En route to the optical nuclear clock

As early as around 15 years ago, the concept of a new atomic clock with unique properties was being developed at PTB in Braunschweig: Instead of a transition frequency between two states in the electron shell being used as the pulse generator of their clock, as is the case in all atomic clocks in use today, they envisaged using a transition frequency in the nucleus. Because the protons and neutrons in the nucleus are packed more densely than the electrons in the shell by several orders of magnitude, they react less sensitively to outside disturbances that can change their transition frequencies – thus providing good conditions for a highprecision clock.

However, the frequencies of nuclear transitions are usually much higher (in the X-ray range); for this reason, they are unusable for atomic clocks. The sole known exception, and the foundation of PTB’s proposal, is the nucleus of thorium-229, which has a transition in the frequency range of ultraviolet light. This transition is within the reach of laser technology that is similar to that used in present-day optical atomic clocks. More than ten research groups around the world are currently working on projects concerning the feasibility of a thorium-229 nuclear clock. In experimental terms, this issue has proven to be extremely difficult. For this reason, no success has been achieved thus far in observing the nuclear transition using optical methods. The resonance bandwidth is, as desired for the clock, very narrow, but its frequency is only roughly known for lack of experimental data. It therefore resembles the proverbial search for a needle in a haystack.

Within the scope of a cooperation project between PTB, Ludwig-Maximilians Universität (LMU) Munich, Johannes Gutenberg University Mainz, the Helmholtz Institute Mainz and GSI in Darmstadt, an important breakthrough has now been achieved: for the first time, it has been possible for basic properties such as the size and shape of the charge distribution to be measured in the excited state of the Th-229 nucleus. To this end, the Th-229 nuclei were not excited from their ground state (as will happen in the future in the clock); instead, in a device developed by LMU, they were obtained in the excited state from the alpha decay of uranium-233, slowed and stored as Th2+ ions in an ion trap. By means of laser systems developed at PTB for the spectroscopy of these ions, it was possible to measure transition frequencies in the electron shell accurately in order to derive information about the properties of the nucleus.

To date, models based solely on theory have not been able to predict the behavior of the structure of the Th-229 nucleus in this unusually low-energy transition. These models can now be refined by means of the experimental data obtained. Furthermore, because the structure of the electron shell is easier to measure using spectroscopy, it has become possible to use it to demonstrate a laser excitation of the nucleus. This, however, does not mean that search for the optical resonant frequency of the Th-229 nucleus as the needle in the haystack has been completed, but now at least, we know what the needle we are looking for actually looks like.