Congratulations to Benny for submitting his master thesis! Benny came from Heidelberg to do an external master thesis with us, working on two-photon excitation of xenon for spectroscopy and spin polarization. Benny will take a few months off to return as a PhD student after the summer break. Very similarly, Thomas had finished his master thesis earlier this year, only to return as a PhD student now: welcome back! Thomas will be setting up the passive ring laser gyroscope for seismological applications, the sensor will form the basis of project P2 in the newly established research unit RING.

The University of Bonn seeks to intensify its cooperation with the Kwame Nkrumah University of Science and Technology (KNUST) in Ghana. The field of quantum technology has been identified as one of the focus areas, and we are glad to contribute to this exciting development. As one of the first actions, Joseph Owusu from Ghana will spend a year with us to work on an external master thesis project. Joseph will upgrade the SrMOT and do spectroscopy on yet unexplored transitions. Welcome Joseph!

It’s the beginning of the summer term, and we welcome new students to our group: Sophia Paul started her master thesis on our quantum frequency conversion project, where she will explore small organic molecules such as methane as nonlinear media for the conversion process. Markus Bayer joined our group as well: Markus will ignite the field of magnetooptics in our group, aiming to interface strong magnetic field with our optical cavities. Good luck to both of you!

The first week of March tends to be the busiest week of the year: it’s the DPG meeting. We went there with almost the entire crowd, presenting a whooping collection of 10 talks and posters. In addition, we had the transportable SrMOT with us, which we displayed at a booth within the exhibition. This was very well received, with many many people enjoying the sight of blue laser-cooled Sr atoms. During the same week, Simon was lecturing at the EDM 2026 workshop in Les Houches. 

Congratulations to Thomas for submitting his master thesis! Thomas installed and characterized an array of passive ring laser gyroscopes that can reconstruct the full three-dimensional rotation vector. We develop such sensors for seismology, where Thomas simulated the wavefield of an earthquake by gently rocking the optical table. Watch out for the paper that will appear in a few weeks! 

In large active ring lasers, the free spectral range is about 100-times smaller than the bandwidth. As a consequence, the laser runs multi-mode on various longitudinal modes: a well-known nuisance that has interfered with precision measurements for decades. We have developed a method based on injection locking, in which we use an external laser to steer the mode index of the active ring laser. The method is remarkable robust and can now be found on the arXiv: https://arxiv.org/abs/2601.19676. 

Quantum frequency conversion is well established between VIS and NIR wavelength, but more challenging if large gaps in wavelength are to be bridged, or if wavelengths in the blue and the UV are involved. Many years ago, we had presented octave-spanning frequency conversion of single photons from the NIR to the UV. This early work could only accept one polarization state of the incoming photon. In an upgraded version of the experiment, we now present polarization-insensitive frequency conversion based on a different type of crystal. Congratulation to Katrin for this nice result! (https://doi.org/10.1364/AO.581771) 

Pound-Drever-Hall locking is the most common approach to lock a laser to an optical cavity, but so-called residual amplitude modulation is a common threat that leads to frequency drift. Ring resonators are sensitive to polarization by design, so it might be straightforward to consider a Hänsch-Couillaud scheme here as well. Combining a Hänsch-Couillaud scheme with lock-in detection allowed us to improve the stability of passive ring laser gyroscopes, now reaching the same performance level as in active operation mode. This work, with Jannik as lead author, has been published with Optics Letters: https://doi.org/10.1364/OL.581271

We have one PhD student less in our group: Thorsten defended his PhD thesis with flying colours! After many years in our group and intensive work on the mercury project, isotope shift spectroscopy became the central piece of his PhD. Congratulations, Dr. Groh!