Thorium nuclear clocks for fundamental tests of physics

Project Information

ThoriumNuclearClock

Grant agreement ID: 856415

DOI

10.3030/856415

EC signature date 28 January 2020

Start date 1 February 2020

End date 31 October 2026

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 13 789 990,00

EU contribution

€ 13 789 990,00

Coordinated by

TECHNISCHE UNIVERSITAET WIEN
Austria

Periodic Reporting for period 3 - ThoriumNuclearClock (Thorium nuclear clocks for fundamental tests of physics)

Reporting period: 2023-02-01 to 2024-07-31

Summary of the context and overall objectives of the project

Time is one of the most basic physical units and probably the one we all have most experience within daily life. The most precise way of measuring time is by using so-called atomic clocks, which make use of extremely well determined quantum levels in the electron shell of atoms or ions to stabilize the “ticking”. These precision clocks have tremendous technological relevance; they constitute the backbone of our satellite-based navigation systems as well as our synchronized digital data traffic.

The Thorium nuclear clock project aims to implement a new type of clock – a nuclear clock. In contrast to electron shell levels used in atomic clocks, it uses quantum states within the atomic nucleus of Thorium-229 as a “ticking” reference. As the nucleus is a thousand times smaller than the electron shell, it reacts much less to perturbations caused by external fields or forces, so the nuclear clock is expected to be dramatically more robust than the current atomic clocks. Furthermore, the nuclear clock transition frequency is determined strongly by all fundamental forces acting inside the nucleus and hence can be used to probe these. A temporal or spatial variation of these forces could be a signature of the existence of dark matter, probably the universe’s greatest standing mystery.

Within the ERC synergy project, we will construct three complementary types of Thorium nuclear clocks and compare them amongst each other (and with conventional atomic clocks) to search for variations in the fundamental forces of nature.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The first reporting period (M20) corresponds roughly to the 1st project phase (first 2 years). The main aim of this phase is a coarse determination of the isomer energy to within < 1 nm precision. This will be the starting point for phase two, which will see the development of dedicated laser systems for high precision spectroscopy and direct optical excitation of the isomer.

Phase 1 is complete. The synergy team has produced and published the most precise isomer energy measurements so far, partially in collaborative work.
Two complementary experiments have yielded compatible (within error bars) results.
A third experiment performed be the team of Piet van Duppen of KU Leuven at CERN with the participation of Synergy partners TU-Wien and LMU-Munich has observed the direct radiative decay of the isomer and performed VUV spectroscopy in this signal.
When this result has been published, the first phase can be considered as terminated successfully.
The two former results have been published in Nature and created significant media attention; we expect a similar or even greater impact from the third result.

Phase 2 is also completed by the demonstration of resonant laser excitation to the Thorium-229 nucleus and a measurement of the transition frequency to 7 GHz precision (limited by laser line width).
Also this result has been realized in collaborative work of the ERC partners PTB and TU Wien, using equipment and samples produced by TU Wien and included into the laser system located at PTB Site in Braunschweig.
This result marks the first ever laser excitation of an atomic nucleus and can be considered a significant breakthrough.

In another experiment performed by TU Wien in collaboration with Okayama University at the Spring-8 synchrotron, the Th-229 isomer was excited through resonant excitation to the 2nd nuclear state at 29 keV. The radiative decay of the Th isomer was observed also in this system. An interesting new phenomenon was observed, preliminarily referred to as "X-ray quenching". The presence of the (non-resonant) X-ray beam seems to open an additional nuclear decay channel depleting the isomer in a timescale up to 50 times faster than the natural decay.

Preparations for the Phase 3 (frequency comb spectroscopy) are progressing with two VUV frequency combs under construction (LMU + TU Wien).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

A nuclear clock

Photo Microchannel detector

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