Cryogenic Society

ALT Doppler cooling of antihydrogen atoms. Baker and colleagues1 trapped atoms of antihydrogen — the simplest stable atoms that consist only of antimatter particles — using magnetic fields. These atoms move at high speeds within the trap. The authors then irradiated the atoms with ultraviolet (UV) laser pulses that had a carefully tuned wavelength. Atoms and laser pulses are not shown to scale. a, Atoms moving towards the laser experience a Doppler effect that shortens the apparent wavelength of the light that interacts with the atoms; the resulting wavelength exactly matches the photon energy that can be absorbed by the atoms. Photon absorption simultaneously excites the atoms and slows them down (cools them). b, Atoms that move away from the laser experience the opposite Doppler effect, increasing the apparent wavelength of the UV light. The atoms cannot absorb photons at this wavelength, and the laser beam therefore passes through the atoms without causing undesirable acceleration.

ALT An artistic rendering of the movement of an antihydrogen atom in the ALPHA magnetic trap, before and after laser cooling [in grey before, and blue after] The images show various lengths of the antihydrogen tracks. Image: Chukman So/TRIUMF