BASE physicist Barbara Latacz in front of the experiment’s cryostat. This cylinder, which is kept at 4 kelvins (-269°C), houses the system of traps that cool and measure the antiprotons and a very strong magnet. (Image: CERN)

In a significant breakthrough for precision physics and antimatter research, in BASE (Baryon Antibaryon Symmetry Experiment), we have successfully demonstrated coherent quantum transition spectroscopy of the spin of a single antiproton [1]. This achievement, conducted at the CERN Antiproton Decelerator/ELENA facility, marks the first time that Rabi oscillations — coherent spin transitions — have been observed in a solitary nuclear spin-½ particle (a quantum bit). This achievement introduces the first step towards implementing powerful new techniques for improving the precision of antiproton magnetic moment measurements, with major implications for tests of CPT symmetry.

Historically, measurements of nuclear magnetic moments relied on incoherent methods. While these have produced high-resolution results, such as for example our 3000-fold improved measurement of the antiproton magnetic moment in 2017 [2], these were constrained by limitations such as resonance line width and suppressed spin transition rate due to decoherence. In contrast, the now implemented coherent schemes that were demonstrated here, enable measurements at much narrower resonance lines and higher spin-flip probabilities, thereby vastly improving measurement sensitivity. 

The BASE experimental setup features a sophisticated cryogenic multi-Penning trap system housed within a 1.945 T superconducting solenoid magnet. This system includes a precision trap (PT) with a highly homogeneous magnetic field for inducing spin transitions, an analysis trap (AT) for spin quantum state detection via the continuous Stern–Gerlach effect, and a cooling trap (CT) that allows for efficient cooling of the particle’s cyclotron motion [3]. 

Fig. 1: Experimental set-up. a, Multi-Penning trap to demonstrate coherent spin quantum transitions with a single trapped antiproton. The trap stack consists of an antiproton reservoir trap, a park trap, a highly homogeneous and shielded precision trap, an analysis trap to apply the continuous Stern–Gerlach effect and a trap to cool the antiproton’s modified cyclotron mode. The trap electrodes (golden) are spaced by sapphire rings (blue shading). b, Schematic of a single-particle detection system. The detector is represented by a parallel RLC circuit, with inductance L ≈ 2 mH, capacitance C_p ≈ 25 pF and R_p ≈ 150 MΩ. c, Magnetic bottle strength in the centre of the precision trap, as a function of current applied to the persistent local superconducting magnet. Error bars are smaller than the size of the data points. d, Non-destructive detection of spin transitions in the centre of the analysis trap by measuring the axial frequency of the single trapped antiproton. Each frequency measurement takes around 120 s. Image from Latacz, B.M., Erlewein, S.R., Fleck, M. et al. Coherent spectroscopy with a single antiproton spin. Nature 644, 64–68 (2025). https://doi.org/10.1038/s41586-025-09323-1

The core innovation is in the unique two particle measurement protocol developed in [1]. One antiproton, the “Larmor particle,” undergoes spin manipulation and state detection, while a second “cyclotron particle” measures the magnetic field via cyclotron frequency detection to precisely set the drive frequency for inducing spin flips. Through careful synchronization and shuttling between traps, we have achieved spin state initialization with nearly 100% fidelity and spin detection error rate below 5%. 

To establish coherent spectroscopy, compared to [2] a significant experiment upgrade was developed, implemented, and successfully characterized. This includes concepts for more efficient cooling performance [3], better magnetic shielding [4], and much improved magnetic field homogeneity [5], which directly impacts coherence time. Given the successful implementation, a development and optimization process that required about five years, we have for the first time observed Rabi oscillations of the antiproton spin state  in time-resolved data. Spin-flip probabilities exceeded 80%, with spin coherence times reaching approximately 50 seconds [1]. Line-shape scans revealed narrow resonance linewidths as low as 200 mHz, over 16 times narrower than in previous incoherent measurements [2], and with signal-to-noise ratios more than 1.5 times higher.  

Figure 2. Observation of coherent Rabi oscillations of the spin of a single trapped antiproton. The blue points represent the measured data and the red line depicts a Monte Carlo fit, which assumes 52 mHz cyclotron frequency measurement decoherence as determined in the related measurements. In grey are the uncertainties of the Monte Carlo simulation. Image from Latacz, B.M., Erlewein, S.R., Fleck, M. et al. Coherent spectroscopy with a single antiproton spin. Nature 644, 64–68 (2025). https://doi.org/10.1038/s41586-025-09323-1 

This enhancement translates, in principle, into a projected 25-fold improvement in statistical precision for determining the g-factor of the antiproton. However, systematic uncertainties arise from particle transport and magnetic field drift. These are partly mitigated by future developments, including our transportable antiproton-trap system (BASE-STEP) [6], which will allow antiprotons to be studied in quieter lab environments outside CERN's AD/ELENA facility.  

The potential implications are profound. With the methods demonstrated, we see perspective that will allow us to approach 10 parts-per-trillion (ppt) precision in future antiproton magnetic moment determinations. This paves the way for the most stringent CPT symmetry tests in the baryon sector, and even for new physics searches — such as interactions between antimatter and hypothetical dark matter particles [7].

Ultimately, this work marks a transformative step in quantum metrology using antimatter, highlighting the power of coherent spectroscopy at the single-particle level [8].