Fig. 1: The ALPHA-2 experiment, magnetic-field profile and level diagram of antihydrogen.

a, The ALPHA-2 central apparatus (adapted from another work¹⁴). The Penning–Malmberg trap electrodes with the superconducting octupole and mirror magnet coils, which form the magnetic minimum trap, are shown to scale. The solenoids at either end boost the field in the preparation traps to 3 T for a more efficient cyclotron cooling of leptons. For this work, they are actively stabilized and kept on continuously, which increases our efficiency in antihydrogen accumulation and reduces magnetic-field drifts during spectroscopy. A cryogenic optical cavity serves to both build up the 243 nm laser light needed to drive the 1S–2S transitions and to provide the counterpropagating photons that cancel the first-order Doppler shift. Blocking potentials applied to the brown-shaded electrodes prevent antiprotons, created by ionizing antihydrogen, from escaping axially. Pulsed laser light at 121.6 nm for laser cooling is produced in a Kr/Ar third-harmonic generation (THG) gas cell immediately outside the ultrahigh-vacuum chamber. The light enters the chamber via a MgF₂ window and travels straight through the atom trap to be detected on the far side using a photomultiplier tube (PMT). The cylindrical silicon vertex detector (SVD) for annihilations (shown in green) surrounds the trapping apparatus. b, Profile of magnetic (B) field on the trap axis. The five mirror coils combine to make the field on axis uniform within ±1.5 Gauss in the centremost 6 cm of the trap (shaded). c, Hydrogen energy levels. Level diagram showing relevant states (S states in black and P states in blue) in the manifolds of the first two principal quantum numbers, n = 1 and n = 2, at B = 0 T and B = 1 T (not to scale). States 1S_a and 1S_b are untrappable and transitions from these states are, therefore, not accessible in our experiment. The pink arrows indicate the optical transitions driven from the trappable states. The two-photon transitions are 1S_d–2S_d and 1S_c–2S_c. The purple arrow identifies the single-photon cooling transition of 1S_d–2P_a.