GoKawiilAntiproton reservoir preparation
The BASE-STEP catching trap is constructed of 15 cylindrical electrodes arranged in a coaxial stack (Extended Data Fig. 1a). Sapphire rings ensure electrical insulation and mechanical alignment. Antiprotons emerging from the degrader foil follow the magnetic field lines (B = 993 mT) into the trap, where three high-voltage electrodes (C01–C03) on the degrader side capture them. These electrodes form a nested axial potential well floated to −142 V with a depth of 16 V (Extended Data Fig. 1b). For capture, we pulse beam-side electrode C01 from 0 V to −158 V in 30 ns using a high-voltage switch. This pulse closes the potential well centred on C02 and traps antiprotons from the low-energy tail of the degraded beam.
After capture, cotrapped electrons—secondary particles forced out of the degrader—sympathetically cool the antiprotons61. We then ramp the C02 voltage to increase the trapping potential to 200 V, which destabilizes most contaminant negative ions while preserving antiproton confinement because of their higher q/m. We subsequently transport the particles to the central region of the catching trap. To this end, we configure electrodes C01–C03 to form a 20-V well and shift the axial potential adiabatically through the stack (typical electrode ramp time: 1 s). Finally, we confine the particles in a compensated, orthogonal five-electrode Penning trap formed by the central ring electrode C08, correction electrodes C07 and C09 and grounded endcaps starting at C06 and C10.
For non-destructive antiproton detection, we remove residual contaminants that would otherwise distort particle motion by means of space-charge effects and prevent formation of a stable axial dip signal. The dominant residual species are electrons and H− ions, which cannot be eliminated by high-voltage cleaning because of their similar or higher q/m.
We remove cotrapped electrons by applying an axial dipolar drive at 9.59 MHz to an endcap electrode (Extended Data Fig. 2a). By reducing the trap depth to 1 V, we tune the electron axial frequency into resonance with the applied drive. We then further reduce the trap depth to 0.5 V, evaporating the excited electrons. We repeat this sequence until no electrons remain. The drive is sufficiently detuned from the motional frequencies of antiprotons and H− ions to avoid unintended excitation. Following injection, we repeat electron cleaning as needed to remove beta-decay-induced electron contamination from activated surfaces (Extended Data Fig. 2b). We then tune the particle cloud into resonance with the axial detector (453 kHz) and apply magnetron sideband cooling. As described in the main text, at thermal equilibrium, the particle–detector interaction produces a characteristic dip in the detector noise spectrum.
Next, we remove heavier residual ions that persist after the initial high-voltage ramp using stored waveform inverse Fourier transform excitation62 applied to the endcap electrode. The drive spans 20–380 kHz, exciting their axial motion. By subsequently reducing the trap depth to 0.5 V and repeating the sequence, we obtain a reservoir containing only antiprotons and H− ions.
Antiprotons and H− ions exhibit nearly identical axial frequencies (separated by about 250 Hz) and indistinguishable dip widths per particle, such that both species contribute to a combined dip signal. At the applied catching parameters, we typically obtain a mixed particle cloud consisting of about 80% antiprotons and 20% H− ions. To discriminate between them, we exploit the difference in modified cyclotron frequency, which amounts to about 16 kHz at B = 993 mT. Using a single trapped proton as a magnetic field probe before injection, we determine
$${
u }_{+,\bar{p}}=15.140\,{\rm{MHz}}\,,\qquad {
u }_{+,{H}^{-}}=15.124\,{\rm{MHz}}.$$ (3)
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