A testing time for antimatter

Spectroscopy is the most accurate branch of science. Optical transition frequencies in isolated atoms and molecules can nowadays be measured to many-digit accuracies by applying the tools developed in the atomic physics community: ultrastable lasers, locked via frequency-comb lasers to atomic clocks, and the techniques to cool and control the motion of atoms. Precision measurements on small quantum systems can be compared with theoretical descriptions of these systems at the most fundamental level, allowing physics theories to be tested and enabling the search for physics beyond the standard model (1). On page 610 of this issue, Hori et al. (2) apply these tricks of the trade to a small atomic quantum system with a built-in antiparticle to perform precise spectroscopic measurement in antiprotonic helium (see the figure). The technique of buffer-gas cooling is demonstrated for the first time on a composite matter–antimatter particle.