approach towards the development of an optical clock of very high accuracy, proposed by PTB scientists in 2003: In the "nuclear clock" the reference frequency is provided by a nuclear transition frequency, in contrast to all present atomic clocks that use a transition in the electron shell. The protons and neutrons inside the nucleus are much more tightly bound than electrons in the atoms' shell and they are therefore much less influenced by external perturbations that could change the resonance frequency. Therefore the nuclear clock promises to provide a higher accuracy.

Usual nuclear transition frequencies are in the range of hard X-rays, sofar unaccessible for the precision technolgy that is used in atomic clocks in the microwave and optical frequency range. One exceptional nuclear transition is known in Th-229 and forms the basis for the concept of the nuclear clock. This nucleus possesses a long-lived excited state at an excitation energy of only 8 eV above the nuclear ground state. The electromagnetic transition between these two states corresponds to vacuum-ultraviolet light at 150 nm wavelength, that is accessible with established laser technology.

The subject of laser nuclear spectroscopy will give access to a novel field at the border between nuclear and atomic physics. It will provide novel tools and methods for the coherent excitation of a nucleus, like for example electronic bridge processes.

More than 10 research groups worldwide are presently investigating the prospects for a Th-229 nuclear clock. Two different preparations of Th-229 are investigated: ions inside a radiofrequency ion trap and thorium-doped transparent crystals. Recent years have seen important progress in the preparation and characterisation of these systems. Within the EU project nuClock (2015-2019) a number of important properties of the Th-229 nucleus have been measured for the first time or with improved accuracy. A central challenge remains: It has not yet been possible to observe a laser excitation of the nucleus, because the value of the transition wavelength is not yet known to sufficient accuracy. New Opens external link in new windowresults from electron- and gamma-spectroscopy have recently improved the situation significantly. Currently the work is funded by an ERC Synergy Grant Opens external link in new windowThoriumNuclearClock in cooperation with TU Vienna, LMU Munich and the Univ. of Delaware. At PTB, a VUV laser system based on four-wave mixing in xenon for direct resonant laser excitation of the nucleus is being developed.

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A selection of literature on the Th-229 nuclear clock can be found on the webpage of the EU project Opens external link in new windownuClock

From 2020 on our work on the Th-229 nuclear clock is supported by a Synergy Grant of the Opens external link in new windowEuropean Research Council ERC.

Homepage of the ERC Synergy project Opens external link in new windowThoriumNuclearClock

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