Chief Scientist Laboratories Fundamental Symmetries Laboratory

Chief Scientist: Stefan Ulmer (Dr. rer. nat)

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Research Summary

The Ulmer Fundamental Symmetries Laboratory is leading the BASE collaboration at the antiproton decelerator of CERN, Geneva, Switzerland. Our goal is to perform high-precision comparisons of the fundamental properties of the proton and the antiproton, such as magnetic moments and charge-to-mass ratios. Our measurements provide stringent tests of CPT invariance, which is the most fundamental symmetry in the Standard Model of Particle Physics. To this end, we have developed the advanced multi-Penning trap system BASE. With our experiments, we have performed the most precise test of CPT invariance in the baryon-sector by comparing the antiproton-to-proton charge-to-mass ratio with a fractional precision of 16 parts in a trillion, the most precise measurement of the proton magnetic moment with a fractional precision of 0.3ppb, as well as the most precise measurement of the antiproton magnetic moment with a fractional precision of 1.5 ppb. Very recently we have reported a substantial step forward and demonstrated the first coherent spectroscopy with a single antiproton spin. To unfold the full potential of our spectroscopy techniques, we have developed a strategy to transport antiprotons to offline laboratory space. To that end, we have developed the autonomous and transportable antiproton trap system BASE-STEP, and have recently demonstrated the first antiproton transport, a major milestone towards the successful implementation of our strategy.
In addition to BASE, the Ulmer Fundamental Symmetries Laboratory is involved in the ASACUSA-CUSP experiment, the PENTATRAP and mu-TEX experiments at the Max Planck Institute for Nuclear Physics in Heidelberg, to demonstrate antihydrogen ground state hyperfine spectroscopy, to measure atomic masses with highest accuracy and to study magnetic charge distributions in nuclei.

All research conducted by the BASE collaboration of CERN, which is led by RIKEN fundamental symmetries laboratory.

Main Research Fields

Mathematical & Physical Sciences

Related Research Fields

Physics

Keywords

High-precision measurements of the fundamental properties of the proton and the antiproton
Percision measurement of the ground state hyperfine splitting of antihydrogen
Development of novel techniques for high-precision Penning trap experiments

Selected Publications

Papers with an asterisk(*) are based on research conducted outside of RIKEN.

1. Latacz, BM, Erlewein, SR, Fleck, M., Jäger, JI, Abbass, F., Arndt, BP, Geissler, P., Imamura, T., Leonhardt, M., Micke, P., Mooser, A., Schweitzer, D., Voelksen, F., Wursten, E., Yildiz, H., Blaum, K., Devlin, JA, Matsuda, Y., Ospelkaus, C., Quint, W., Soter, A., Walz, J., Yamazaki, Y., Smorra, C., & Ulmer, S.:
"Coherent spectroscopy with a single antiproton spin."
Nature, 664, 64-68 (2025)
2. Leonhardt, M., Schweitzer, D., Abbass, F., Anjum, K. K., Arndt, B., Erlewein, S., Endoh, S., Geissler, P., Imamura, T., Jäger, J. I., Latacz, B. M., Micke, P., Voelksen, F., Yildiz, H., Blaum, K., Devlin, J. A., Matsuda, Y., Ospelkaus, C., Quint, W., Soter, A., Walz, J., Yamazaki, Y., Ulmer, S., & Smorra, C.:
"Proton transport from the antimatter factory of CERN."
Nature, 641, 871–875 (2025)
3. Latacz, B. M., Fleck, M., Jäger, J. I., Umbrazunas, G., Arndt, B. P., Erlewein, S. R., Wursten, E. J., Devlin, J. A., Micke, P., Abbass, F., Schweitzer, D., Wiesinger, M., Will, C., Yildiz, H., Blaum, K., Matsuda, Y., Mooser, A., Ospelkaus, C., Smorra, C., Soter, A., Quint, W., Walz, J., Yamazaki, Y., & Ulmer, S.:
"Orders of magnitude improved cyclotron-mode cooling for nondestructive spin quantum transition spectroscopy with single trapped antiprotons."
Physical Review Letters, 133(5), 053201 (2024)
4. Borchert, M. J., Devlin, J. A., Erlewein, S., Fleck, M., Harrington, J. A., Latacz, B., Wursten, E., Mooser, A. H., Bohman, M. A., Grunhofer, V., Smorra, C., Wiesinger, M., Will, C., Blaum, K., Matsuda, Y., Ospelkaus, C., Quint, W., Walz, J., Yamazaki, Y., & Ulmer, S.:
"A 16 parts-per-trillion measurement of the antiproton-to-proton charge-mass ratio."
Nature, 601, 53-57 (2022)
5. Bohman, M. A., Grunhofer, V., Smorra, C., Wiesinger, M., Will, C., Borchert, M. J., Devlin, J. A., Erlewein, S., Fleck, M., Harrington, J. A., Latacz, B., Wursten, E., Mooser, A. H., Blaum, K., Matsuda, Y., Ospelkaus, C., Quint, W., Walz, J., Yamazaki, Y., & Ulmer, S.:
"LC circuit mediated sympathetic cooling of a proton via image currents."
Nature, 596, 514 (2021)
6. Smorra, C., Stadnik, Y., Bohman, M., Borchert, M. J., Erlewein, S., Harrington, J. A., Devlin, J. A., Blessing, P. E., Higuchi, T., Wiesinger, M., Schneider, G., Mooser, A., Blaum, K., Matsuda, Y., Quint, W., Walz, J., Budker, D., & Ulmer, S.:
"Direct limits on the interaction of antiprotons with axion-like dark matter."
Nature, 575, 310 (2019)
7. Borchert, M. J., Blessing, P. E., Devlin, J. A., Harrington, J. A., Higuchi, T., Morgner, J., Wursten, E., Bohman, M., Wiesinger, M., Smorra, C., Blaum, K., Matsuda, Y., Quint, W., Walz, J., & Ulmer, S.:
"Measurement of ultra-low heating rates in a cryogenic Penning trap."
Physical Review Letters, 122(4), 043201 (2019)
8. G. Schneider, A. Mooser, M. Bohman, N. Schön, J. Harrington, T. Higuchi, H. Nagahama, S. Sellner, C. Smorra, K. Blaum, Y. Matsuda, W. Quint, J. Walz, S. Ulmer.:
"Double-trap measurement of the proton magnetic moment at 0.3 parts per billion precision"
Science Vol. 358, Issue 6366, pp. 1081-1084 (2017)
9. C. Smorra, S. Sellner, M. J. Borchert, J. A. Harrington, T. Higuchi, H. Nagahama, T. Tanaka, A. Mooser, G. Schneider, M. Bohman, K. Blaum, Y. Matsuda, C. Ospelkaus, W. Quint, J. Walz, Y. Yamazaki and S. Ulmer.:
"A parts-per-billion measurement of the antiproton magnetic moment"
Nature 550, 371-374 (2017)
10. *S. Ulmer, C. C. Rodegheri, H. Kracke, K. Blaum, A. Mooser, W. Quint, and J. Walz.:
"Observation of spin flips with a single trapped proton"
Phys. Rev. Lett. 106, 253001 (2011)