Antiprotons have the same mass as protons, but carry an opposite electric charge. Both particles behave like tiny bar magnets: their so-called spin points in one of two directions, similar to a compass needle. The precise measurement of the associated magnetic moment, in particular by controlled ‘flipping’ of the spin, is one of the central tools of modern quantum measurement technology. This is because it allows fundamental laws of nature to be tested experimentally.
The study presented in Nature used the method of ‘coherent spin quantum transition spectroscopy’. This enables the high-precision manipulation and observation of individual spin states. The background to the measurements was the test of the so-called CPT symmetry (charge, parity, time reversal): it requires that matter and antimatter – apart from their opposite charges – behave exactly the same, meaning that they should occur with equal frequency in the universe. In reality, however, the world shows considerable asymmetry: it consists almost entirely of matter. This remains an unsolved mystery of modern physics to this day.
Until now, such coherent quantum transitions have been detected, for example, in macroscopic particle ensembles or in the hyperfine structure of stored ions. The BASE collaboration has now demonstrated and observed such a spin transition coherently for the first time in a single, free nuclear spin of an antiproton – which is an enormous challenge both physically and technically.
‘A good analogy for this is a playground swing,’ explains BASE spokesperson Prof. Dr. Stefan Ulmer from University Düsseldorf: ‘If it is pushed at the right frequency, it swings back and forth rhythmically. In our case, the swing is the spin of a single antiproton, which we set into oscillation using electromagnetic fields. We were also able to achieve a coherence time of 50 seconds."
The antiprotons required for the experiment were produced in CERN's Antimatter Factory (AMF) and stored in so-called Penning traps – high-precision electromagnetic instruments for precise particle control. They were then transferred individually to a separate multiple trap system, where their spin states can be measured and manipulated. ‘This is nothing more than a qubit consisting of a single antiproton spin,’ emphasises CERN scientist Dr Barbara Maria Latacz, the lead author of the study.
Greater precision for atomic clocks and quantum computers in Lower Saxony
Quantum bits, or qubits, play an important role at the Physikalisch-Technische Bundesanstalt (PTB) and Leibniz University Hannover. Prof. Christian Ospelkaus' working group, which is involved in the work of the BASE collaboration, is developing quantum computers based on stored ions. These methods could be used to further improve the measurement accuracy of protons and antiprotons by applying quantum computer calculations to the ‘antiproton qubits’. ‘Such “quantum gates” could be used to manipulate the antiproton via a stored ion and transfer the quantum state of the antiproton to a stored ion,’ says Ospelkaus. At PTB, this method is also used for atomic clocks and for extremely precise spectroscopy on molecular ions and highly charged ions.
Through comprehensive improvements to the setup, the BASE collaboration succeeded in systematically suppressing decoherence mechanisms, thereby enabling the first coherent spectroscopy of an antiproton spin. In doing so, the research team not only created a stable antimatter qubit, but also enabled completely new measurement methods.
The next major step has already been planned: with the newly developed BASE-STEP system, antiprotons will in future be transported from the AMF environment to specially prepared precision laboratories in transportable precision traps. There, spin coherence times up to ten times longer can be achieved, resulting in far greater measurement accuracy.
The BASE Collaboration
Founded in 2012 and based at the AMF at CERN, the collaboration includes research institutes in Germany, Japan, the United Kingdom and Switzerland. Its members include:
- Physikalisch-Technische Bundesanstalt, Braunschweig
- GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt
- Heinrich-Heine-Universität Düsseldorf
- CERN, Genf
- Leibniz Universität Hannover
- Max-Planck-Institut für Kernphysik, Heidelberg
- Imperial College London
- Johannes-Gutenberg-Universität Mainz
- RIKEN, Japan
- Universität Tokio
- ETH Zürich
Original publication
B. M. Latacz, S. R. Erlewein, M. Fleck, J. I. Jäger, F. Abbass, B. P. Arndt, P. Geissler, T. Imamura, M. Leonhardt, P. Micke, A. Mooser, D. Schweitzer, F. Voelksen, E. Wursten, H. Yildiz, K. Blaum, J. A. Devlin, Y. Matsuda, C. Ospelkaus, W. Quint, A. Soter, J. Walz, Y. Yamazaki, C. Smorra, and S. Ulmer. Coherent Spectroscopy with a Single Antiproton Spin. Nature XXX (2025)
B. M. Latacz, S. R. Erlewein, M. Fleck, J. I. Jäger, F. Abbass, B. P. Arndt, P. Geissler, T. Imamura, M. Leonhardt, P. Micke, A. Mooser, D. Schweitzer, F. Voelksen, E. Wursten, H. Yildiz, K. Blaum, J. A. Devlin, Y. Matsuda, C. Ospelkaus, W. Quint, A. Soter, J. Walz, Y. Yamazaki, C. Smorra, and S. Ulmer. Coherent Spectroscopy with a Single Antiproton Spin. Nature (2025)
